One of the complicated parts of assessing the hazards and risks of glyphosate is that the product that everyone uses (for example, Round-Up) is not just glyphosate. The active ingredient is glyphosate, but the final formulation sold in stores is a combination of glyphosate and other “inert” ingredients.
[Note: I’m going to stubbornly use quotation marks around the words “inert” throughout this article, to emphasize my point that this is not an accurate characterization. “Inert” implies inactive, which is not true. Read on for more.]
These “inert” ingredients are subject to essentially no testing, disclosure, and regulatory requirements, even though they almost always make up a larger percentage of the final product than active ingredients. And, evidence indicates that combinations of the “inert” and active ingredients can actually be more toxic than the pure active compound (for example, see here, here, and here).
A new publication by Mesnage et al. in Food and Chemical Toxicology reviews the problems with the status quo and the implications for health effects research. Given the relevance of this topic to my previous blog posts on glyphosate (see here and here) and pesticides in general, I’ll summarize some of the authors’ key points below.
But first, some terminology: what is the difference between active and “inert” pesticide ingredients?
Under the U.S. Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), an active ingredient is one that is intended to be toxic to the target species. For example, glyphosate, the active ingredient used in glyphosate-based herbicides (GBHs), blocks an essential enzyme pathway in plants. All other ingredients in a pesticide product, often added to improve effectiveness, are classified as “inert.”
But, it’s challenging to get extensive and accurate information about these chemicals because:
Neither the “inert” ingredients nor the final formulations (the combination of active + “inert” ingredients) are subject to most standard regulatory toxicity tests, such as evaluation of cancer and reproductive effects. As a result, pesticide approvals are based on the unrealistic scenario of exposure to the active ingredient alone.
Companies can routinely claim the identity and concentration of these “inert” ingredients as confidential business information (CBI). That is why you’ll often see labels like the extremely vague one below. As a result, it’s difficult – actually, essentially impossible – for scientists to independently evaluate possible toxicity. We are kept blind to these final products.
Because we don’t know the identity of these “inert” ingredients, there are essentially no monitoring data on environmental or human exposure.
So, in summary, we don’t know how toxic the “inert” ingredients or final formulations are; the identity of these “inert” ingredients is kept secret from the public; and we aren’t monitoring any of these chemicals for their presence in our bodies or the environment.
All of this makes it challenging for the EPA to conduct accurate pesticide risk assessments, which require information on both hazard (ie: toxicity) and exposure.
How are we supposed to evaluate the health effects of such a moving target? Robust epidemiological studies require precise definitions of exposure (referred to as the “consistency” principle) to prove causality. In essence, the exposure under investigation should be defined very specifically, such that it is not possible for variations in versions of the exposure to have different effects, which could muddy the overall conclusion of the study.
(As a concrete example, think about investigating the impact of “exercise” on health. Exercise is very broad, so it wouldn’t be helpful or informative to evaluate the effect of general “exercise,” which could span everything from a 30-minute walk once per month to a 2-hour run each day. The effects of these different types of exercise could have very different impacts on health. So, a better study question would be focused on a more specific type of exercise.)
For pesticide epidemiology, all of these changing formulations make it very challenging to draw conclusions on health effects across time and space. It’s quite likely that one study based in multiple locations could be evaluating the effects of different products at the same time. A study looking at one region over a period of several years also faces the same problem. As the authors of the recent publication stated, “formulated GBHs with the same product name from different countries may not be the same mixture of chemicals, nor the same as the brand-name product bought previously, or in the future.”
This is one possible reason for differing conclusions about hazard, and it makes reproducibility nearly impossible.
The authors put forth a few suggestions to improve this murky situation. Some can be acted on by researchers now, such as including detailed product information (ex: trade name, dates of manufacture, product ID number) in the methods sections of their papers, to facilitate reproducibility and comparison across studies.
Other proposals will need to wait until there is political will for policy change. Most important is the need for full public disclosure of pesticide product composition. (By the way, back in 1997, the American Medical Association urged Congress to “support all efforts to list both active and inert ingredients on pesticide container labels.”) The authors also suggest monitoring of food and feed for concentrations of the “inert” ingredients (that is, if we can get access to information about their identities!), so we can understand patterns of exposure.
Additionally, it is essential to revise the pesticide approval processes to include full testing of “inert” ingredients as well as the final formulated products. We urgently need a regulatory system that accounts for these real-world exposures.
It’s high time for transparency on these formulations and their effects on human health and the environment.
Last week, we published a meta-analysis that found that high exposure to glyphosate-based herbicides was associated with an increased risk of non-Hodgkin Lymphoma (NHL). There was a lot of discussion about this paper in the news, on social media, and across internet forums (as expected, given the ongoing controversy and high stakes of this conclusion). Most articles focused on the specific risk estimate that we reported, with headlines such as:
A common critique of these headlines (and our article) was that they (and we) were being misleading, because we reported the 41% increased relative risk of NHL – which sounds very scary!—rather than a 0.8% increased absolute risk of NHL – which sounds less scary.
At the risk of more undue attention on the 41% number (which as I said in my previous post, is less important than the finding of a significant association itself), let me explain a few things about (1) how we report results in epidemiological research, (2) why small increases in risk matter, and (3) how agencies like the Environmental Protection Agency (EPA) regulate on risk.
Relative risks vs. absolute risks
In epidemiology, we are trying to understand whether an exposure is associated with a disease. To do this, we compare the disease rate in the exposed group with the disease rate in the unexposed group. This ratio gives us the relative risk of disease between the two groups.
[Side note: this is why it is crucial for researchers to select an appropriate comparison group! The relative risk depends entirely on this decision! If your comparison group has an unusually high rate of cancer, you will get a very skewed (and wrong) answer about the effects of the exposure.]
This relative risk, however, does not give us any information on the absolute risk of the disease at the individual level. It only tells us whether the exposed group has a higher or lower chance of developing the disease than the comparison group. In our paper, we report that individuals with high exposure to glyphosate-based herbicides (for example, people who spray it daily for many years) have a 41% increased risk of developing NHL over their lifetimes, compared to those who were not highly exposed (infrequent or no history of use).
The absolute risk, by contrast, tells us the actual risk of the disease for a given level of exposure. This is much more intuitive. For example, on average in the US, approximately 2 out of every 100 people develop NHL during their lifetime. So, the absolute risk of NHL over a lifetime is 2%. Therefore, when our study reports a 41% increased risk for those who are highly exposed, that is equivalent to saying that these individuals now have an absolute risk of 2.8% risk of NHL.
These statistics are communicating the same basic information, but they sound very different. In our epidemiology courses, we learn that absolute risk is better for communicating to the public because it is easier to understand. But, because of the way that epidemiological studies are designed (comparing disease rates in one group vs. the other), our default is to report relative risks. And because we are used to thinking about these ratios, we don’t always realize that this information can be misinterpreted, misunderstood, and confusing. Maybe we should report both metrics in our abstracts.
