Four Port Authority police officers working at ground zero have been reassigned after tests showed elevated levels of mercury in their blood, officials said Friday.

All four were in good health and had no symptoms of mercury poisoning, said Port Authority of New York and New Jersey spokesman Allen Morrison. High levels of mercury can damage the brain, kidneys and lungs.

The source of the mercury was not known.

"We can't be certain whether working at ground zero caused the elevation in those four cases, but we didn't want to take a chance," Morrison said.

An increased level of heavy metals, including mercury, was also found in preliminary tests done at a building across from the World Trade Center site where the Legal Aid Society was housed before the Sept. 11 attack, spokeswoman Pat Bath said. More tests are being done.

Environmental Protection Agency regional spokeswoman Bonnie Bellow said any elevated levels of mercury in the blood of workers at ground zero should be taken seriously, and she urged work crews to continue wearing respirators.

But so far, she said, there is no definitive evidence the mercury came from ground zero.

Early air and water samples taken at the site failed to turn up any detectable levels of mercury, and officials were unable to find any large source of mercury in or around the twin towers, she said.

"So the critical question," Bellow said, "is what would the source be?"

Morrison said the Port Authority began testing ground zero workers in November because of the possibility of toxins at the site. Out of 58 tests conducted, 49 results came back and four were elevated, Morrison said.

Health officials told the agency that an acceptable mercury level is 0 to 13 micrograms per liter of blood. Two of the four officers had 14 micrograms per liter, one had 18 and another 24, Morrison said.