Countless New Yorkers, police included, on Wednesday night began donning masks to filter the air as winds shifted north and choked residential neighborhoods miles from the disaster area, near the foot of Manhattan island.

In neighborhoods like Greenwich Village and Murray Hill, some created their own filters, stealing breaths through T-shirts and handkerchiefs.

"When I came to work this morning, about half the people on the street were wearing masks, and I asked myself at first, 'What's going on?' Then I remembered the Trade Center," said Deborah DiNapoli, a hostess at Merchants wine bar near 17th Street in the Chelsea neighborhood.

She said the pollution cleared up considerably by Thursday afternoon, but even so, DiNapoli is a little dubious about venturing much further south.

"I wouldn't go below 14th Street," the hostess said, for fear of being reconfronted with too much dust and smoke.

CATCH YOUR BREATH

So, is the air really safe?

Hans Hallman, a spokesman for the New York State Emergency Management Office, said a clear answer won't be available until an analysis of air quality samples arrives perhaps later today.

The agency, which is coordinating information from city and state rescue efforts, said the air is clogged with pulverized concrete, glass, and other debris as well as fumes from nearby buildings that are still smoldering.

Some rescuers, sifting through the rubble in search of survivors, have wondered aloud whether they are being exposed to asbestos, a cancer-causing mineral used as insulation decades ago.

Hallman said he was unsure whether asbestos was indeed present, adding that he assumed air-quality samples and other tests would soon give an authoritative answer.

Asked whether the feared thousands of bodies buried in the twisted steel and concrete are a health hazard, Hallman said, "I can't even speculate on that" until better information is in hand.

HEART RISKS CITED

Joel Schwartz, a professor at the Harvard School of Public Health, said studies show particles from smoldering combustion are associated with increased risk of blood clots that can trigger heart attacks.

"More people will die this week in New York because of air pollution than would of died otherwise. There will be extra deaths because of the smoke," Schwartz said.

"Air pollution is not just aesthetic, it does kill people. They should keep windows shut and turn air conditioners on."

Paul Brandt-Rauf, a professor of environmental health sciences at Columbia University's Mailman School of Public Health, said asbestos particles would indeed be a major concern, if they are really blowing in the wind.

Other than asbestos, he said his second biggest concern is burning plastics. "Some of the worst chemicals come from the smoldering after the fire is out," Brandt-Rauf said.

"The lower exposure you have, and the shorter the exposure, the better off you are." He speculated people keeping at least a 10-block distance from the smoking rubble suffer little risk of harm.

Rescuers are obviously at greatest risk because of their direct exposure to toxic fumes such as carbon monoxide and cyanide, and to potential carcinogens, Brandt-Rauf said.