By Maggie Fox, Health and Science Editor

WASHINGTON, April 7 (Reuters) - Nuclear war between India and Pakistan would cause more than slaughter and destruction -- it would knock a big hole in the ozone layer, affecting crops, animals and people worldwide, U.S. researchers said on Monday.

Fires from burning cities would send 5 million metric tonnes of soot or more into the lowest part of Earth's atmosphere known as the troposphere, and heat from the sun would carry these blackened particles into the stratosphere, the team at the University of Colorado reported.

"The sunlight really heats it up and sends it up to the top of the stratosphere," said Michael Mills of the Laboratory for Atmospheric and Space Physics, who chose India and Pakistan as one of several possible examples.

Up there, the soot would absorb radiation from the sun and heat surrounding gases, causing chemical reactions that break down ozone.

"We find column ozone losses in excess of 20 percent globally, 25 percent to 45 percent at midlatitudes, and 50 percent to 70 percent at northern high latitudes persisting for five years, with substantial losses continuing for five additional years," Mills' team wrote in the Proceedings of the National Academy of Sciences.

This would let in enough ultraviolet radiation to cause cancer, damage eyes and skin, damage crops and other plants and injure animals.

Mills and colleagues based their computer model on other research on how much fire would be produced by a regional nuclear conflict.

"Certainly there is a growing number of large nuclear-armed states that have a growing number of weapons. This could be typical of what you might see," Mills said in a telephone interview.

SMOKE IS KEY

Eight nations are known to have nuclear weapons, and Pakistan and India are believed to have at least 50 weapons apiece, each with the power of the weapon the United States used to destroy Hiroshima in 1945.

Mills said the study added a new factor to the worries about what might damage the world's ozone layer, as well as to research about the effects of even a limited nuclear exchange.

"The smoke is the key and it is coming from these firestorms that build up actually several hours after the explosions," he said.

"We are talking about modern megacities that have a lot of material in them that would burn. We saw these kinds of megafires in World War Two in Dresden and Tokyo. The difference is we are talking about a large number of cities that would be bombed within a few days."

Nothing natural could create this much black smoke in the same way, Mills noted. Volcanic ash, dust and smoke is of a different nature, for example, and forest fires are not big or hot enough.

The University of Colorado's Brian Toon, who also worked on the study, said the damage to the ozone layer would be worse than what has been predicted by "nuclear winter" and "ultraviolet spring" scenarios.

"The big surprise is that this study demonstrates that a small-scale, regional nuclear conflict is capable of triggering ozone losses even larger than losses that were predicted following a full-scale nuclear war," Toon said in a statement.

Mills noted the United States is currently working on a controversial deal that would give India access to U.S. nuclear fuel and equipment for the first time in 30 years even though India refused to join nonproliferation agreements.

Nonproliferation advocates believe it undermines the global system designed to prevent the spread of nuclear weapons.

(Editing by Cynthia Osterman)

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