Student Research: Lindsay Smith

Plausibility of Direct Exposure to Paranitrophenol in the Chicago Methyl Parathion Sprayings Case


Incidents involving indoor residential application of the agricultural pesticide methyl parathion (MP) were reported in several states, including Mississippi, Ohio and Illinois in the 1990's. Exposure to methyl parathion from its illegal use indoors can have serious health consequences. Fatalities were reported after similar applications in Mississippi in the 1980's. As a result a substantial and expensive response to the more recent cases was mounted by EPA and ATSDR. Remedial decisions were based in part on observed paranitrophenol (PNP) levels in urine of residents of affected homes. PNP is a metabolite of MP and is typically used as a biomarker of exposure. Generally, it is assumed that all PNP in the urine originates from exposure to the MP parent compound. However, observation of asymptomatic individuals with very high urinary PNP levels led to the hypothesis that direct exposure to PNP could account for a large portion of urinary PNP in some cases. Since limited resampling of residences revealed similar MP and PNP residues on surfaces (mass/area basis), disproportionate direct exposure to PNP would require that the environmental availability of PNP be greater than that of MP. Three processes that could lead to higher exposure from PNP than MP residues are: (1) greater transport from treated surfaces to high contact surfaces, (2) greater transfer from those high contact surfaces to human skin and (3) faster dermal absorption. More rapid transport of PNP than MP from treated surfaces to non-treated surfaces is consistent with the higher vapor pressure of PNP. Whether PNP would transfer more easily from surfaces on which it was deposited from vapor to skin than MP is unclear. Once on skin however, dermal absorption of PNP would be expected to be much faster than absorption of MP. A probabilistic exposure assessment has been conducted incorporating differences in the properties of MP and PNP as well as available data from an MP exposure incident in Chicago, Illinois. Differential availability of environmental PNP does appear to be one possible explanation for anomalous urinary PNP.