Humans spend about 90% of their lives indoors, where they are exposed to most of their chemical exposure from a “cocktail” of numerous synthetic chemicals (1, 2). A large fraction of these chemicals originates from the volatilization, abrasion, and leaching of commercial products (3). Some of these contaminants can exert toxic effects by entering human bodies via air inhalation, dermal exposure, or dust ingestion (3). For example, bisphenols, including bisphenol A (BPA), a significant class of endocrine-disrupting chemicals widely detected in indoor environments, have been associated with a range of adverse health effects, including disorders of the reproductive, neuroendocrine, and immune systems (4–6). It is essential to investigate the source, presence, and environmental fate of indoor contaminants.
In response to potential human exposure risks, substantial effort has been invested in determining the concentrations of commercial chemicals in the indoor environment (e.g., in the air, in dust, and on surfaces) and their toxicological effects (7–11). However, it has recently been recognized that commercial chemicals may undergo reactions with oxidants, such as ozone (O3), once released into the indoor environment (12–16). The transformation products are of great concern given their potentially higher toxicities than those of their precursor compounds, as exemplified by the formation of carcinogenic carbonyls from unsaturated lipids through reactions with O3 (17–20). Also, nicotine from tobacco smoke adsorbed onto indoor surfaces is converted to carcinogenic nitrosamines via a heterogeneous reaction with indoor gaseous nitrous acid (HONO) (21). Although very high mixing ratios [up to 14 parts per billion (ppb)] of HONO have been detected inside (22, 23), little is known about the gas-surface heterogeneous reactions in which it participates. Together, these results underscore the need to consider chemical transformations when assessing the health risks arising from indoor synthetic chemicals.