Air pollution exposure increases the risk of illness and death among people with heart conditions. Understanding whether air pollution contributes to the development of diseases of the heart and blood vessels can help with preventing these diseases and helps justify strong air pollution standards. Our study of just over 4,000 people from six U.S. communities did not find evidence that living for 20 years in more polluted areas contributes to worsening function of the arteries, suggesting that air pollution may damage health through other pathways. Exercising, not smoking, eating a healthy diet, controlling weight, and supporting efforts to clean up the environment remain important strategies for maintaining good general and cardiovascular health.