BANGKOK, Thailand - Countries that start battling global warming now won't have to wait generations to see the rewards: Burning cleaner fuels can yield immediate health benefits that save lives and money, world health experts say.

A climate-change conference in Bangkok this week has centered on the high cost of promoting greener policies. But governments should consider how much they will save in medical costs by adopting policies that minimize heat waves, disease and water scarcity resulting from rising temperatures, the scientists said.

Big developing countries like China and India can play a huge role in improving health by expanding their use of cleaner energy sources, said Diarmid Campbell-Lendrum, an expert on climate change and health at the World Health Organization.

"The policy options that you choose to try to cut emissions also have very important health effects," he said from the agency's headquarters in Geneva, Switzerland. "If you choose the right ones, then you can certainly have a win-win at cutting CO2 emissions and directly benefiting health."

Urban air pollution, for example, kills about 800,000 people a year globally, according to WHO. More than half of those deaths occur in China, the world's second-largest greenhouse-gas emitter after the United States, the agency said.

Promoting walking or bicycling instead of driving could further reduce major health problems like diabetes and heart disease that are striking many rapidly developing countries, Campbell-Lendrum said.

A report by the Intergovernmental Panel on Climate Change, a U.N. network of 2,000 scientists, was being hammered out last night by conference delegates from more than 120 countries. A final version was expected today.

"Prevention is cheaper than cure," said Hisashi Ogawa, regional adviser for healthy settings and environment at WHO's office in the Philippines. Besides illness and death, "the economy will be affected because sick people cannot produce services and products."

Mosquito-borne illnesses like dengue fever are expected to spike with rising temperatures, in addition to diseases caused by diarrhea that are associated with flooding. Malnutrition from lower crop yields also is predicted to plague the developing world, along with disease and injury from heat waves, storms and droughts.

The problems have already started, the scientists warned. WHO conservatively estimated that 150,000 people died worldwide in 2000 from health issues related to climate change, with nearly 90 percent of those fatalities in Asia and Africa. The figures do not include deaths linked to air pollution.

An ongoing outbreak of malaria in Papua New Guinea has been blamed on global warming. Mosquitoes that spread the disease normally can't breed in the highland's cool climate, but rising temperatures have changed that, Ogawa said.

"The projections are overwhelmingly negative," said Kristi Ebi, a Virginia epidemiologist on the scientific panel. "Most of these diseases and health outcomes we worry about, we've dealt with before. The question is how to get everything in place as quickly as possible to reduce these impacts now."

She said poor countries, mainly in Asia and Africa, will be forced to bear the brunt of damage already caused by rising temperatures.

Countries can start working to reduce the impact of climate change on health by revving up resources to tackle existing diseases that are expected to worsen, Campbell-Lendrum said.