Nevertheless, both ways of talking about risk give us the same answer to the central question of carcinogenicity: evidence suggests that glyphosate exposure is associated with an increased risk of cancer.
Why seemingly low risks are still important
Some environmental exposures have very high relative risks. Individuals exposed to high levels of asbestos in their homes, for example, have an 800% increased risk of developing mesothelioma, a very rare type of lung cancer.
Most common environmental exposures, however, are associated with relatively small increased relative risks. Let’s take a look at air pollution, a very common exposure. And more specifically, fine particulate matter (PM2.5), very tiny particles emitted from vehicles, industrial facilities, and fires. While exact estimates vary based on the population studied, an increased concentration (of 10 ug/m3, to be exact) in 24-hour average PM2.5 has been associated with a 0.4%-1.0% increased risk of death (mostly from cardiovascular disease). An increase (again, of 10 ug/m3) in long term average PM2.5 has been associated with an overall 10% increased risk of death.
Those seem like small changes in risk. So, can we stop worrying about air pollution?
No, definitely not.
Low relative risks applied to large populations can be extremely consequential. We are all exposed to air pollution. Everyday. And all of those exposures add up. In fact, PM2.5 was ranked as the 5th most important cause of death around the world in 2015, accounting for approximately 4.2 million deaths.
Glyphosate-based herbicides are the most heavily used herbicides in the world, with an estimated 1.8 billion pounds applied in 2014. Most of this usage is on commercial agricultural farms by workers with potentially high cumulative exposures over their lifetimes. Given the large number of people possibly exposed, any significant increase in risk – especially the 41% estimate that we report – is meaningful to consider at the population level.
Finally, I want to bring up a point about cancer risk in relation to regulations. The US EPA and Food and Drug Administration (FDA), among other agencies, have to manage and regulate risks for the population. For most scenarios, they have decided that an “acceptable risk” for a carcinogen in the general population is between 1 in a million and 1 in 10,000 (over a lifetime). In other words, EPA and FDA are supposed to take action to prevent exposure to carcinogens that would result in risks higher than those rates (the specific threshold depends on the scenario and, sometimes, technologic feasibility).
Our findings suggest that the absolute risk of NHL over a lifetime might shift from approximately 2% to 2.8% with high exposure to glyphosate-based herbicides. This difference represents an increase of 8/1000 – certainly above EPA’s threshold of concern for the general population.
Note, however, that some of the studies in our meta-analysis were focused on people using glyphosate in commercial agricultural settings. EPA usually allows a higher risk of cancer in occupational scenarios, approximately 1 in 1000. Even with that standard, however, our results would suggest a need for action.
I’m just using these comparisons to put our results in context, because many people seemed to discount this work because of the small absolute risk estimates. Before any actual regulatory action, EPA would need to consider extensive evidence on hazard and exposure in a formal risk assessment.
In closing, I hope that I’ve clarified a few points about risk that were raised in the aftermath of the glyphosate publication. But once again, let me emphasize that you should not focus too much on the specific numerical estimates above but rather use them to better understand that:
Relative risks are different than absolute risks. Epidemiologists usually use relative risks, so that is what you will see in published papers (and, likely, the headlines as well).
Exposures with low relative risks can still have huge impacts at the population level.
Regulatory agencies set certain benchmarks for acceptable lifetime cancer risk in the population. You might not agree with the thresholds, but those are the standards. Keep that in mind when you are reading about risks from environmental exposures.
Apologies for the long blog absence. I’ve been busy PhD-ing (including preparing for and passing my oral general exam!) and working on various side projects.
One of those side projects has been focused on glyphosate. Glyphosate, the active ingredient in Monsanto’s (now owned by Bayer) Roundup, is the most widely used herbicide in the world. First marketed in 1974, its usage skyrocketed after the introduction of “Roundup-ready” (i.e.: Roundup resistant) crops in 1996 and the practice of “green-burndown” (i.e.: using the chemical as a desiccant shortly before harvest) in the mid-2000s. In 2014, global usage was estimated to be 1.8 billion pounds.
But these staggering statistics are not the only claim to fame for glyphosate. It has also been the subject of intense international regulatory and scientific scrutiny in recent years, for its possible link to cancer. The stakes are high (billions of dollars for Monsanto, related to sales of both the herbicide itself and its line of herbicide-resistant crops), and the conclusions are controversial.
Carcinogenic or not, that is the question.
In 2015, the International Agency on Cancer (IARC) declared that glyphosate was a “probable human carcinogen” (relevant links: explanation of IARC classifications; official summary for glyphosate; IARC webpage with follow-up links). However, that same year, the European Food Safety Authority (EFSA) concluded that “glyphosate is unlikely to pose a carcinogenic hazard to humans, and the evidence does not support classification with regard to its carcinogenic potential.” In 2016, the US Environmental Protection Agency (EPA) determined that glyphosate was “not likely to be carcinogenic to humans at doses relevant for human health risk assessment.”
Ok, so that’s confusing. How did these agencies, all of which are supposed to conduct unbiased reviews of all of the evidence come to such different conclusions? There have been several recent publications that explain these inconsistencies (for example, see here and here). In essence, it boils down to: 1) differences in how the agencies weighed peer-reviewed, publicly available studies (most show adverse health effects) versus unpublished regulatory studies submitted by manufacturers (most do not show adverse health effects); 2) whether the agencies focused on studies of pure glyphosate or the final formulated glyphosate-based product that is used in agricultural applications (which is known to be more toxic); and 3) whether the agencies considered dietary exposures to the general population only or also took into account elevated exposures in occupational scenarios (i.e. individuals who apply glyphosate-based herbicides in agricultural settings).
Meanwhile, as the debate continues… 27 countries (as of November 2018) have decided to move forward with implementing their own bans or restrictions. And, Monsanto/Bayer faces more than 9,000 lawsuits in the US from individuals who link their cancer to the herbicide. (The courts ruled the first case in favor of the plaintiff, though Monsanto is appealing the decision).
This highly contentious area is outside the topic of my dissertation research, but I got involved because my advisor was a member of the EPA scientific advisory panel that reviewed the agency’s draft assessment of glyphosate in 2016. The panel’s final report raised a number of concerns with EPA’s process and conclusions, including that the agency did not follow its own cancer guidelines and made some inappropriate statistical decisions in the analysis.
Because of their dissatisfaction with EPA’s report, my advisor and two other panel members decided to pursue related research to dig further into the issues. I enthusiastically accepted the invitation to join.
Our collaborative group recently published two review papers on glyphosate. I’ll provide brief highlights of both below.
