Workers spent several days digging through World Trade Center debris recently in a search for human remains before health officials tested the air and soil, officials said Tuesday.

Though the mayor said the environmental tests performed Monday revealed no toxins, some critics said they should have been done immediately.

The tests on material pulled from manholes on the site's edge were negative for asbestos and other threats, Mayor Michael Bloomberg said.

"We always err on the side of making sure that the people that are working there are safe," Bloomberg said.

But that doesn't satisfy some workers who have claimed that exposure to the debris at the trade center site has sickened thousands since Sept. 11, 2001.

"How many times you gotta learn?" said Marvin Bethea, a paramedic who said he got chronic asthma from working at the lower Manhattan site in the days after the terrorist attack. "They should have been properly geared up. They should have first have had someone come in there and test the air."

A group of Consolidated Edison utility employees began work Wednesday on a manhole at the site; construction workers discovered human bones in the debris on Thursday. Since then, about two dozen workers daily have been digging through several manholes in the area for possible remains.

The Port Authority of New York and New Jersey, which owns the site and had overseen the manhole work, called in the U.S. Occupational Safety and Health Administration on Monday "to ensure the safety of the project for the workers," spokesman Steve Coleman said.

Officials said the tests weren't sought until the work became more extensive.

As of Tuesday, the searchers were wearing protective suits and face masks, officials said. The search ended a couple of hours early on Monday to complete tests of air and soil, officials said. The search resumed Tuesday.

OSHA spokesman John Chavez said federal standards require protection from asbestos and other toxic substances, and "that would appear to be a common concern at this site."

A recent study has concluded that 70 percent of ground zero workers developed respiratory problems after working there; a lawsuit filed by thousands of workers and residents charges they were sickened by debris and dust that contained pulverized concrete, asbestos and other toxins.