A bike ride a day…. might not keep the doctor away?

An abbreviated version of this post was originally published in The Crosscut with the title “Is biking a Catch-22 situation?

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 flameretardant 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.

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Heading home from school on my bike. Photo credit: Alex Kritchevsky

While I do ride on the BurkeGilman 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)?

What am I exposed to?

The short answer is: a toxic brew of traffic-related air pollution, at higher levels when biking on busier roads, and probably in higher doses than while driving in a car.

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?

  • Particulate matter (PM): PM is a mix of dust, dirt, and soot particles. Sources of PM (and gases that trigger formation of PM) include wood stoves, fires, power plants, vehicles, industrial facilities, and construction sites, among others. PM can also include heavy metals (such as lead, cadmium, copper, and zinc) from tires, brake wear, and diesel exhaust, as well as black carbon. Smaller particles, such as PM2.5 (≤2.5 micrometers) and ultrafine (UF) PM (≤100 nanometers), can penetrate more deeply into our bodies (and are thus likely more dangerous) than PM10 (≤10 micrometers). PM has been linked to adverse respiratory, cardiovascular, cognitive (neurodevelopmental and neurodegenerative), and reproductive outcomes.
  • Nitrogen oxides (NOx): Nitrogen oxides, reactive gases emitted from vehicles and power plants, are highly irritating to the respiratory system.
  • Volatile organic compounds (VOCs): Released from fuels and vehicle exhaust, this large class of compounds contributes to ozone formation. Many have also been classified as “hazardous air pollutants” by the US EPA because of their link to cancer, reproductive, or adverse environmental effects.
  • Ozone (O3): Ozone is formed through chemical reactions of NOx and VOCs in the presence of sunlight and triggers respiratory health effects, like reduced lung function and asthma attacks.
  • Carbon monoxide (CO): Colorless and odorless, carbon monoxide is produced through incomplete combustion from cars, trucks, and machinery. While the major concern with CO exposure at high levels indoors is its potential to interfere with transport of oxygen in the body and cause acute neurological and cardiac effects, the likely consequences of lower level outdoor exposures are more subtle effects on the heart and brain.
  • 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.
Linear (solid line) vs. supralinear cure (dashed line). Image from: Goodkind, Andrew L., et al. “A Spatial Model of Air Pollution: The Impact of the Concentration-Response Function.” Journal of the Association of Environmental and Resource Economists, vol. 1, no. 4, 2014, pp. 451–479. JSTOR, JSTOR, http://www.jstor.org/stable/10.1086/678985.
  • 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).

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Risk Space & Public Perception. Morgan, M. Granger. “Risk Analysis and Management.” Scientific American, vol. 269, no. 1, 1993, pp. 32–41. JSTOR, JSTOR, http://www.jstor.org/stable/24941545.

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.

Consolatory actions

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.

As you can see, it’s personal now.