Reviewing our reviews, part 1: exposure to glyphosate
In January 2019, we published a review of the evidence of worldwide exposure to glyphosate. Even though glyphosate-based products are the most heavily used herbicides in the world, we were surprised (and dismayed) to find less than twenty published studies documenting exposure in only 3721 individuals.
So, our paper mostly serves to highlight the limitations of the existing data:
These studies sampled small numbers of individuals from certain geographic regions, mostly in the US and Europe, and therefore are not representative of the full scope of global exposures
Most studies relied on a single urine spot sample, which does not represent exposure over the long term and/or in different agricultural seasons
The occupational studies only covered 403 workers in total, a serious deficiency given its widespread agricultural use. Few assessed exposure before and after spraying; and no studies evaluated patterns related to seasonality, crop use, etc.
Only two small studies evaluated how population exposure has changed over time. So, we definitely don’t know enough about whether the dramatic increases in global usage have resulted in similarly dramatic increased concentrations in our bodies. (Presumably, yes).
In addition to highlighting the need to address the points above, we specifically recommended incorporating glyphosate into the National Health and Nutrition Examination Survey (NHANES), a national survey that monitors exposure to many chemicals – including other common pesticides. This is an obvious and fairly straightforward suggestion; in reality, it’s quite bizarre that it has not already been incorporated into NHANES. Testing for glyphosate would allow us to better understand exposure across the US – which is not reflective of global levels, of course, but an important start.
Reviewing our reviews, part 2: glyphosate & non-Hodgkin Lymphoma (NHL)
Our second paper, published earlier this week, was a meta-analysis of the link between glyphosate exposure and non-Hodgkin Lymphoma (NHL). Yes, diving right in to the controversy.
There had already been several prior meta-analyses that showed an association between glyphosate and NHL, but ours incorporates new research and applies a method that would be more sensitive to detecting an association.
A meta-analysis combines results from separate studies to better understand the overall association. While they technically do not generate any “new” data, meta-analyses are essential in the field of public health. A single study may have certain weaknesses, focus only on selected populations, or reflect a chance finding. In drawing conclusions about hazards (especially in this scenario, affecting millions of people and billions of dollars), we want to look across the collection of data from many studies so we can be confident in our assessment.
We were able to include a newly published follow-up study of over 54,000 licensed pesticide applicators (part of the Agricultural Health Study (AHS)). Compared to an earlier paper of the same cohort, this updated AHS study reports on data for an additional 11-12 years. This extension is important to consider, given that cancer develops over a long period of time, and shorter studies may not have followed individuals long enough for the disease to arise.
We conducted this meta-analysis with a specific and somewhat unusual approach. We decided to focus on the highly exposed groups in order to most directly address the question of carcinogenicity. In other words, we would expect the dangers (or, proof of safety: is it safe enough to drink?) to be most obvious in those who are highly exposed. Combining people who have low exposure with those who have high exposure would dilute the association. IMPORTANT NOTE: this approach of picking out the high exposure groups is only appropriate because we are simply looking for the presence or absence of a link. If you were interested in the specific dose-response relationship (i.e.: how a certain level of exposure relates to a certain level of hazard), this would not be ok.
Our results indicate that individuals who are highly exposed to glyphosate have an increased risk of NHL, compared to the control/comparison groups. This finding itself is not entirely earth-shattering: the results from prior meta-analyses were similar. But, it adds more support to the carcinogenic classification.
More specifically, we report a 41% increased risk. For comparison, the average lifetime risk of NHL is about 2%. However, I want to emphasize that because our analytical method prioritized the high exposure groups, the precise numerical estimate is less important than the significant positive correlation. Basically, the purpose of this and other related assessments (like IARC’s) is to understand whether glyphosate is carcinogenic or not: this is a yes/no question. It is up to regulatory agencies to judge the scale of this effect and decide how to act on this information.
As with any scientific project, there are several limitations. In particular, we combined estimates from studies that differed in important ways, including their design (cohort vs. case-control), how they controlled for confounding by exposure to other pesticides, and which reference group they chose for the comparison (unexposed vs. lowest exposed). When studies are very different, we need to be cautious about combining them. This is another reason to focus more on the direction of the effect rather than the exact numerical estimate.
Beyond the headlines
The news coverage of this work has focused on the overarching results (especially the 41% statistic), as expected. But I want to highlight a few other aspects that have been overlooked.
To better understand the timing of these studies in relation to glyphosate usage, we put together a timeline of market milestones and epidemiological study events.
Of note is that all of the studies conducted to date evaluated cancers that developed prior to 2012-2013, at the latest. Most were much earlier (80s, 90s, early 00s). As illustrated in the timeline, we’ve seen a huge increase in glyphosate usage since green burndown started in the mid-2000s. Yet none of these studies would have captured the effects of these exposures, which means the correlation should be easier to see in newer studies if/when they are conducted.
Also, as I mentioned above, we included the newly published AHS cohort study in our meta-analysis. One might expect the old and new AHS studies to be directly comparable, given that they were conducted by the same research group. However, our deep dive into both papers elucidated important differences; consequently, they are not directly comparable (see Table 8 of our paper). An in-depth discussion of these issues (and some of their potential implications) is a topic for a separate post, but there’s a clear lesson here about how important it is to carefully understand study design and exposure assessment methods when interpreting results.
Finally, two brief points on the animal toxicology studies, which we also reviewed in our paper because they provide complementary evidence for assessing hazard in humans. We discuss these data but did not conduct a formal pooled analysis (to combine results from separate but similarly designed animal studies), which would allow us to better understand overarching results from the animal studies. Anyone ready for a project?
Additionally, in future animal toxicology studies, researchers should use the formulated glyphosate product that is actually used around the world rather than the pure glyphosate chemical that has been the focus of prior testing. There is growing evidence to suggest that the final formulated product is more toxic, perhaps due to the added adjuvants and surfactants. And this would allow for better comparisons to the human epidemiological studies, which assess effects of exposure to the formulated product.
Reflecting on the process
I had followed the evolving story on glyphosate with great interest for several years, so it was exciting to be part of these projects. Contributing to research with a real-world public health impact has always been a priority for me, and this high-profile research (affecting millions of people, billions of dollars) certainly fits the bill.
That being said, it was not an easy process. These two papers represent years of work by our group, which we did on top of our regular commitments. Collaborating with three researchers whom I had never met also proved challenging, since we did not have established rapport or an understanding of each other’s work and communication styles. So, in addition to gaining skills in conducting literature reviews and meta-analyses, I learned valuable lessons in group dynamics. 🙂
Given the high-stakes and high-profile nature of this work, we were extra meticulous about the details of this project. We knew that it would be scrutinized carefully, and any error could damage our credibility (especially worrisome for me, since I’m just establishing myself in my career). It took many, many rounds of review and editing to get everything right. A good lesson in patience.
