One of China's lesser-known exports is a dangerous brew of soot, toxic chemicals and climate-changing gases from the smokestacks of coal-burning power plants.

In early April, a dense cloud of pollutants over northern China sailed to Seoul, sweeping along dust and desert sand before wafting across the Pacific. A U.S. satellite spotted the cloud as it crossed the West Coast of the United States.

Researchers in California, Oregon and Washington noticed specks of sulfur compounds, carbon and other byproducts of coal combustion coating the silvery surfaces of their mountaintop detectors. These microscopic particles can work their way deep into the lungs, contributing to respiratory damage, heart disease and cancer.

Filters near Lake Tahoe, in the mountains of eastern California, "are the darkest that we've seen" outside smoggy urban areas, said Steven Cliff, an atmospheric scientist at the University of California at Davis.

Unless China finds a way to clean up its coal plants and the thousands of factories that burn coal, pollution will soar both at home and abroad. The increase in global-warming gases from China's coal use will probably exceed that for all industrialized countries combined over the next 25 years, surpassing by five times the reduction in such emissions that is called for under the Kyoto Protocol.

The sulfur dioxide produced in coal combustion poses an immediate threat to the health of China's citizens, contributing to about 400,000 premature deaths a year. It also causes acid rain that poisons lakes, rivers, forests and crops.

The sulfur pollution is so pervasive as to have an extraordinary side effect that is helping the rest of the world, but only temporarily: It actually slows global warming. The tiny, airborne particles deflect the sun's hot rays back into space.

But the cooling effect from sulfur is short-lived. By contrast, the carbon dioxide emanating from Chinese coal plants will last for decades, with a cumulative warming effect that will eventually overwhelm the cooling from sulfur and deliver another large kick to global warming, climate scientists say.

A warmer climate could lead to rising sea levels, the spread of tropical diseases in previously temperate climates, crop failures in some regions and the extinction of many plant and animal species, especially those in polar or alpine areas.

In these respects, coal is indeed a double-edged sword: the new Chinese economy's black gold and the fragile environment's dark cloud.

Already, China uses more coal than the United States, the European Union and Japan combined. And it has increased coal consumption 14 percent in each of the past two years in the broadest industrialization ever. Every 7 to 10 days, another major coal- fired power plant opens somewhere in China.

To make matters worse, India is right behind China in stepping up its construction of coal-fired power plants and has a population expected to outstrip China's by 2030.

Aware of the country's growing reliance on coal and of the dangers from burning so much of it, China's leaders have vowed to improve the nation's energy efficiency. No one thinks that effort will be enough.

To make a significant improvement in emissions of global-warming gases and other pollutants, the country must install the most modern equipment equipment that for the time being must come from other nations.

Industrialized countries could help by providing loans or grants, as the Japanese government and the World Bank have done, or by sharing technology. But Chinese utilities have in the past preferred to buy cheap but often-antiquated equipment from well-connected domestic suppliers instead of importing costlier gear from the West.

The Chinese government has been reluctant to approve the extra spending. Asking customers to shoulder the bill would set back the government's efforts to protect consumers from inflation and to create jobs and social stability.

But each year that China defers buying advanced technology, older equipment goes into scores of new coal-fired plants with a lifespan of up to 75 years.

"This is the great challenge they have to face," said David Moskovitz, an energy consultant who advises the Chinese government. "How can they continue their rapid growth without plunging the environment into the abyss?"

Wu Yiebing and his wife, Cao Waiping, used to have very little effect on their environment. But they have tasted the rising standard of living from coal-generated electricity, and they are hooked, even as they suffer the vivid effects of the damage their new lifestyle creates.

Years ago, the mountain village where they grew up had electricity for only several hours each evening, when water was let out of a nearby dam to turn a small turbine. They lived in a mud hut, farmed by hand from dawn to dusk on hillside terraces too small for tractors, and ate almost nothing but rice on an income of $25 a month

Today, they live here in Hanjing, a small town in central China where Wu earns nearly $200 a month. He operates a large electric drill in a coal mine, digging out the fuel that has powered his own family's advancement. He and his wife have a stereo, a refrigerator, a television, an electric fan, a phone and light bulbs, paying just $2.50 a month for all the electricity they can burn from a nearby coal-fired power plant.

They occupy a snug house with brick walls and floors and a concrete foundation the bricks and concrete are products of the smoking, energy-ravenous factories that dot the valley.

One-fifth of the world's population already lives in affluent countries with air-conditioning, refrigerators and other appliances. This group consumes a tremendous amount of oil, natural gas, nuclear power, coal and alternative energy sources.

Now China is trying to bring its fifth of the world's population, people like Wu and Cao, up to the same standard. One goal is to build urban communities for 300 million people over the next two decades.

