The Department of Homeland Security is set to issue guidelines that will likely change the way emergency workers would respond to a dirty bomb attack. NPR received a preview of the new safety standards, which significantly increase the level of radiation exposure considered safe for emergency workers and residents.

For instance, the guidelines advise that residents should only be evacuated if they are in danger of getting a radiation dose greater than 1,000 dental X-rays; that's about four times the exposure a person gets each year from natural resources.

As NPR's David Kestenbaum reports, the new guidelines suggest that a dirty bomb does not pose as great a risk as the guidelines drawn up by many emergency services have suggested.

An overview of the new draft "protective action guidelines" recommended by the Department of Homeland Security:

First Responder Exposure: Over the course of the initial event, the new guidelines say it's safe for firemen, police and EMTs to receive a total exposure of five rem. That's the equivalent of 5,000 dental X-rays, or 20 times the radiation people normally are exposed to in a year from natural sources.

Evacuation: Residents do not need to be evacuated in the days immediately following the attack unless exposure surpasses one rem, or the equivalent of 1,000 dental X-rays. In some cases, exposure as high as five rem may be allowed.

Relocation: More permanent relocation would only be ordered if over the course of first year the total additional dose to a resident would be two rem -- eight times the radiation dose people normally get in a year. For subsequent years, the allowable additional radiation dose would be 500 millirem, which is twice the average annual background radiation dose from natural sources.

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