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Date: May 14, 1997
Contacts: Dan Quinn, Media Relations Associate
Amy Kushner, Media Relations Assistant
(202) 334-2138      


Cold War Chemical Tests Over American Cities
Were Far Below Dangerous Levels

WASHINGTON -- A series of secret tests conducted by the U.S. Army in the 1950s and 1960s did not expose residents of the United States and Canada to chemical levels considered harmful, according to a new report* from a committee of the National Research Council.

The U.S. Army released the chemical compound zinc cadmium sulfide from airplanes, rooftops, and moving vehicles in 33 urban and rural areas as part of a Cold War program to test the way biological weapons might disperse under different conditions. Zinc cadmium sulfide, a fine fluorescent powder, was chosen because its particles are similar in size to germs used in biological warfare, and because its fluorescence under ultraviolet light made it easy to trace. It is not a biological weapon, nor was it thought at the time to be toxic. But residents in affected cities -- including Minneapolis; St. Louis; Winnipeg, Manitoba; Corpus Christi, Texas; and Fort Wayne, Ind. -- became concerned about possible health effects after details of the tests became widely known in the 1990s.

"After an exhaustive, independent review requested by Congress we have found no evidence that exposure to zinc cadmium sulfide at these levels could cause people to become sick," said committee chair Rogene Henderson, senior scientist, Lovelace Respiratory Research Institute, Albuquerque, N.M. "Even when we assume the worst about how this chemical might behave in the lungs, we conclude that people would be at a higher risk simply from living in a typical urban, industrialized area for several days or, in some cases, for months."

Information on how zinc cadmium sulfide affects human health is sparse, but data from animal tests indicate that the compound, when taken orally, has no short-term toxic effects; nor was it found to be a skin or eye irritant. Because limited laboratory research on the toxicity of zinc cadmium sulfide has been performed on animals, and no data exist on humans, the committee based its conclusions about the ability of the compound to cause cancer on what it called a "worst-case" assumption: that the compound is every bit as toxic as its most toxic component -- cadmium. High doses of cadmium over long periods of time could cause bone and kidney problems and lung cancer, but the Army's tests involved small doses of a less toxic compound over short periods of time, the report says. The committee estimated that the excess maximum lifetime cancer risk for the most heavily exposed residents of St. Louis is 1.5 in 1 million; in Minneapolis it is one in 2.5 million; one in 1 million in Winnipeg; seven in 100 million in Fort Wayne; and one in 100 million in Corpus Christi.

For non-cancer toxicity, the committee based its conclusions on what is known about cadmium sulfide, a compound that has some properties similar to zinc cadmium sulfide. The committee estimated that an average-size male could inhale as much as 500 micrograms of cadmium sulfide over a few days without causing toxicity in the lungs. Even in populated areas where exposures from the Army's tests were the highest, residents were exposed to far more cadmium in their normal daily contact with soil, water, food, and air (between 12 and 84 micrograms) than they were potentially exposed to from the Army's tests. The maximum estimated cadmium dose from all tests combined was 24.4 micrograms in St. Louis; 14.5 in Winnipeg; 6.8 in Minneapolis; 1.1 in Fort Wayne, and 0.1 in Corpus Christi. In Biltmore Beach, Fla. -- a remote unpopulated island location at the time of the tests -- the total may have been as high as 390 micrograms, but very few people if any were believed to have been exposed there.

The conclusions are based on data from the published scientific literature, information supplied by the Army and its contractors, and testimony from citizens in public meetings held by the committee in Minneapolis, Fort Wayne, and Corpus Christi. Some of the exposure data from the Army's tests have been lost or misplaced over the years since the tests were conducted, but the committee "feels confident in the large amount of data that it reviewed and does not believe it likely that the additional missing data would alter its conclusions."

Beyond looking at information on the toxicity, exposure data, and physical and chemical properties of the substance used in the experiments, the committee also evaluated whether it is feasible to perform a follow-up study to track the health status of those exposed. Three barriers would make such a study unfeasible, according to the report. Since the Army tests are now 30 to 40 years old, it would be extremely difficult to identify the people who were affected and to determine their past exposures to zinc cadmium sulfide. Even if they were found, there is a lack of data on their health before, during, and after exposure. And it would take a huge sample of exposed residents to detect even a small increase in health problems.

The Army should conduct studies to determine whether inhaled zinc cadmium sulfide breaks down into toxic cadmium compounds, which can be absorbed into the blood to produce toxicity in the lungs and other organs. Such research would strengthen the database needed for assessing the risk from the substance, the report says, and would eliminate the reliance on estimates of exposure to cadmium or cadmium compounds.

In some cities the Army dispersed microorganisms -- either alone or in combination with zinc cadmium sulfide. Recent research indicates that the microbes used do not produce disease in healthy people, but could produce disease in people with weak immune systems. The committee was not asked to assess the possible health effects of these tests, nor was it asked to address the ethical and social issues raised by the testing program as a whole.

A committee roster follows. This study was funded by the U.S. Department of the Army. The National Research Council is the operating arm of the National Academy of Sciences and National Academy of Engineering. It is a private, non-profit organization that provides independent advice on science and technology under a congressional charter granted to the National Academy of Sciences.

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