Speaking of patience, I know that scientific research and related policy decisions take time. But I hope that these two projects can contribute to moving forward in a direction that protects public health.
Or, should she use infant formula, which avoids the problem of breast milk contaminants but does not offer the same benefits to her newborn and may also contain toxic chemicals (because of lax food safety regulations or if contaminated water is used to reconstitute the formula, for example).
Last month, two papers (from the same group of collaborators) published in Environmental Health Perspectives attempted to address these issues by reviewing decades of relevant research. These papers are both quite extensive and represent impressive work by the authors – but it’s unlikely that non-scientists will wade through the details. So, I’ll do my best to help you out.
Breast milk vs. infant formula: What chemicals are in each?
The first paper starts by documenting all of the chemicals detected in either breast milk or infant formula, based on studies published between the years 2000-2014 (mostly in the United States). Below is a highly simplified table, with just the chemicals rather than other details (refer to the paper if you’re interested in more).
What can we learn from these data, other than that it looks like complicated alphabet soup?
Well, toxic chemicals have been detected in both breast milk and infant formula, but there are some differences in the types of chemicals found in each. Breast milk is more likely to contain lipophilic (fat-loving/stored in fat) and long-lasting chemicals, such as dioxins and certain pesticides. By contrast, breast milk and formula both have some common short-lived chemicals, such as bisphenol-A (BPA) and parabens.
While the paper also provides information about the average and range of concentrations of chemicals in each medium (and how they compare to acceptable levels of exposure for infants), it’s hard to draw general conclusions because there are such limited data available. It is complicated, expensive and invasive to get samples of breast milk across wide segments of the population, and relatively few studies have looked at chemicals found in infant formula. We need more information before we can accurately understand the patterns of exposure across the population.
Nevertheless, the presence of all of these chemicals seems concerning. No one wants to deliver toxic milk to children during their early months of life, when they are more vulnerable because their organ systems and defense mechanisms are still developing.
But, what do the data indicate about the health consequences of these exposures?
Early dietary exposures and child health outcomes
That’s where the second paper comes in. Here, the same group of authors reviewed the literature on the association between chemicals in breast milk and adverse health outcomes in children. (Note: they had planned to ask the same question for infant formula, but there were not enough published studies). They looked at many chemicals (such as dioxins, PCBs, organochlorine pesticides, PBDEs) and many outcomes (including neurological development, growth & maturation, immune system, respiratory illness, infection, thyroid hormone levels).
Overall, when looking across various chemicals and health outcomes, the current literature is actually… inconclusive. Many studies reported no associations, and studies asking similar questions often reported conflicting results. Furthermore, studies that reported significant effects often evaluated health outcomes at only one or two periods in early life, and we don’t know if those changes really persist over time.
A glass half full…of challenges
In the end, the authors ended up with more questions than answers – and a long list of challenges that prevent us from understanding the effects of breast milk-related chemical exposures on children’s health. For example:
Chemicals in breast milk are often also present in the mother during pregnancy. How can we disentangle the effects of exposures during the prenatal period from exposures due only to breast milk in early postnatal life?
Many of these studies represent a classic case of “looking for your keys under the lamppost.” We can only study chemicals and outcomes that we choose to focus on, so we could be missing other important associations that exist.
On a related note, most studies focused on exposure to only one or a small group of chemicals, rather than the real-world scenario of the complex mixtures in breast milk.
There was little study replication (ie: more than one study looking at the same question). Generally, we feel more confident drawing conclusions based on a larger pool of studies.
The few studies that did ask the same questions often used different experimental designs. These distinctions also pose challenges for interpretation, since differences in how researchers measure exposures and outcomes could affect their results.
Most studies evaluated levels of chemicals in breast milk using one or two samples only. How accurate are these exposure assessments, given that levels in the milk may change over time?
Measuring chemicals in breast milk is just one aspect of exposure, but it doesn’t tell us how much the infant actually received. Mothers breastfeed for different amounts of time, which affects how much is delivered to the infant. These person-to-person differences within a study could make it challenging to see clear results in an analysis.
Filling in the gaps
Perhaps the only certain conclusion from these publications is that much work remains. Not only do we need more studies that document the levels of chemicals in breast milk and infant formula (as the first paper highlighted), but we also need more data on the links between these exposures and health outcomes – including targeted research to address the challenges and key gaps noted above.
Importantly, because breastfeeding is associated with many key health benefits (such as improved neurodevelopment and reduced risk of obesity, diabetes, infections, and more), any study that looks at the impact of chemical exposures in breast milk should also ask a similar question in a comparison group of formula-fed infants. It is likely that the positive effects of breast milk far outweigh any potential negative impacts from the chemicals in the milk, and that the infants would actually be worse off if they were fed formula that had the same level of chemicals (but did not receive the benefits of breast milk).
I’ll be the first to admit: it is scary to think about all of these chemicals in breast milk. But, all decisions have trade-offs, and here, when weighing the risks and benefits, the balance still seems to favor breastfeeding in most situations.
My friends think that I’m slightly obsessed. I prefer to think of myself as extremely passionate.
I’m getting my PhD in toxicology, but my interests don’t stop when I leave campus. I live and breathe this stuff: I love reading, learning, and writing about all things environmental health.
So, naturally, I’m worried about my own exposures to the overwhelming number of pollutants that we’re surrounded by. Though much is out of my control, I do what I can to minimize my exposures by buying organic (even on a student budget, and sometimes to an extreme that annoys my friends and family), avoiding processed and packaged foods, minimizing my use of plastics, choosing fragrance-free products, obsessively searching for flame–retardant free furniture, etc. The list goes on. My environmental health knowledge and concern for my health (as well as the health of my potential future children?) drive my lifestyle and purchasing decisions.
Yet, there’s one lifestyle choice that I’m not willing to give up, especially while living in Seattle: biking. I bike commute to school almost every day of the year. (I only missed 2 days this winter, when the roads were icy). I love riding to campus; it’s my morning and evening meditation/reflection time and my exercise.
While I do ride on the Burke–Gilman trail for most of the way, with gorgeous views of the water, mountains and city skyline, there are also several segments on roads. Obviously, biking on busy, car-filled streets presents immediate physical dangers, like car-bike collisions (my housemate has been hit twice in 6 months) and getting doored (ouch! Looks so painful). But, there’s also all that disgusting air pollution I inhale as take deep breaths alongside the ever-steady stream of cars. Luckily, at least for now, I’m not affected by asthma or other respiratory conditions. Yet, air pollution has been linked to many health problems, including cardiovascular disease and dementia (the latter is the subject of my PhD dissertation); I can’t help but think about those dangers on my daily rides.
Am I causing more harm than good to myself by commuting by bike? Why am I willing to impose such strict controls on other parts of my life (ie: purchasing decisions), but I allow myself to take deep breaths of noxious miasma every single day? Sure, exercise is good for me – for both my physical health and my brain health. But, do the negatives (pollution, collisions) outweigh the positives (physical and emotional health)?
