The debate about perchlorate (search) contamination in drinking water is getting more heated as environmentalists object to a report claiming the widespread toxin is far less dangerous than was thought.

A National Academy of Sciences panel said Monday that perchlorate, a toxic chemical used in rocket fuel and explosives, is safe for consumption at levels 20 times the standard being considered by the Environmental Protection Agency.

The study is expected to influence the EPA (search) as it develops its first national standard for perchlorate in drinking water. But environmentalists contended such a high standard could endanger children's health while letting defense contractors off the hook for cleanup costs.

"Wherever this standard is applied, most perchlorate contamination sites will be wiped off the map," said Lenny Siegel, director of the Center for Public Environmental Oversight in Mountain View, Calif. "Millions of children and pregnant women will continue to be guinea pigs in the great perchlorate exposure experiment."

The study comes after years of disagreement over how dangerous it is for people to drink water tainted with perchlorate, a pervasive leftover of Cold War defense manufacturing that has been found in drinking water in 35 states. The chemical, which leaches easily into groundwater from defense and manufacturing sites, can inhibit thyroid function (search) and is considered particularly dangerous to children.

While the chemical also is found in nature, the panel said its presence in the environment primarily comes from the manufacture and use of rocket fuels as well as explosives and fireworks.

The NAS panel recommended a level for safe human consumption that translates to approximately 20 parts per billion in drinking water. Two years ago, the EPA issued a preliminary recommendation of 1 part per billion.

"The committee disagrees with EPA's conclusion and thinks that perchlorate exposure is unlikely to lead to thyroid tumors in humans," the panel said in a statement accompanying its report.

The academy study was ordered by the Bush administration in 2003 to review the stricter standard the EPA had proposed in 2002. The Pentagon had criticized that standard as too stringent and recommended one as high as 200 parts per billion.

The Natural Resources Defense Council contended that documents obtained under Freedom of Information Act (search) requests showed the Pentagon and the White House had sought to influence the scope of the academy's study in order to get a weaker standard.

Local governments around the country already have begun trying to hold defense contractors and the Pentagon liable for huge cleanup costs to rid groundwater of the toxin.

Bob Hopkins, spokesman for the White House's Office of Science and Technology Policy, said accusations of improper influence by administration officials "couldn't be further from the truth."

The academy defended its work. "The government had no influence over the conduct or outcome of this study," said E. William Colglazier, the academy's executive officer. "The committee members were highly competent, there were no conflicts of interest, and we have full confidence in the report."

A few states have defined their own proposed limits on perchlorate contamination in drinking water, though none is finalized. California's standard is 6 parts per billion, while Massachusetts' is 1 part per billion.

Although California's standard would remain in place even if the national standard were more lax, Californians would still be affected. Contamination in the Colorado River, which provides drinking water to 20 million people, comes from a site in Nevada.