Indoor/outdoor relationships of PM2.5-associated toxic metals/metalloids at a rural residence in North China: Sources and probabilistic health risks

Levels of toxic metals/metalloids (TMs) in indoor air are of vital importance to the health of residents due to the high toxicity of TMs and the fact that the majority of the time is spent indoors. Currently, the relationship between indoor and outdoor TMs at rural residences remains unclear. In this study, 24-h samples of indoor and outdoor fine particulate matter (PM2.5) were simultaneously collected at a rural residence during the heating season in North China. The results revealed that the concentrations of PM2.5 and certain TMs indoors were higher than those outdoors (p < 0.05), and the low correlation between the indoor and outdoor concentrations indicates the slight influences of indoor and outdoor TMs. Among the TMs, the As concentration exceeded its environmental threshold (6 ng/m3) both indoors and outdoors. An integrated approach comprising positive matrix factorization (PMF), infiltration factor analysis, correlation analysis, and polar plot analysis was used to identify the sources of TMs. Regarding indoor emissions, indoor coal combustion, indoor cooking sources and fugitive dust were identified, while in regard to outdoor emissions, the sources included traffic emissions, coal combustion, fugitive dust, and industrial emissions. The probabilistic risk estimated via Monte Carlo simulations indicated that both the noncancer health risk (HI) and cancer risk (TCR) were unacceptable as the corresponding safety thresholds were exceeded (HI > 1 and TCR>10(-4)). In particular, As dominated both the noncancer and cancer risks (nearly 40%). Therefore, more efforts are needed to reduce the As concentration and coal combustion source in rural areas.