Particulate matter (PM) air pollution less than 2.5 microm in diameter (PM(2.5)), which is now an all-pervading element of modern-day society, is associated with heightened cardiovascular morbidity and mortality. Not only can short-term PM(2.5) exposure trigger acute cardiovascular events, but longer-term exposure over years augments cardiovascular risk to an even greater extent. One biological mechanism capable of explaining this observation is that chronic exposure may promote the progression and vulnerability of atherosclerotic plaques. Indeed, recent epidemiologic studies have demonstrated an association between ambient PM(2.5) exposure and the presence or extent of atherosclerosis in humans. Several animal experiments have provided corroborating evidence that chronic exposures in fact do enhance the progression and perhaps vulnerability of atherosclerotic lesions. Due to the billions of people continually exposed to PM(2.5), the long-term pro-atherosclerotic effects of this ubiquitous air pollutant are likely to be of enormous and growing global public health importance.