Already, China has more than tripled the number of air conditioners in the past five years, to 84 per 100 urban households. And it has brought modern appliances to hundreds of millions of households in small towns and villages like Hanjing.

The difference from most wealthy countries is that China depends overwhelmingly on coal. And using coal to produce electricity and run factories generates more global-warming gases and lung-damaging pollutants than relying on oil or gas.

Indeed, Wu's family dislikes the light gray smog of sulfur particles and other pollutants that darkens the sky and dulls the dark green fields of young wheat and the white blossoms of peach orchards in the distance. But they tolerate the pollution.

"Everything else is better here," Wu said. "Now we live better, we eat better."

Large areas of north-central China have been devastated by the spectacular growth of the local coal industry. Severe pollution extends across Shaanxi Province, where the Wus live, and a neighboring province, Shanxi, which produces even more coal.

Not long ago, the northeastern city of Datong, long the nation's coal capital, was branded one of the world's most-polluted cities. Desert dust and particulate matter in the city had been known to force the pollution index into warning territory, above 300, which means people should stay indoors. On Dec. 28, the index hit 350.

"The pollution is worst during the winter," said Ji Youping, a former coal miner who now works with a local environmental protection agency. "Datong gets very black. Even during the daytime, people drive with their lights on."

Of China's 10 most polluted cities, four, including Datong, are in Shanxi Province. The coal-mining operations have damaged waterways and scarred the land. Because of intense underground mining, thousands of acres are prone to sinking, and hundreds of villages are blackened with coal waste.

There is a Dickensian feel to much of the region. There are growing concerns about the impact of this coal boom on the environment. The Asian Development Bank says it is financing pollution control programs in Shanxi because the number of people suffering from lung cancer and other respiratory diseases in the province has soared over the past 20 years. Yet even after years of government-mandated cleanup efforts, the region's factories belch black smoke.

The government has promised to close the foulest factories, and to shutter thousands of illegal mines, where some of the worst safety and environmental hazards are concentrated. But no one is talking about shutting the region's coal-burning power plants, which account for more than half the pollution. In fact, Shanxi and Shaanxi are rapidly building new coal-fired plants to keep pace with soaring energy demand.

One decision facing China lies in how efficiently the heat from burning coal is converted into electricity. Western countries' coal- heated steam can generate 20 percent to 50 percent more kilowatts than older Chinese power plants, while emitting the same carbon- dioxide emissions and potentially lower sulfur emissions.

China has limited the construction of small power plants, which are inefficient, and has required the use of somewhat higher steam temperatures and pressures. But Chinese officials say few new plants use the highest temperatures and pressures, which require costly imported equipment.

Another choice facing China involves whether to pulverize coal and then burn the powder, as is done now, or convert the coal into a gas and then burn the gas, in a process known as integrated gasification combined combustion.

One advantage of this approach is that coal contaminants like mercury and sulfur can be easily filtered from the gas and disposed.

Another advantage is that carbon dioxide can be separated from the emissions and pumped underground, although this technology remains unproved.

Leading climate scientists like this approach to dealing with China's rising coal consumption.

"There's a whole range of things that can be done; we should try to deploy coal gasification," said Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change, which is affiliated with the United Nations.

The World Bank in 2003 offered a $15 million grant from the Global Environment Facility to help China build its first state-of- the-art power plant to convert coal into a gas before burning it. The plan called for pumping combustion byproducts from the plant underground.

But the Chinese government put the plan on hold after bids to build the plant were higher than expected. Chinese officials have expressed an interest this spring in building five or six power plants with the new technology instead of just one. But they are in danger of losing the original grant if they do not take some action soon, said Zhao Jianping, the senior energy specialist in the Beijing office of the World Bank.

Another stumbling block has been that China wants foreign manufacturers to transfer technological secrets to Chinese rivals, instead of simply filling orders to import equipment, said Anil Terway, director of the East Asia energy division at the Asian Development Bank.

"The fact that they are keen to have the technologies along with the equipment is slowing things down," he said.

Andy Solem, vice president for China infrastructure at General Electric, a leading manufacturer of coal gasification equipment, said he believed that China would place orders in 2007 or 2008 for the construction of a series of these plants. But he said some technology transfer was unavoidable.

Western companies could help Chinese businesses take steps to reduce carbon-dioxide emissions, like subsidizing the purchase of more efficient boilers. Some companies already have such programs in other countries, to offset the environmental consequences of their own carbon-dioxide emissions at home, and are looking at similar projects in China. But the scale of emissions in China to offset is enormous.

For all the worries about pollution from China, international climate experts are loath to criticize the country without pointing out that the average American still consumes more energy and is responsible for the release of 10 times as much carbon dioxide as the average Chinese. While China now generates more electricity from coal than does the United States, America's consumption of gasoline dwarfs China's, and burning gasoline also releases carbon dioxide.