RWJF Scholar Finds Lead in Soil Can Harm Children
Sammy Zahran, PhD, is a Robert Wood Johnson Foundation (RWJF) Health & Society Scholar (2012 - 2014). He is assistant professor of demography in the Department of Economics at Colorado State University, assistant professor in the Department of Epidemiology in the Colorado School of Public Health, and co-director of the Center for Disaster and Risk Analysis at Colorado State University. This blog is based on his study: "Linking Source and Effect: Resuspended Soil Lead, Air Lead, and Children's Blood Lead Levels in Detroit, Michigan."
RWJF Health & Society Scholars lead the field of environmental health. This is part of a series highlighting their 2013 research.
Human Capital Blog: Tell us about your recent study, published in Environmental Science and Technology. What questions did you set out to answer? And what did you find?
Sammy Zahran: We sought to understand a mysterious statistical regularity in blood lead (Pb) data obtained from the Michigan Department of Community Health. The dataset contained information on the dates of blood sample collection for 367,800 children (<10 years of age) in Detroit. By graphing the average monthly blood Pb levels (μg/dL) of sampled children, we found a striking seasonal pattern (see Figure 1). Child blood Pb levels behaved cyclically. Compared to the reference month of January, blood Pb levels were 11-14 percent higher in the summer months of July, August, and September.
We determined that any theory of Pb exposure must account for the striking seasonal variation in child blood Pb levels. Our research aimed to reconcile a series of facts. First, like many other post-industrial cities, Detroit has remarkably high concentrations of Pb in neighborhood soils (>400 mg/kg). Outdoor soils are a reservoir for legacy Pb emissions. Second, studies show that child blood Pb levels correlate strongly with spatial variation of Pb in residential soils (see Figure 2). With pioneering urban toxicologist, Howard Mielke, PhD, we’ve published work showing that a 100 mg/kg increase in neighborhood soil Pb increases the odds of a child’s blood Pb level exceeding 5μg/dL by around 77 percent. Third, research shows that atmospheric concentrations of Pb spike during summer months in many U.S. cities. In fact, the seasonal levels of Pb found in children’s blood and in the atmosphere are very similar in peridocity and amplitude. Taken together, we suspected a soil to air dust to child pathway of Pb exposure, where Pb-contaminated urban soils are re-suspended as dust subject to seasonal weather changes, with higher quantities of air Pb dust inhaled by children in summer months.
To test this intuition of a soil to air dust to child pathway of Pb exposure, we collected daily air monitor data on both soil and Pb aerosols from the Interagency Monitoring of Protected Visual Environments. By analyzing the joint behavior of atmospheric Pb and soil, we found that a substantial fraction of the daily variation in atmospheric Pb could be explained by the amount of soil in the air (see Figure 3).
With the first link (soil to air dust) corroborated, we then analyzed whether blood Pb levels could be explained by atmospheric Pb, adjusting for demographic and neighborhood characteristics, as well as child residential proximity to Pb-emitting facilities. With a series of strategic comparisons involving the age and sex of a child, we found that the seasonal variation in child blood Pb can be explained by seasonal fluctuations in atmospheric Pb. We also found that the statistical effect of child exposure to Pb aerosols disappeared when the underlying soil source of air Pb was statistically removed. Meaning, absent the seasonal re-suspension of contaminated urban soils, the leftover effect of Pb aerosols on child blood Pb levels was negligible.
HCB: Do you think the scientific community and/or policy-makers pay enough attention to environmental health risks of lead? What do you hope your research will teach or show them?
Zahran: The negative health consequences of child Pb exposure are well understood and appreciated by scientific and policy communities. Because elevated blood Pb levels are associated with impaired cognitive, motor, behavioral, and physical abilities, Pb has been eliminated from most product streams (e.g., gasoline for automobiles, paint, water pipes and solder used to seal canned goods). With removal of Pb from gasoline, child blood Pb levels in the United States declined dramatically.
The social and economic benefits of limiting Pb exposure are substantial in terms of hospital costs averted, and increased future earnings following from lower levels of cognitive impairment in the population. Our study shows that work remains with respect to reducing child Pb exposure. The surface soils of our cities—including areas frequented by children like schools, playgrounds, and daycare centers—often contain unacceptably high levels of Pb. Soil remediation efforts are likely to yield great benefits.
HCB: Are you planning any additional studies on this set of issues, or on any aspect of environmental or children’s health?
Zahran: Yes, I’m working on two studies. The first paper examines maternal eclampsia risk by exposure to contaminated urban soils in New Orleans. We have data on 80,000 completed pregnancies in New Orleans before and after Hurricanes Katrina and Rita that we’ve merged with neighborhood soil Pb data.
The second paper examines variation in child blood levels in Michigan by proximity to airports. Leaded gasoline is still used in small aircraft. With detailed data from the FAA on departures, arrivals, and seat count of piston engine aircraft (that use leaded aviation gasoline) for airports in Michigan, we’ve developed a series of strategic comparisons to determine if the deposition of leaded gasoline from aircraft poses a health risk to children. This second paper is generously supported by the Robert Wood Johnson Foundation Health & Society Scholars program at Columbia University.