Two California scientists have been honored for their research into air pollution, outdoor and indoor. This year's winners of the $200,000 Tyler Prize for Environmental Achievement, John Seinfeld and Kirk Smith, have shown the far-reaching nature of the problem.

Nearly half of the world's people use biomass fuels such as wood or dried dung to cook their food, and many cook indoors.

Kirk Smith of the University of California, Berkeley studies the impact of the environment on human health, and wondered about the impact of smoke on the families. In the early 1980s he was studying energy use in rural Asia.

"And during that time, I noted the very smoky conditions in village households," said Smith. "I came back and I thought, well somebody must have looked at the health effects of this, and I could find nothing in the literature. No one had and later measurements showed that household cooking can produce as much smoke per hour as 1,000 cigarettes burning. His studies show that this leads to nearly two-million premature deaths a year, especially among women and children. Smith has also shown that the emissions contribute to climate change.

Air pollution in one part of the world affects the air in another, says the other recipient of this year's Tyler Prize, John Seinfeld of the California Institute of Technology.

"Emissions from Asia will make it across the Pacific, will be in the air over the United States, and even in some cases be tracked out over the Atlantic heading to Europe," said Seinfeld. "And so you can think of the northern hemisphere as a big backyard."

He says the southern hemisphere has the same mixing, and there is long-term interaction between the two hemispheres.

Seinfeld says natural and man-made substances interact.

"Every particle in the air anywhere on earth is a little kitchen sink of compounds that come from everywhere."

The scientists say environmental research requires careful measurement. In Guatemala, India, China, and other countries, Kirk Smith has overseen studies to measure household emissions and assess the long-term effects on those exposed to smoke from cooking.

Teams are also assessing the effectiveness of low-pollution stoves, and Smith foresees widespread use of that technology when the results are in.

He says stoves that are proven to be effective at reducing emissions will benefit families and communities and help to clear the air around the world.

dung: the excrement of animals 动物粪便

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（来源：VOA 编辑：旭燕）