Everyone is constantly and unavoidably exposed to PM2.5 from both natural and manmade sources. Natural sources include dust, pollen, mould, pet dander, forest fires, sea spray and volcanoes. Manmade sources primarily are smoking, fossil fuel burning, industrial processes, wood stoves, fireplaces and indoor cooking. Indoor exposures to PM2.5 can easily exceed outdoor exposures — by as much as a factor of 100. Although EPA claims that almost 25% of annual U.S. deaths are caused by PM2.5, no death has ever been medically attributed to PM2.5.
Of course that is not to say that we should not try to reduce these micro particles, simply that the effect of inhaling them does not seem to be as serious as we are led to believe.