The countries involved would be devastated as well as fall victim to the ozone disaster, the scientists' analysis says.
Massive fires resulting from even a limited conflict would blast enough soot into the atmosphere to create an ozone hole over heavily populated areas, the researchers warned in a paper published Monday in the online edition of Proceedings of the National Academy of Sciences.
A region of depleted ozone over the Antarctic, known as an ozone hole, has been a worry to scientists for years as it allows damaging ultraviolet radiation from the sun to reach Earth's surface. Some chemicals have been banned from use to help eliminate that hole.
Unlike the Antarctic, a nuclear-induced ozone hole would affect much of the world, causing damage to plants and animals and adding to skin cancer, eye damage and other effects in millions of people, according to researchers led by Michael J. Mills of the University of Colorado.
Mills' team used complex computer programs to model what would happen in the atmosphere in the event of a war between India and Pakistan in which each detonated 50 Hiroshima-sized nuclear explosives.
They calculated that the blasts would send as much as five million metric tons of soot as high as 50 miles into the atmosphere.
The soot and the heat from solar radiation would cause a series of chemical reactions that would break down the stratospheric ozone layer that protects Earth from harmful ultraviolet radiation, they said.
"We would see a dramatic drop in ozone levels that would persist for many years," Mills said in a statement. "At mid-latitudes the ozone decrease would be up to 40 percent, which could have huge effects on human health and on terrestrial, aquatic and marine ecosystems."
They calculated that a 40 percent ozone decrease would result in a 132 percent increase in light damage to plants and a 213 percent increase in DNA damage associated with skin cancer.
The mid-latitudes are the regions between the tropics and the Arctic and are home to the largest numbers of people.