Is the Relation Between Ozone and Mortality Confounded by Chemical Components of Particulate Matter? Analysis of 7 Components in 57 US Communities

Epidemiologic studies have linked ozone to adverse health outcomes including hospitalizations (1, 2) and death (1, 3–5), and this association is supported by toxicologic studies (1, 6). Evidence of these effects resulted in regulations aimed at lowering ozone (6). Particulate matter has been associated with heart failure, myocardial infarction, hospitalizations, and deaths (7–10). Historically, most research studied particulate matter by mass (e.g., particulate matter with aerodynamic diameter ≤2.5 µm (PM2.5) or ≤10 µm (PM10)) (11–16). Several studies found that observed ozone-mortality associations are robust to control for particulate matter mass (PM2.5, PM10) (3, 5, 17–22); however, the chemical composition of particulate matter varies geographically and seasonally (12, 23–31). Variations can depend on changes in sources (27, 32) (e.g., home heating fuel) and with weather (e.g., changes in mixing height (32), photochemical formation (26, 33)). Epidemiologic studies found health effects of particulate matter mass to vary by season and location (12–14, 28–30), which suggests that particulate matter composition might modify its health impact. Some studies measured the health impacts of specific PM2.5 chemical components, either locally (24, 30, 32, 34–38) or nationally (12, 39, 40), and found health effects for organic carbon (34–37, 40), elemental carbon (24, 30, 32, 34–40), sulfate (12, 24, 32, 35, 37, 38), nitrate (24, 34, 35, 40), ammonium (40), and silicon (12, 30, 37). Although there is evidence that the ozone-mortality relation is robust to particulate matter mass, specific PM2.5 components could potentially be confounders. Ozone precursors originate from the same sources as some particulate matter components. Additionally, PM2.5 components and ozone can covary with weather and can have related complex atmospheric reactions (6, 41). Little research has considered whether particulate matter components confound the relation between ozone and mortality, partially because data on joint ozone and particulate component concentrations are scarce. The US Environmental Protection Agency (EPA) began a network of chemical component monitors in 2000, which collect data in either 1 of 3 or 1 of 6 days, while many ozone monitors operate only during the warm season. One study found evidence that the ozone-mortality relation is confounded by sulfate PM2.5 but not by nitrate or organic carbon matter (42). However, this study had limited data (18 communities over 3–6 years/community). Although particulate matter is composed of dozens of components, 7 components (sulfate, nitrate, silicon, elemental carbon, organic carbon matter, sodium ion, and ammonium) constitute 79%–85% of PM2.5 in communities across the United States (23). They were the only components to contribute >1% to PM2.5 mass, either seasonally or throughout the year (23). Here, we investigate whether the ozone-mortality relation is confounded by these 7 main components of PM2.5 in 57 US communities over a 6-year period. This study does not aim to improve on earlier estimates of ozone effects (3, 20, 22), but rather investigates whether these effects are confounded by PM2.5 components.