"Collateral Damage: The Health and Environmental Costs of War on Iraq" was prepared largely by Medact, the British affiliate of International Physicians for the Prevention of Nuclear War. The U.S. affiliate, Physicians for Social Responsibility, also was involved. Most of the estimated casualties would be Iraqi civilians caught in the bombing, said Bob Schaeffer, a spokesman in Massachusetts for the International Physicians organization. It was awarded the Nobel Peace Prize in 1985 for what the committee called its "considerable service to mankind by spreading authoritative information and by creating an awareness of the catastrophic consequences of atomic warfare.''

The study also looks at the impact of an invasion on the public health system and necessities such as agriculture, water and energy, he said.

"We're saying that there'll be a very large short-term impact and an even more profound longer-term impact," Schaeffer said. "The report uses the word `human catastrophe' even if it does not escalate to the level of poison gas, civil war or nuclear weapons.''

The estimates of casualties, he said, range from a low of 50,000 up to 250,000.

James Snyder, spokesman in Washington for Physicians for Social Responsibility, said the report utilizes information about likely Iraq invasion scenarios as well as knowledge gleaned from study of the 1991 Persian Gulf War and U.S. actions in Somalia and Panama.

Schaeffer said physicians associated with the international organization also had made some inspection tours, and their findings were factored in.

"The estimates and ranges are based on sound science and previous experience," Snyder said.

There has not been much public data on the extent of possible casualties from an invasion of Iraq.

A number of experts at the Pentagon and elsewhere have discussed the possibility of Iraqi leader Saddam Hussein using human shields and placing military targets within civilian sites, such as hospitals and schools.

Ibrahim Al-Marashi, an analyst at the Monterey Institute of International Studies in California who favors an invasion, said such estimates are extremely difficult because there are so many variables about how a war might unfold.

"The way Saddam would respond is such a wild card in this," he added.