This report (prepared for the National Institute for Transportation and Communities) has a good summary of the research (as of 2014) about traffic-related air pollution exposure to bicyclists. Since then, more studies (for example, in Salt Lake City, Utah; Minneapolis, Minnesota; and Montreal, Canada (here and here)) have also quantified exposures to city cyclists; others (like this one in New York City) are in process now. All of these assessments are specific to each city, season, time, and route. It will take much more research to develop a body of information that reflects the average and range of possible exposures to cyclists.
Such research with individual level measurements is crucial. We routinely track ambient air pollution across the country with a surprisingly few number of monitors (check out this interactive map to explore your area). These devices, which are often located away from major roadways or pollution sources, would definitely underestimate my own exposure, especially when I’m biking right behind a bus.
Exact quantification pending, what am I breathing in on my morning and evening commutes?
Sulfur dioxide (SO2): Sulfur dioxide is released from industrial facilities and vehicles burning fuel with high sulfur content. It is linked to respiratory problems (like asthma attacks and airway irritation) and can contribute to formation of PM.
Of course, we are all exposed to these air pollutants when we walk outside (and while driving in cars). But, during vigorous exercise, like biking up Seattle’s killer hills, we breathe in at 2-5x higher rates, and also more deeply, than at rest. So, I’m inhaling much more of all this bad stuff when I bike – in traffic – each day.
What are the consequences?
Research on the effects of air pollution exposure during exercise and active transportation (ie: walking and cycling) is beginning to emerge. According to one recent study, walking along busy streets reduced the short term cardiovascular benefits of the exercise compared to walking in a park. In studies of cyclists, researchers have found that biking in traffic is associated with various physiological changes, such as increases in certain inflammatory blood cells, alterations in heart rate variability (see here, here, and here) and other cardiovascular measures, and decreases in lung function. The implications of these changes are still unclear, however. As usual, we need more research on the short- and long-term health effects of cycling in traffic.
Several studies (see selected examples from 2017, 2016, 2015, 2014, and 2010) have tried to examine the overall health trade-offs of cycling in cities. The general conclusion is that the long-term benefits of active transportation (ie: namely, physical activity) outweigh the potential risks from traffic accidents and air pollution. However, I think these assessments are limited in several ways:
Most focus on the impact on mortality only, rather than the other myriad of health effects from air pollution that could lead to decreased quality of life and then, indirectly, mortality.
Most only consider the effects of a single pollutant (usually PM2.5) rather than the effects of combined exposures to multiple traffic-related air pollutants (ie: what happens in the real world).
On a more technical level, they assume a linear dose-response (solid line, below), where the relationship between exposure and outcome is the same across all levels of exposure. However, some evidence suggests that this may not actually be the case for PM2.5. Instead, the curve might be supralinear (dashed line, below), where the risk increases more steeply at lower levels of exposure. In this scenario, there might be greater benefits to health per unit decrease in exposure at lower ends of the spectrum, which would alter the modeling calculations.
The alternative scenario (in epidemiology speak, the “counterfactual”) used in these cost-benefit assessments is decreased physical exercise. In other words, they are roughly comparing: [exercise + pollution] vs. [no exercise + pollution]. Because the benefits of physical activity are so enormous, this equation tips towards the [exercise + pollution] side. However, if I didn’t commute by bike, I would replace this exercise with alternative activities (with less exposure to air pollution, presumably). If my equation is instead [exercise + pollution] vs. [exercise + less pollution], it would likely tip in the other direction.
So, in summary, we don’t fully understand all of the physiological impacts of biking in traffic-related air pollution, and I think that the current cost-benefit analyses may actually underestimate the long-term costs to my health.
Hmmm…. Should I re-evaluate my decision?
Last summer, I bought myself an air pollution mask to wear while biking. But, while I hate to admit it, I don’t use it every day. (It often causes my sunglasses to fog up!). I really should use it – assuming it is as effective as it claims?
Even though Seattle has the reputation of having fairly good air quality, the 2018 American Lung Association State of the Air Report indicates that there is still enormous room for improvement. (And, as I noted above, this city-level ranking, based on ambient air monitors, likely definitely underestimates my exposure while biking).
Everyone makes risk-related choices differently, based on their own calculations and priorities; risk perception and decision-making is complex and not entirely rational. But, in general, people are more likely to accept risks that they perceive as controllable, familiar and natural compared to those they perceive as imposed by others, uncontrollable, and unfamiliar (see image, below).
I’m still trying to understand the calculations that led me to my decision to expose myself to substantial pollution every day. Maybe it is related to the fact that I have control in this situation, since it is my choice to bike? Maybe it is because Seattle appears to have relatively clean air, compared to other places I’ve lived, like Atlanta and Bangkok, where the pollution is more directly visible?
Maybe… maybe… I just love biking too much, and this is where I draw my personal line. While it is definitely important to me to minimize harmful exposures and prioritize my health, I cannot and do not want to live in a complete bubble (though sometimes it seems to others that I already do). Life involves risk, and I’ve somehow decided that this is one I’m willing to take. Biking every day brings me too much happiness to give up (at least for now). Plus, cars are no safe haven; there’s plenty of dirty air inside from both internal and external sources.
While I take this risk, which is perhaps ironic given my PhD research on air pollution and dementia, there are some things I can do to mitigate my exposures. In addition to wearing my mask more consistently, I can check local air quality (like through this pollution app) and avoid riding on particularly bad days (like last summer, when Seattle was choked by horrific wildfire smoke). When bike paths are not available, I can do a better job of altering my route to prioritize low traffic roads, where I will be less exposed than on busier routes.
However, like for all pollutants, individuals only have limited abilities to control their own exposures. In the end, we need systemic, societal changes to make cities safer and healthier for people: stricter controls of vehicle emissions, increased utilization of electric cars and buses, improved public transportation, better bicycling infrastructure (eg: off-street bike paths), more greenspace, etc. The intersection of urban planning and public health definitely intrigues me (my next PhD? No, just kidding).
Ultimately, I hope that my own research can demonstrate the importance of strengthening air quality regulations and help motivate policies to reduce exposures across the population.
You don’t have to search long to find eye-catching headlines about CRISPR/Cas9, a technique that allows scientists to more easily edit specific parts of the genome. It’s particularly exciting for the same reason that it’s particularly concerning: it opens up a whole new world of possibilities for gene manipulation in humans, animals, and plants. The technology offers hope for real scientific, societal and medical advances, from engineering oranges to be resistant to citrus greening and controlling mosquito populations by making them infertile to removing genetic mutations that cause disease in humans (as demonstrated on a mutation that causes heart muscle to thicken dangerously).
