[The following is an exact transcript of this podcast.]

There's a huge, gunky brown cloud that lingers over south Asia and the Indian Ocean each winter. It’s been known to cause respiratory diseases and even cancers in India and China. But scientists didn't really know what was in it. Now they do. Researchers from Stockholm University and colleagues published the results in the January 23rd issue of the journal Science.

The cloud contains black particles called carbonaceous aerosols—basically carbon soot. The team used radiocarbon analysis to figure out what parts of the soot come from biomass and what comes from fossil fuels. Biomass typically comes from burning forests for agriculture or burning wood in stoves. And fossil fuel particulates come from sources such as diesel engines or burning coal.

The investigators were surprised to discover that a large percentage of the soot, from almost half to two-thirds, comes from burning biomass like wood and dung for cooking and heat, rather than from coal power plants. Scientists say the good news is that these particles only remain in the atmosphere for a few days or weeks at a time. So once societies can figure out how to reduce biomass burning, that brown sooty haze—and the illnesses it causes—might disappear.

—Cynthia Graber 

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