Thursdays are one of my favorite days of the week. Not because the weekend is approaching, but because it is when the UW Department of Environmental and Occupational Health Science seminar series takes place. Each week, an outside speaker joins us to discuss his/her research. I often leave inspired, with broadened interests, and a renewed excitement and passion for the environmental health field.
This week was no exception. I had the privilege of hearing Dr. Howard Mielke discuss his area of expertise: lead contamination in cities.
Lead has been front and center in the news recently. From the tragedy in Flint to emerging concerns about lead contamination in schools around the country, we are all now highly aware of the fact that our water supply may not be appropriately protected from outdated and dangerous lead pipes.
However, Mielke’s presentation did not focus on lead pipes. Nor the other common exposure source that I was familiar with, lead paint. Instead, he emphasized lead in soil.
Digging into the facts about lead in soil
How did lead end up in the soil?
Before leaded gasoline was phased out in the US in 1996, lead was emitted from tailpipes as a volatile compound (PbBr2) but quickly reacted to form a non-volatile compound (PbSO4) that precipitated to the ground (for more on the atmospheric chemistry of lead, see here and here). Thus, for years, we had millions of cars spewing lead not only into the air but also onto the ground all around us.
These automobile-related lead emissions resulted in several trends, including:
- soil in larger cities is more contaminated than soil in smaller cities
- soil in city centers is more contaminated than soil in the suburbs, and
- soil near high-traffic areas (and buildings) is more contaminated than soil farther from these areas. (note: lead paint from buildings likely contributes to this last pattern)
Such widespread lead contamination in the soils around our homes was a surprise to me, and these distinct patterns emphasize the terrible, lingering legacy of leaded gasoline.
But, could this really be an important exposure source, given all of the attention on lead paint and pipes?
Part of answering this question involves understanding that child blood lead levels exhibit a well-documented seasonal pattern: higher levels in the summer, and lower levels in the winter.
What could account for this variation? Mielke and others suggest that in the summer months, children spend more time outside, playing in the yard. Lead-contaminated soil ends up on their hands, on their faces, and, likely also, in their mouths. In addition, soil dust tends to be drier in the late summer and can be easily inhaled. The observed seasonal patterns suggest that lead in soil accounts for a large part of child lead exposure. If lead paint were the main source of exposure, then we would probably see peaks in the winter, when kids are cooped up inside.
A solution for soiled soil?
While replacing leaded pipes seems like a tall task, eliminating lead in the soils around us feels even more overwhelming. Mielke has led efforts in the New Orleans area to bring in clean dirt to layer on top of lead-laden dirt (for example, in children’s playgrounds). But widespread implementation of massive soil-shifting projects seems unlikely.
Perhaps a better solution is to invest in emerging bioremediation techniques, using plants and microbes?
Big picture, though, Mielke advocates for a “Clean Soil Act,” analogous to our Clean Water Act or Clean Air Act, to provide the appropriate protections for the earth beneath our feet. (In fact, he helped develop a Clean Soil Act for Norway).
In the meantime, parents should be aware of this often overlooked source of child lead exposure. In addition, it is important for urban gardeners take appropriate precautions, since lead and other heavy metals can be absorbed in plants – thereby posing potential risks through dietary intake. (For more on heavy metals in gardens, check out Environmental Health Perspectives’ Urban Gardening: Managing the Risks of Contaminated Soil)
A lesson from the past?
In closing, I’ll share one of my favorite slides from Mielke’s presentation (see image below).
He displays a quotation from Yandell Henderson, a Yale professor who vehemently protested against adding lead to gasoline during a hearing in 1925. He warned that society was at a crossroads, facing “the question whether…the action of the Government is guided by [scientific] advice; or whether commercial interests are allowed to subordinate every other consideration to that of profit.”
Unfortunately, we know how that particular story ended up.
And while there have been numerous similarly discouraging stories in the realm of chemicals and children’s health, I’m still hopeful that one day soon, our country will be ready to take the other road – prioritizing public health and environmental protections over profits.