However, since I’m a PhD student in Toxicology, I investigate the role of environmental exposures on human health. What happens in your body when you inhale polluted air or drink contaminated water, and how does this ultimately affect your risk for certain health effects?
So, while I had casually followed the news about CRISPR/Cas9 over the years, I had not seriously considered its application to my field until listening to the recent National Academies of Sciences workshop, The Promise of Genome Editing Tools of Advance Environmental Health Research. This workshop brought together experts from across the disciplines of environmental health to understand how these technologies can improve our understanding of the impacts of exposures.
It turns out, there’s a lot to be excited about. Here’s a snapshot of where they saw potential.
Identifying affected biological pathways
Much of the work of toxicologists is focused on understanding the specific biological mechanism, or “adverse outcome pathway,” that is affected by a pollutant when it enters the body. If the pollutant changes the normal signaling for a gene or protein, it could set off a domino effect of problems (like when bisphenol-A (BPA), an estrogen mimic, prompts inappropriate estrogen signaling in the body). CRISPR/Cas9 technology could allow researchers to alter different genes in different adverse outcome pathways of their model systems (either in vitro, cell-based, or in vivo, with whole animals) and then see how response to the exposure changes. If changing a specific gene leads to a change in biological response, it would provide insight to how the chemical exerts its effects.
For example, imagine that a pollutant activates a certain receptor by fitting into it perfectly, like a lock and key. Researchers could use CRISPR/Cas9 to alter the gene that determines the shape of the receptor so that the pollutant no longer fits – and thus could not activate the receptor and prompt associated changes in the cell. If, after this genetic modification, the pollutant does not trigger a response, we would know that it acts through the pathway that includes that specific receptor.
Incorporating and understanding human variability
Current toxicity testing approaches are usually based on genetically identical model systems, like mice, that do not represent the genetic diversity in humans. So, unsurprisingly, results from the lab do not reflect the full spectrum of effects that would be expected in the actual population. To remedy this issue, researchers could use CRISPR/Cas9 to introduce relevant human genetic variation into their models and then see how response to the exposure changes accordingly.
Information about which genetic variants are more vulnerable to specific exposures can be used to improve risk assessment. And, policy-makers could set regulations to protect even those individuals with highly sensitive genetic variants from harmful health effects. (This situation has come up with occupational beryllium exposure limits, since we know about a specific mutation that makes people much more likely to get chronic beryllium disease)
Elucidating effects of epigenetic changes
In recent years, toxicologists have become increasingly concerned about the potential for pollutants to make subtle changes in how genes are expressed without actually changing the genes themselves. This field of research is called “epigenetics” and is particularly relevant to the effects of early-life exposures. However, in many cases, we don’t fully understand the long-term implications of these changes. CRISPR/Cas9 could allow researchers to artificially induce epigenetic alterations of interest in the lab and then carefully track the consequences over time.
A slow start, but lots of potential
The applications described above, among others, clearly indicate that CRISPR/Cas9 gene editing technologies could hold great promise for advancing our understanding of the effects of environmental exposures across the population. While there are some studies that have already utilized this approach (for example, a 2016 study about the effects of the antimicrobial triclosan on human liver cells), its use across toxicology have been limited so far. Hopefully, the discussions and excitement following this workshop can prompt further widespread adoption.
Yet, tricky ethical questions also exist for CRISPR/Cas9’s potential environmental health applications. If we have the ability to eliminate a gene that makes someone susceptible to an environmental exposure, should we act to remove it from future generations? Once we start, where do we draw the line? Undoubtedly, these issues will be debated over the coming years. Nevertheless, when used for basic and translational toxicological research in the short term, CRISPR/Cas9 should be seen as an important tool to help us address the huge gaps in our understanding about the chemicals that we are exposed to on a daily basis.
Last month, the Lancet Commission on Pollution and Health released a striking report estimating that pollution caused 9 million deaths worldwide in 2015 – 3 times more deaths than caused by AIDS, tuberculosis, and malaria combined. Air pollution was responsible for the vast majority of these deaths, but water and chemical pollution also contributed substantial burdens.
Well before the new Lancet report was released, the international community had recognized the dangers of mercury and had been working to develop policies to minimize exposure to this pollutant. In fact, on August 16, 2017, after sixteen years of work and negotiations, the Minamata Convention on Mercury entered into force.
This global treaty aims to protect human health and the environment from the toxic effects of mercury through restriction of mercury products and processes. It is the first new international convention in almost 10 years focused specifically on health and the environment. (Other previous treaties include the Basel Convention for hazardous waste, the Rotterdam Convention for pesticides and industrial chemicals, and the Stockholm Convention for highly persistent global pollutants).
The convention is named after the decades-long environmental health tragedy in Minamata, Japan. Residents and animals in this area developed severe neurological syndromes after eating seafood that had been highly contaminated with mercury from industrial pollution.
Mercury is a naturally occurring metal, and certain chemical forms (specifically, methylmercury and metallic mercury vapor) are highly toxic. According to the World Health Organization (WHO), mercury is one of the top ten chemicals of public health concern. The nervous system – and in particular, the developing brain – is highly vulnerable to mercury. Exposure can result in permanent neurological damage. (Remember the Mad Hatter from Alice In Wonderland?) Other organ systems, such as the lungs, kidneys, and immune systems, may also be affected. The United Nations Environment Programme (UNEP) has stated that there is no safe level of mercury exposure.
How Are We Exposed Today?
Mercury is emitted through both natural and industrial processes. Examples of natural processes that release mercury include rock weathering, forest fires, and volcanic eruptions.
However, this global treaty targets mercury from industrial and human processes. These include coal burning, waste incineration, consumer products, and small-scale gold mining. Because mercury emissions travel through air and water without regard to political borders, only an international treaty could truly be effective in addressing this pollutant.
Reduce or eliminate the use of mercury in chemical manufacturing processes
The convention also provides guidance for safe storage of mercury, waste disposal, and contaminated sites.
Threats to U.S. Progress and Compliance
The U.S. Environmental Protection Agency (EPA) aims to address mercury pollution through numerous programs and regulations. But now, some of those efforts are under attack or subject to delay – threatening our prospects for reducing mercury exposure and complying with the convention.
For example, the Mercury and Air Toxics Standards (MATS) rule, passed under the Obama administration, limits the amount of mercury released from coal-fired power plants. The D.C. Circuit Court of Appeals had planned to review the cost-benefit analysis for this regulation but recently decided to delay the case instead. The Trump administration may actually decide to repeal the regulation altogether rather than defend the rule in court.
The administration’s vocal support for revitalizing the coal industry and the proposed repeal of the Clean Power Plan would further reverse progress that we have made in reducing mercury emissions. Recent shifts away from coal in this country have led to decreased mercury emissions and declining mercury contamination in tuna – historically, a significant exposure route for the population.
