WASHINGTON - A nuclear war between Pakistan and India could dwarf any catastrophe in world history, killing up to 12 million people in South Asia.

But the radioactive fallout probably would not harm Americans half a world away. In fact, because of the combined effects of distance, dispersion, and dilution, the increased amount of radiation in US air would be barely measurable, health specialists say.

''As concerned as we need to be for the Indian and Pakistani populations, the concern for ourselves here is not proportionate to the risk,'' said Cham Dallas, a University of Georgia toxicologist.

Dallas, who is helping the Centers for Disease Control and Prevention coordinate medical response to possible nuclear attacks on the United States, said, ''The effects here in the United States will be minimal.''

Other specialists say there would be no real risk for the United States.

''In terms of health effects, I think the risks are vanishingly small,'' said Arthur Upton, former director of the National Cancer Institute and a leading authority on the health effects of radiation. ''I don't think one can say it's a public health threat.''

That is because by the time the radiation travels across the globe, it would spread out in the air so much that it would register only a minuscule increase over normal background radiation, he said.

Ordinary background radiation - which emanates from space, seeps up from Earth's core, and is ingested in food - exposes the average American to about 100 millirems of exposure per year, he said. (A millirem is a unit of absorbed radiation; one millirem is the amount absorbed by the average television viewer over one year.)

A nuclear exchange between Pakistan and India would add less than one millirem a year to the average American's exposure, Upton predicted.

America would benefit as well from prevalent weather patterns at this time of year. The South Asia wind is quite stagnant during the summer, said Tony Barnston, forecast operations chief at the International Research Institute for Climate Prediction in New York.

But if India and Pakistan exchanged nuclear bombs in winter, winds would blow east much faster, and radiation could reach the West Coast in less than a week, he said.

Still, a Pakistan-India nuclear exchange would set a new standard for human horror: A Pentagon intelligence report estimates that the dead could total 9 million to 12 million, based on population centers that could be targeted.

''You would have a very significant death rate, depending on where they'd use them,'' said Dallas, who spent a decade studying the aftermath of Chernobyl.

There would also be hundreds of thousands of burn victims, with few medical supplies and few doctors able to treat them, Dallas said.

A lot would depend on whether the bombs explode in the air or on the ground. If they explode in the air, the fallout would be limited and local. But if they explode on the ground, that would contaminate more dirt, plants, and trees, and the radioactivity could then spread further via the wind, said John Boice, a former physicist and current professor of medicine at Vanderbilt University's International Epidemiology Institute in Bethesda, Md.

When the United States bombed the Japanese cities of Hiroshima and Nagasaki in 1945 to end World War II, the explosions were in the air, and the amount of radiation and resultant cancers dropped off dramatically a few miles beyond ground zero, Boice said. The 1986 Chernobyl nuclear power plant disaster spewed radioactivity closer to the ground and the wind spread it widely, Boice said.

A lot would depend on how many nuclear weapons the two countries used. Pakistan is believed to have 15 to 45 warheads, and India 30 to 150. If the two countries exchanged 40 warheads with 20 megatons of explosives each (about the size of the bomb that the United States dropped on Nagasaki) the world already has endured much more radiation dispersion.

This story ran on page A5 of the Boston Globe on 6/4/2001. © Copyright 2002 Globe Newspaper Company.