Air Pollution and Atherosclerosis: New Evidence to Support Air Quality Policies

Increasing attention is being paid to the subject of air pollution, with the bulk of previous work being done in animal models suggesting that exposure to air pollution causes atherosclerosis—stiffening and calcification of the arteries—in rabbits and mice [1,2]. The new findings of Sara Adar and colleagues, published in this week’s PLOS Medicine [3], take us a step forward in clarifying the broader implications of air pollution by offering further evidence in humans that ambient particulate matter (PM) contributes to the development of cardiovascular disease (CVD). CVDs are now the world’s leading causes of death [4]. The most important underlying pathology resulting in CVD is atherosclerosis, known to be a life-long process with a long and silent pre-clinical phase. It has been known since the last century that ambient air pollution can trigger acute cardiovascular morbidities [5], and a comparative risk assessment of established triggers of myocardial infarctions concluded that a rather substantial fraction of these acute and life-threatening events can be attributed to current levels of air pollution [6]. However, it is of importance to understand the causes of atherosclerosis, given that its prevention or deceleration could drastically delay and reduce the burden of CVDs. The hypothesis that ambient air pollution could play a role in these chronic processes was first addressed and confirmed in rabbits and mice [1,2]. Adar et al. have now provided important humanbased results based upon the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA). It is only the second longitudinal study about the impact of air pollution on the progression of atherosclerosis and is the first one based on a general population sample free of CVD at study onset. The authors here share their early findings of MESA, which examined associations of common carotid artery intima-medial thickness (IMT), a surrogate of atherosclerosis, with long-term PM2.5 concentrations among over 5,300 people over an average of two-and-a-half years. They report that higher long-term PM2.5 concentrations are associated with increased IMT progression and that greater reductions in PM2.5 are related to slower IMT progression. MESA replicates findings observed in the first longitudinal study, which was based on data from five clinical trials conducted in volunteers from Los Angeles [7]. Both research teams used IMT as a marker of atherosclerosis [8]. With only two longitudinal studies, it is worth discussing three important issues that need to be addressed in the future, namely the contributing role of noise, the link between cross-sectional and longitudinal findings, and the identification of susceptibility factors.