These steps are hugely disappointing. Tackling this global pollution problem requires global action, and therefore the U.S. must continue to take strong steps to reduce mercury use and releases.
During these tumultuous times in particular, the ratification of this global treaty is an important victory for human health and the environment – and a reminder that we can still come together to make progress towards global health and sustainability. But, the realization of these goals requires political will and cooperation from all parties, and only time will tell if they can follow through on these targets.
Lead is a well-known neurotoxicant, and children are particularly vulnerable to its devastating and irreversible impacts. The U.S. Centers for Disease Control and Prevention (CDC) says there is no safe level of blood lead in children. The EPA estimates that approximately 16 million people, including 3 million children, live or attend school within one mile of airports using leaded avgas. Researchers have found that children living close to airports with planes using avgas have higher blood lead levels than children living farther from those airports. Workers who service or refuel the aircrafts may also be exposed.
There are alternatives to avgas, and it is estimated that about 80 percent of the current piston fleet across the country could operate safely on these fuels without retrofitting. Europe already implemented policies to promote the use of unleaded alternatives. Yet, without regulatory updates in the U.S., there is little incentive for industries to change or for airports to provide alternatives.
What is the roadblock to these policy changes? To regulate lead under the Clean Air Act, the EPA must make an “endangerment finding” that documents the hazard of lead released from aviation gasoline. Despite petitions from multiple advocacy groups, however, the EPA has declined to make this determination and has insisted on the need for more data.
In the meantime, the Federal Aviation Administration (FAA) formed the Piston Aviation Fuel Initiative, a collaboration between FAA and industry to spur the development of additional avgas substitutes by 2018. Whether this effort delivers on its promise remains to be seen. And even if a replacement is “certified,” the FAA estimated that a complete phase out of leaded fuel could take 11 years.
To spur changes in the absence of efficient federal progress, action at the state and local levels is needed. For example, requiring airports to provide unleaded gasoline or adopting taxes on leaded gasoline to promote use of alternatives. Revenue generated could be used for soil lead testing or remediation at homes, schools, and parks near airports using leaded gasoline. We urge local policymakers to consider such initiatives in the coming legislative sessions.
A recent report from the Pew Charitable Trusts calculated that removing lead from aviation fuel would prevent a 5.7 percent increase in child blood lead across the country and result in $262 million in gross future benefits.
Given the known hazards of lead exposure and the existence of alternative aviation fuels, we have an ethical responsibility to eliminate the use of avgas and protect our population from such a significant source of lead pollution.
Toxicology’s founding father, Paracelsus (1493-1541), is famous for his paraphrased proclamation: “the dose makes the poison.” This phrase represents a pillar of traditional toxicology: Essentially, chemicals are harmful only at high enough doses.
Standard toxicity tests don’t always detect the effects that chemicals can have at lower levels. There are many reasons for this shortcoming, including a focus on high dose animal testing as well as failure to include endpoints relevant to low dose disruption. And, even when the data do suggest such effects, scientists and policymakers may not act upon this information in a timely manner.
Recognizing these challenges, the U.S. Environmental Protection Agency (EPA) asked the National Academy of Sciences convene a committee to study the issue in detail. How can we better identify whether chemicals have effects at low doses? And how can we act on this information to protect public health?
After several years of work, the committee’s report was released in July. This landmark report provides the EPA with a strategy to identify and systematically analyze data about low-dose health effects, as well as two case study examples. It is an evidence-based call to action, and scientists and policymakers should take notice.
Delving into definitions
Before discussing the report, let’s review some definitions…
We know that animal experiments usually use high doses, but in comparison, what is a “low dose?”
This issue was a matter of considerable debate, but ultimately, the committee decided to proceed with a fairly general definition of low dose as “external or internal exposure that falls with the range estimated to occur in humans.” Therefore, any dose that we would encounter in our daily lives could be included, as well as doses that would be experienced in the workplace.
The committee also clarified the meaning of “adverse effects.” When a chemical produces a visible malformation, it is easy to conclude that it is adverse. But, when a chemical causes a small change in hormone levels, it is more difficult to conclusively state that the change is adverse. Are all hormone changes adverse? If not, what is the threshold of change that should be considered adverse?
In this context, an adverse effect was defined as “a biological change in an organism that results in an impairment of functional capacity, a decrease in the capacity to compensate for stress, or an increase in susceptibility to other influences.”
A strategy to identify low dose toxicity
With these semantics settled, the committee developed a 3-part strategy to help with timely identification, analysis, and action on low-dose toxicity information:
(1) Surveillance: Active monitoring of varied data sources and solicitation of stakeholder input can provide information on low dose effects of specific chemicals, especially since EPA’s standard regulatory testing framework may not always identify such effects. Human exposure and biomonitoring data should also be collected to help define relevant exposure levels of concern across the population.
(2) Investigation & Analysis:Systematic review and related evidence integration methods can be used to conduct targeted analysis of the human, animal, and in vitro studies identified in the surveillance step. Each of these approaches has different strengths and weaknesses, so examining the evidence together offers insight that a single approach could not provide.
(3) Actions: New evidence can be incorporated into risk assessments or utilized to improve toxicity testing. For example, protocols could be updated to include newly identified outcomes relevant to endocrine disruption.
Leading by example: systematic review case studies
To put their strategy into practice, the committee conducted two systematic reviews of low dose EDC effects.
The first case study looked at phthalates, chemicals that increase the flexibility of plastic products such as shower curtains and food wrapping.
The committee found that diethylhexyl phthalate and other selected phthalates are associated with changes in male reproductive and hormonal health. Overall, the data were strong enough to classify diethylhexyl phthalate as a “presumed reproductive hazard” in humans.
The second case study focused on polybrominated diphenyl ethers (PBDEs), flame retardants used for over 30 years. Though they are now being phased out, these chemicals remain a concern for humans. They are still present in older products and can persist in the environment for many years.
Based on data showing the impact of these chemicals on learning and IQ, the panel concluded that developmental exposure is “presumed to pose a hazard to intelligence in humans.”
Questions and challenges for the future
During its review, the committee encountered a variety of barriers that could impede similar investigations into specific chemicals.
First, when reviewing evidence, it’s important to assess any systematic errors – also known as biases – that might have led to incorrect results. These errors can arise from study design flaws, such as failure to properly blind the researchers during analysis.
Some journals have strict guidelines for reporting details related to bias, but many do not. Better adherence to reporting guidelines would improve scientists’ ability to assess the quality of evidence.
Second, the committee noted a discrepancy between the concept of doses used in human and animal studies, which made it difficult to compare data from different sources.
For example, most toxicologists simply report the dose that they delivered to animals. But some of that administered dose might not actually be absorbed. The actual internal dose of chemical circulating in the body and causing harm may differ from the amount that was administered. By contrast, epidemiologists usually think about dose as the level of chemical they detect in the body, but they may not know how much of the chemical an individual was actually exposed to.
