10 μg/m3 (darker blue) is WHO "safe" levelMillions die every year of respiratory diseases. A prime cause, epidemiologists suspected, is air pollution by "fine particulates" aka aerosols, smog, soot, smoke from traffic & kitchens, oil & gas flaring; or, when it reaches high altitudes, the "brown cloud". This map, published by Dalhousie University researchers Aaron van Donkelaar and Randall Martin in Environmental Health Perspectives (June 2010) and online by NASA shows ground-level readings of fine particulate matter 2.5 micrometers or less in diameter, about a tenth the fraction of human hair(PM2.5).
These small particles penetrate deep into the lungs, past the body’s normal defenses. More than 80 percent of the world's population breathe polluted air that exceeds the World Health Organization's recommended level (10 micrograms per cubic meter). Though most of North America does not exceed the "safe" level, hotspots can be seen in the Midwest and Eastern cities.
Sandstorms lift huge amounts of dust from the Arabian and Saharan deserts; it falls as far away as Brazil. The Asian brown cloud includes sulfate and soot particles from coal plants, factories, burning agricultural waste, kitchen fires and motor traffic.
Vehicles create significant amounts of nitrates and other particles. Untuned diesel engines are among the worst offenders, yielding dark sooty particles scientists call black carbon -- highly visible on the streets of Third World cities. But the total pollution is largely invisible.
These mix and create hybrid particles that threaten health. "There are still big debates about which type of particle is the most toxic," says epidemiologist Arden Pope. "We're not sure whether it's the sulfates, or the nitrates, or even fine dust that's the most problematic." No one is quite sure of the total impact in disease and death worldwide, adds Randall Martin: "Most of the epidemiology has focused on developed countries in North America and Europe." That is why this new map is so important.
See also Wikipedia on the Asian brown cloud, aerosol, particulates,