The government issued cleanup standards Tuesday for a "dirty bomb" terrorist attack that would in some cases be far less rigorous than what is required for Superfund sites, nuclear power plants and nuclear waste dumps.

After such an attack, long-term radiation exposure could remain at levels that would be expected to produce cancers in one of every four people who return to the contaminated sites, anti-nuclear watchdog groups said after analyzing the new federal guidelines.

Dirty bombs — largely theoretical terrorist weapons — would use conventional explosives to disperse radioactive material without a nuclear explosion. Such weapons, which could use Cesium 137 or other radioactives, would be useful as terror devices because they can render an area dangerous or uninhabitable.

The guidelines issued by the Homeland Security Department say the impact from detonating a crude nuclear device or a dirty bomb could vary widely, from contaminating a small area, such as a single building or city block, to conceivably many square miles. So, it said, cleanup requirements also could vary widely.

In some cases, the document suggested, long-term radiation exposures of as much as 10,000 millirems per year — a level equivalent to hundreds of chest X-rays a year or 30 times the annual exposure to radiation from natural "background" sources — could be allowed for areas that are returned to general use.

If there is widespread contamination from a dirty bomb or an "improvised nuclear device" — which could cause a crude nuclear explosion — some areas may have to be put off limits permanently, the guidelines said.

The guidelines, which go into effect immediately but could be modified after a public comment period, brought sharp criticism from some environmental groups and nuclear watchdog organizations.

Long-term radiation levels of 10,000 millirems a year as would be permitted by the guidelines in some cases can be expected to produce a cancer in one of every four people exposed, said Diane D'Arrigo, of the Nuclear Information and Resource Service, a Washington-based nuclear industry watchdog group, citing government radiation risk assessments.

The federal guidelines do not establish specific numerical standards for cleanup, but they cite radiation "benchmarks" established by other agencies or international organizations that would be acceptable.

Among those benchmarks that could be used under the guidelines is one established by the International Commission on Radiation Protection which cites a long-term release of 10,000 millirems a year as an acceptable exposure standard after cleanup.

By comparison, the Nuclear Regulatory Commission does not allow exposure to the public of more than 100 millirems per year in its cleanup standard. The exposure at a future Yucca Mountain nuclear waste site has a limit of 15 millirems. Background radiation from natural sources averages about 350 millirems, while exposure from a chest X-ray is about 6 millirems.

Donald Tighe, a spokesman for the White House Office of Science and Technology, said the guidelines specifically avoided setting a numerical cleanup standard for long-term radiation exposure. Instead, he said, it is hoped the guidelines will help state, local and federal officials choose appropriate cleanup standards depending on circumstances.

"It would be very inaccurate for anyone to characterize this as leaning toward any one side of the range of (cleanup) standards" that might be available, said Tighe.

But Daniel Hirsch, of the Committee to Bridge the Gap, a nuclear watchdog group in California, said the guidelines are so lax that it opens the way for cleanup efforts to fall short of what is needed to protect public health.

The Department of Homeland Security "is proposing a nuclear Katrina, a formal policy of allowing the public to be exposed to massive radiation doses from a dirty bomb while the government does nothing to protect them," he said.