Fine particles of pollution lodge deep in the lungs, where they cause serious health problems. Scientists are working to tally the global consequences.
Solid particles and liquid droplets in the air come in a range of sizes, but those smaller than 2.5 micrometers pose the greatest risk to human health.
Smoke from fires in southwestern Oregon drifted over much of Oregon in this true color image from late September 2009. The associated aerosol image reveals that the smoke reached as far east as eastern Montana and central Canada, illustrating that pollution travels long distances.