Polycyclic aromatic hydrocarbons are enriched but bioaccessibility reduced in brownfield soils adhered to human hands

The health risk associated with exposure to urban brownfields is often driven by the incidental ingestion of soil by humans. Recent evidence found that humans likely ingest the fraction of soil that passes a 45-microm sieve, which is the particle size adhered to the hands. We evaluated if PAH concentrations were enriched in this soil fraction compared to the bulk soil and if this enrichment lead to an increase in bioaccessibility and thus an increase in incremental lifetime cancer risk for exposed persons. Soils (n=18) with PAH concentrations below the current Canadian soil quality guidelines for human health were collected from an Arctic urban site and were sieved to pass a 45-microm sieve. Soil PAH profiles were measured and bioaccessibility was assessed using the Simulator of the Human Intestinal Microbial Ecosystem (SHIME). PAHs were significantly enriched in the <45 microm size fraction (3.7-fold) and this enrichment could be predicted according to the fugacity capacity of soil (Enrichment=2.18-0.055Zsoil, r2=0.65, p<0.001). PAH release in the stomach and small intestine compartments of the SHIME was low (8%) and could not be predicted by PAH concentrations in 45-microm sieved soil. In fact, PAH release in the SHIME was lower from the <45 microm size fraction despite the fact that this fraction had higher levels of PAHs than the bulk soil. We postulate that this occurs because PAHs adsorbed to soil did not reach equilibrium with the small intestinal fluid. In contrast, PAH release in the colonic compartment of the SHIME reached equilibrium and was linked to soil concentration. Bioaccessibility in the SHIME colon could be predicted by the ratio of fugacity capacity of soil to water for a PAH (Bioaccessibility=0.15e(-6.4x10E-7Zsoil/Zwater), r2=0.53, p<0.01). The estimated incremental lifetime cancer risk was significantly greater for the <45 microm soil fraction compared to the bulk fraction; however, when bioaccessible PAH concentrations in a simulated small intestine were used in the risk assessment calculations, cancer risk was slightly lower in the <45 microm soil fraction for these soils. Our results highlight the importance of using a small soil size fraction, e.g. 45 microm, for contaminated site human health risk assessment. However, further work is needed to estimate the bioavailability of this size fraction in an in vivo model and to assess the correlation between in vitro and in vivo gastrointestinal models.