Biological modeling techniques can help scientists draw the connection between administered and internal doses and more closely compare results from animal and human studies.
Finally, many toxicology studies focus on only a single chemical. This is a valuable way to identify how one chemical affects the body. However, given that we are all exposed to chemical mixtures, these procedures may be of limited use in the real world.
The committee suggested that toxicologists incorporate real-world mixtures into their studies, to provide more relevant information about the risk to human health.
Leveraging toxicity testing for answers about low dose effects
This report demonstrates one of the challenges facing the field of toxicology and environmental health: How well can existing and emerging laboratory techniques predict adverse outcomes in humans? (If you’ve read some of my previous posts, you know that this issue is of particular interest to me.)
Traditional animal experiments usually use high doses, which don’t necessarily reflect the real world. These studies can be an important first step in identifying health hazards, but they cannot accurately predict how or at what levels the chemicals affect humans. The committee noted that more relevant doses and better modeling could help mitigate this problem.
Emerging high-throughput testing techniques use cell-based methods to detect how a chemical changes specific molecular or cellular activities. These newer methods are increasingly used in toxicology testing. They have the potential to quickly identify harmful chemicals, but have yet to be fully accepted or validated by the scientific community.
For these two case studies, the committee noted that high-throughput tests were not particularly helpful in drawing conclusions about health effects. Many of these studies are narrowly focused – looking at, for example, just a single signaling pathway, without indicating a chemical’s overall influence on an organism. Nevertheless, these methods could be used to prioritize chemicals for further in-depth testing, since activity in one pathway may predict a chemical’s capacity to cause harm.
Putting the report into action
Despite the imperfections of our testing methods, there’s already ample evidence about low-dose effects from many chemicals (including the two cases studies from the committee). The EPA should implement this new strategy to efficiently identify and act on problematic endocrine-disrupting chemicals. Only through such strong, science-based efforts can we prevent adverse effects from chemical exposures – and allow everyone to live the healthy lives that they deserve.
Toxicologists are no strangers to mysteries. In fact, understanding unexpected results caused by unintentional chemical contamination in the laboratory has a storied history in the environmental health field.
Following in this tradition, researchers at the University of Massachusetts (UMass) Amherst recently conducted some important toxicology detective work after they noticed that mammary glands of adult male mice raised in a commercial laboratory (that supplies mice for scientists) were larger and more developed than the mammary glands of the same type of mice raised in their own laboratory.
Scientists use rodent mammary glands as models of human breasts, which allow them to better understand growth and development as well as risk factors and treatments for diseases like breast cancer. Toxicologists have paid particular attention to how mammary glands change after exposure to EDCs, since the chemicals can interfere with hormonal activity and mammary glands are especially responsive to hormonal signals.
So, when the UMass Amherst researchers noticed a difference in the mammary glands between the two groups of theoretically similar mice, it set off some alarm bells. Could an unidentified EDC exposure in the commercial lab be the culprit? And if so, could this potential EDC exposure impact the ability of scientists and policy-makers to draw conclusions from toxicological experiments?
Comparing Gland Growth
To find out, the researchers carefully compared the mammary glands and blood hormones between the two groups of mice. One group was ordered directly from a commercial supplier. The other group was ordered from the same commercial supplier and then bred for two generations to produce offspring that were raised in their own lab under controlled conditions to minimize exposure to EDCs.
The findings supported their preliminary observations: the mammary glands in the male mice raised in the commercial lab were larger and more developed than those of male mice raised in their own lab. The researchers noted that the commercially-raised mouse mammary glands actually mirrored those of mice from different experiments that had been intentionally exposed to BPA during early development.
By contrast, female mice had smaller and less developed mammary glands at puberty. While this difference in response may at first seem counter-intuitive, endocrine disruptors are complicated. Because they interfere with hormones, which are gender-specific, EDCs have the potential to affect males and females in different ways.
Contemplating the Culprit
Although the researchers found differences between the two sets of mice, they didn’t immediately know why. A simple explanation would have been that different amounts of circulating hormones at the time they examined the mice might have influenced their body composition. Yet, they detected no significant differences in estrogen, the primary hormone that drives mammary gland development, between the two groups.
It is also unlikely that genetic differences could have contributed to the distinct mammary gland growth, given that the non-commercially raised animals were actually only two generations removed from the original commercial strain. Such drastic genetic changes do not usually occur over such short cycles.
Therefore, the researchers hypothesized that the difference was more likely due to the effects of EDC exposures during their early life in the commercial laboratory. (However, they were not able to confirm this theory through specific tests.) Exposures to EDCs, among other chemicals, during sensitive windows of development in early life have the potential to cause long-lasting changes, including in the mammary gland.
Considering the Consequences
These findings, which suggest that animals raised in commercial labs may be exposed to EDCs, could impact how we interpret toxicological studies.
One main reason is what researchers call the “two-hit” model, which suggests that an exposure in early life makes an individual more sensitive to the effects of a second exposure later in life. In this context, early exposures to EDCs might prime laboratory animals for more pronounced responses when treated with a test chemical in an experiment later in life. In other words, some laboratory experiments may be erroneously linking the test chemical to an outcome that is actually due to a combination of the test chemical and being exposed to EDCs earlier in life.
This study only evaluated animals from one commercial lab, and conditions may differ in other labs. The possibility that animals could be subject to different unintentional exposures may affect our ability to compare studies and pool data from diverse labs to make science-based policies. This issue may also partially explain why research and policy on EDCs has been so highly controversial, with distinct labs generating very different results about the same chemicals.
These differences also have implications for a controversial practice in the field of toxicology: the use of “historical controls.” Sometimes, scientists compare the changes in treated animals from one study to a database of untreated animals from a collection of previous studies. This practice can provide researchers with a better sense of whether the treated animals they are studying are truly different from normal. Yet, this study suggests that control animals raised in some laboratories may be exposed to EDCs, and therefore it would not be appropriate to compare them to treated animals raised in different environments. The presence of too many differing variables would make it difficult to make an accurate comparison.
Taken together, these findings suggest that both scientists and the public should be cautious when interpreting certain studies. While the researchers did not conclusively confirm the linkage to early life EDC exposure, this highly likely explanation illustrates that, at least in some cases, there may be factors behind the scenes that could be influencing the results of toxicology experiments.
How should the scientific community address the implications of this study? While researchers could try to mitigate the impact of these exposures on their subsequent toxicological experiments by screening animals prior to beginning their work, a better approach would be to improve handling of animals in commercial facilities through strict standards. Exposures to EDCs, among other chemicals, should be minimized. If such exposures continue, it may be hard to trust the results of these toxicological studies, which could impede the development of appropriate, evidence-based environmental health policies and protections for the population.