Ambient air pollution is the largest environmental health problem in the United States and in the world more generally. Fine particulate matter smaller than 2.5 millionths of a meter, known as PM2.5, was the fifth-leading cause of death in the world in 2015, factoring in approximately 4.1 million global deaths annually. In the United States, PM2.5 contributed to about 88,000 deaths in 2015 – more than diabetes, influenza, kidney disease or suicide.
Current evidence suggests that PM2.5 alone causes more deaths and illnesses than all other environmental exposures combined.
Developed countries have made progress in reducing particulate air pollution in recent decades, but much remains to be done to further reduce this hazard. And the situation has gotten dramatically worse in many developing countries – most notably, China and India, which have industrialized faster and on vaster scales than ever seen before. According to the World Health Organization, more than 90 percent of the world's children breathe air so polluted it threatens their health and development.