CHAMPAIGN, Ill., Sept. 7 (UPI) -- Air pollution over the Indian subcontinent shows some surprising seasonal changes, say U.S. scientists analyzing 10 years of satellite data.

University of Illinois atmospheric scientists have studied trends in aerosol pollution concentration, distribution and composition using instruments on a NASA satellite launched in 1999, a university release said Tuesday.

Aerosols detrimental to human health can come from natural sources, such as dust and pollen carried on the wind, but the most hazardous aerosols, like soot and other hydrocarbons released from burning various fuels, are generated by human activity, Larry Di Girolamo, a professor of atmospheric sciences at U of I, says.

"The man-made aerosols tend to have a nastier effect on human health," he says.

The data shows very high levels of both natural and man-made aerosol pollutants over the Indian subcontinent, but the study also revealed some surprising seasonal shifts in man-made versus natural aerosols.

This is particularly true in the monsoon season, Di Girolamo says.

"Just before the rains come the air gets really polluted, and for a long time everyone blamed the dust," he says, "but (satellite data) has shown that not only is there an influx of dust, there's also a massive buildup of man-made pollutants that's hidden within the dust."

During the monsoon season, rains wash some of the dust and soot from the air, but other man-made pollutants continue to build up.

During the post-monsoon season, dust transport is reduced but man-made pollutant levels skyrocket as biomass burning and the use of diesel-fueled transportation soar.

During the winter, seaward breezes disperse both natural and human-generated pollution across the subcontinent and far out to sea until the pre-monsoon winds blow again, Di Girolamo says.