A terrorist attack using a crude nuclear device or a "dirty bomb" could leave anywhere from a building to many square miles contaminated -- perhaps even uninhabitable.

Now the government has issued cleanup guidelines that some critics maintain would expose people returning to such a site to high cancer risk.

The guidelines issued Tuesday by the Homeland Security Department would allow cleanup standards that in some cases would be far less stringent than what is required for Superfund sites, commercial nuclear power plants and nuclear waste dumps.

Long-term radiation exposure using some of the cleanup standards in the guidelines could be as high as 10,000 millirems a year, equal to more than 1,600 chest X-rays or 30 times the average background radiation from natural sources.

By comparison, the Nuclear Regulatory Commission limits public exposure from the facilities it licenses to no more than 100 millirems a year. The radiation exposure limit proposed for the future Yucca Mountain nuclear waste site is 15 millirems per year.

The exposure allowed by the new Homeland Security guidelines "could cause one in four people to get cancer" if they return to the site of the attack, said Diane D'Arrigo of the Nuclear Information and Resource Service, a watchdog group that has lobbied for more rigorous federal requirements. The Environmental Protection Agency limits the risk of additional cancer to 1 in 10,000 at Superfund toxic waste sites when they are cleaned up.

The guidelines, which have been several years in the making, are designed to help local, state and federal officials plan how they would deal with a terrorist attack where radioactivity was released.

An attack using a crude nuclear device that produces an actual nuclear detonation could contaminate many square miles. A dirty bomb -- mixing a conventional explosive with a radiation source, such as the cesium-137 used in medicine -- probably would have a more confined impact, perhaps a building or several city blocks, the guidance says.

Donald Tighe, a spokesman for the White House Office of Science and Technology, said the guidelines specifically avoided setting a numerical cleanup standard because there is such a wide range of potential cleanup scenarios.

In the long term a community would "have to evaluate not only public health, but the health of the community as well," said Tighe. "This is the feedback we've gotten from state and local officials. (They want) a flexible approach."

So the guidelines direct local, state and federal officials to various benchmarks used by other agencies as well as international organizations.

One benchmark that could be used is the standard established by the International Commission on Radiation Protection, a London-based group, which maintains a long-term release of 10,000 millirems a year is an acceptable exposure standard after cleanup.

Daniel Hirsch, president of the watchdog group Committee to Bridge the Gap, said the task force clearly favors that standard.

Hirsch accused the Homeland Security Department of "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."