NEW YORK — They dug in the toxic World Trade Center dust for survivors, and later for the dead. Their feet were burned by white-hot debris. But unlike thousands of others who toiled at ground zero after Sept. 11, these rescue workers aren't sick.

Scientists have spent years studying the health of search-and-rescue dogs that nosed through the debris at ground zero, and to their surprise, they have found no sign of major illness in the animals.

They are trying to figure out why this is so.

"They didn't have any airway protection, they didn't have any skin protection. They were sort of in the worst of it," said Cynthia Otto, a veterinarian at the University of Pennsylvania, where researchers launched a study of 97 dogs five years ago.

Although many ground-zero dogs have died — some of rare cancers — researchers say many have lived beyond the average life span for dogs and are not getting any sicker than average.

Owners of the dogs dispute the findings, saying there is a definite link between the toxic air and their pets' health.

Otto has tracked dogs that spent an average of 10 days after the 2001 terrorist attacks at either the trade center site, the landfill in New York where most of the debris was taken, or the heavily damaged Pentagon.

As of last month, she said, 30 percent of the dogs deployed after Sept. 11 had died, compared with 22 percent of those in a comparison group of dogs who were not pressed into service. The difference was not considered statistically significant, Otto said.

But she added: "We have to keep looking."

A separate study, to be published soon by a doctor at New York's Animal Medical Center, focused on about two dozen New York police dogs, and comes to similar conclusions.

The results have baffled doctors. A study released last month found that 70 percent of the people who worked at ground zero suffer severe respiratory problems; scientists thought that the dogs might have similar health problems.

The dogs' owners and scientists have many theories why dogs aren't showing the same level of illness as people. Their noses are longer, possibly serving as a filter to protect their lungs from toxic dust and other debris, they say. The dogs were at the site an average of several days, while many people who report lung disease and cancer spent months cleaning up after the attacks.

The research isn't persuasive to many owners of dogs that died after working at the trade center site.

Joaquin Guerrero, a police officer in Saginaw, Mich., took two dogs, Felony and Rookie, to ground zero for 10 days after the attacks. While Felony remains healthy, Rookie died at age 9 in 2004 of cancer of the mouth. Guerrero believes his death was caused by exposure to ground zero.

"If the people are getting it, you know dogs are showing signs of it," Guerrero said.

Scott Shields' golden retriever, Bear, located the body of a fire chief and many other victims at ground zero. The 11-year-old dog died a year after the attacks of several types of cancer.

"He had never been sick a day in his life" before going to the site, where he sustained a wound to his back from steel debris, Shields said.

Shields, who heads a search-and-rescue dog foundation named after Bear, said Bear "died from bad government" and the toxic air at ground zero. He said that studies under way should have included every dog that worked at the site, and that the Penn study is flawed because it tries to compare dogs that worked at the Pentagon as well as in New York.

Otto said that some of the dogs that worked at the sites could not be found and other dogs' owners were not willing to subject their pets to annual blood tests and X-rays.

Mary Flood, whose 11 1/2-year-old black Labrador, Jake, is completely healthy five years after working at ground zero, said that dogs' much shorter life span may also make it harder to track long-term illness.

"Maybe there's not enough time to develop these things before they're no longer with us," she said.

On the Web:

www.9-11dogs.org