Measurement and mechanism of airborne PAH uptake in wheat grain as a model edible plant

Many polycyclic aromatic hydrocarbons (PAHs) are toxic to humans and animals. They are primarily emitted to the air; from there they can deposit on and partition to vegetation surfaces including edible crops. Although inhalation exposure is important, previous studies have shown dietary intake of PAHs can be a significant route of exposure for the general population where grains and vegetables are an important source of dietary PAHs. PAHs in wheat grain harvested in California were investigated and considerable differences by locations were observed. Controlled exposure chamber experiments were conducted to investigate uptake and clearance behavior of wheat grain for volatile and semivolatile PAHs that were found in California grain. Stable PAH generation systems that we developed enabled us to conduct uptake studies for 196 days. Grain uptake was observed to occur over a long time period and was due to being kinetically limited. Computer models that describe sorption of these PAHs to grain were developed for both bare grain and grain with husk. These models provided insights on how the chemical reaches the grain, which could explain uniqueness of grain uptake of airborne PAHs. Case studies of applying the grain model to a real world exposure scenario were conducted. For the first case study, we employed model predicted grain/air concentration ratios to estimate relative importance of oral exposure through eating wheat grain to inhalation exposure. The analysis showed that the average daily dose of fluoranthene and pyrene from eating wheat was expected to be higher than inhalation. Comparison of risks of contracting cancer from inhalation and ingestion of wheat for fluoranthene showed the ingestion risk was eight times higher than the inhalation risk for a given air concentration. For the second case study, we applied the grain/air concentration ratios to air concentration data available in the literature to estimate possible gain contamination levels in different regions in the world. Air concentrations reported for some regions showed the ingestion risk level exceeding 1 x 10-5. Based on these initial calculations and analyses, the plant pathway appears to be an important route for human PAH exposure.