Scientists at the United States Environmental Protection Agency (EPA) have estimated that, in the United States, 130,000 deaths each year are attributable to ambient (i.e., outdoor) particulate matter (PM) exposure. Neal Fann et al., Estimating the national public health burden associated with exposure to ambient PM(2.5) and ozone, 32 Risk Analysis 81, 88 (2012). Given these types of mortality estimates, EPA and other public health agencies frequently recommend that people stay indoors during high PM days to reduce their exposure to ambient particles. Ironically, scientific findings demonstrate that levels of indoor PM often exceed those outdoors due to common everyday indoor PM generation sources. That exposure is further exacerbated by the fact that people in the United States spend the majority of their time indoors where they are often in close proximity to indoor PM sources. Yet, relatively few epidemiology and toxicology studies have addressed the potential health effects of indoor PM as compared to ambient PM. It has been generally assumed that indoor PM is somehow less toxic than ambient PM.
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