Modeled estimates of chlorpyrifos exposure and dose for the Minnesota and Arizona NHEXAS populations

This paper presents a probabilistic, multimedia, multipathway exposure mode l and assessment for chlorpyrifos developed as part of the National Human E xposure Assessment Survey (NHEXAS). The model was constructed using availab le information prior to completion of the NHEXAS study. It simulates the di stribution of daily aggregate and pathway-specific chlorpyrifos absorbed do se in the general population of the State of Arizona (AZ) and in children a ged 3-12 years residing in Minneapolis-St. Paul, Minnesota (MSP). Pathways included were inhalation of indoor and outdoor air, dietary ingestion, non- dietary ingestion of dust and soil, and dermal contact with dust and soil. Probability distributions for model input parameters were derived from the available literature, and input values were chosen to represent chlorpyrifo s concentrations and demographics in AZ and MSP to the extent possible. Whe n the NHEXAS AZ and MSP data become available, they can be compared to the distributions derived in this and other prototype modeling assessments to t est the adequacy of this pre-NHEXAS model assessment. Although pathway- spe cific absorbed dose estimates differed between AZ and MSP due to difference s in model inputs between simulated adults and children, the aggregate mode l results and general findings for simulated AZ and MSP populations were si milar. The major route of chlorpyrifos intake was food ingestion, followed by indoor air inhalation. Two-stage Monte Carlo simulation was used to deri ve estimates of both interindividual variability and uncertainty in the est imated distributions. The variability in the model results reflects the dif ference in activity patterns, exposure factors, and concentrations contacte d by individuals during their daily activities. Based on the coefficient of variation, indoor air inhalation and dust ingestion were most variable rel ative to the mean, primarily because of variability in concentrations due t o use or no-use of pesticides. Uncertainty analyses indicated a factor of 1 0-30 for uncertainty of model predictions of 10th, 50th, and 90th percentil es. The greatest source of uncertainty in the model stems from the definiti on of no household pesticide use as no use in the past year. Because chlorp yrifos persists in the residential environment for longer than a year, the modeled estimates are likely to be low. More information on pesticide usage and environmental concentrations measured at different post-application ti mes is needed to refine and evaluate this and other pesticide exposure mode ls.