Soot is in the general category of airborne particulate matter, and as such is considered hazardous to the lungs and general health when the particles are less than five micrometres in diameter, as such particles are not filtered out by the upper respiratory tract.[citation needed] Smoke from diesel engines, while composed mostly of carbon soot, is considered especially dangerous owing to both its particulate size and the many other chemical compounds present.[citation needed]Soot can stain clothing and can possibly cause illness if inhaled. Breathing common urban air pollution (containing soot) is much more deadly than previously thought, according to a major study and an editorial published in New England Journal of Medicine on February 1, 2007.
