By: Thomas E. McKone
In the period 2005 to 2006, I was working with a group that gained important new insight on the complex interactions among the ambient environment, indoor environment, and the human exposome. This was an EPA/NIEHS-funded project at the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) to characterize exposure to organophosphorus (OP) pesticides in an agricultural region of California. I led an effort to use models to evaluate relationships among different data sets relating to 600 pregnant Latina women who were farmworkers or members of farmworker families in the Salinas Valley. This is John Steinbeck country, the “long valley” with the Salinas River running down the middle from south to north and coastal mountain ranges providing boundaries on the south, east, and west. From a modeler’s perspective, it is a relatively isolated multimedia system.
The key data sets from this study were 600 urine samples evaluated for levels of the dimethyl and diethyl metabolites of OP pesticides, which were then heavily used in the valley. We compared probability plots of the two metabolites for the CHAMACOS to plots of 1000 age-matched women from the national probabilistic samples collected for the National Health and Nutrition Evaluation Survey (NHANES). We saw a surprising result. Both the CHAMACOS and NHANES sets showed logarithmic variations over several orders of magnitude for both metabolites but had essentially the same standard deviations. The CHAMACOS curves were systematically higher—by 10 nmol/L for the diethyl metabolites and 25 nmol/L for dimethyl metabolites. We expected higher metabolite levels due to the pesticide use but, because of the large number of factors that intervene between pesticide application and intake, we expected a random distribution of higher exposures, not a systematic increase. It was as though each woman was getting the same added daily dose, regardless of location, month, diet, etc. We crafted models to explain this–a regional multimedia fate model to track the temporal distribution among air, water, soil, sediments, vegetation; an indoor model link to ambient air and soil; simple pharmacokinetic models; and dietary intake models for both populations. We had detailed pesticide use data. We had environmental samples that confirmed our expectation of the short OP half-lives of OPs outdoors and long half-lives indoors.
Dietary data confirmed that dietary intake was a major contributor in both the Salinas and national samples and accounted for most of the interindividual variability. After testing several hypotheses using combinations of models and ground-truthing with measurements, we found what appeared to be a consistent explanation. Because OPs have a short lifetime in both the ambient environment and the human body, neither system could be controlling the levels in human urine. But the indoor environments, in which OPs can persist more than 100 days, were moderating the added exposure to the Salinas population. More interestingly, the indoor environment levels seemed to be in chemical equilibrium with the spatial and long-term average of concentrations throughout the Salinas valley. In effect the indoor environment was acting to integrate regional average levels—acting like a sampling medium. This explained why we saw urinary levels so consistent in space and time. It taught us an important lesson about understanding coupling among human end environmental systems and the effective use of models and measurements. The results were published in the journal Environmental Science and Technology in 2007.
T.E. McKone, R. Castorina, Y. Kuwabara, M.E. Harnly, B. Eskenazi, and A. Bradman, “Merging Models and Biomonitoring Data to Characterize Sources and Pathways of Human Exposure to Organophosphorous Pesticides in the Salinas Valley of California”, Environmental Science & Technology 2007 41 (9), 3233-3240. DOI: 10.1021/es0618447