Particulate matter (PM), a heterogeneous mixture that includes chemicals, metals, and soils, is an air pollutant that contributes to multiple poor health outcomes; small particles, which are able to reach deep into the lungs, cause the greatest harm. Sources of fine PM emissions into the air include motorized vehicles, diesel-powered equipment, industrial and residential fuel combustion, and other industrial processes. Reviews of the health effects of PM2.5, which is the fraction of airborne particles less than 2.5 μm in diameter, have established that short- and long-term exposure has causal effects on cardiovascular outcomes such as ischemic heart disease and premature mortality and likely has effects on respiratory morbidity.1 Toxicological evidence from animal and human studies supports this epidemiologic evidence, demonstrating the physiological effects of PM2.5 on the cardiovascular system. The association between ambient PM2.5 concentration and ischemic stroke reported in this issue adds to the already strong evidence linking PM2.5 to cardiovascular effects (Wellenius et al2), and the analysis on cognitive function shows that we may not fully understand the breadth of PM health burdens (Weuve et al3). The strong and growing evidence on the harms of PM2.5 demands scrutiny of societal efforts to reduce exposure.