A 5-year-old gray, part-Siamese cat named Tom was a hero yesterday, responsible for alerting 10 residents of an Arlington condominium to the presence of deadly carbon-monoxide gas.

"It was the cat that did it," said Bruce E. De Jarnette, explaining why he decided that his headache, nausea and dizziness yesterday morning were not just a touch of the flu.

When the cat became ill and vomited, De Jarnette and a friend, Margaret Edmunds, the cat's owner, who was also ill, decided to call the Arlington Fire Department. An ambulance came to The Arlington, the 500-unit condominium complex where they live in Clarendon, and took them to National Hospital for Orthopaedics and Rehabilitation.

Within minutes of their arrival, Dr. Susan Brennan, chief of the emergency room, diagnosed the problem as carbon-monoxide poisoning and notified the fire department to have the building checked immediately. Eight apartments were temporarily evacuated.

It wasn't long before eight other residents from the complex on South 28th Road went to the hospital, five by ambulance and three on their own. Many of them apparently were staying home that day because they had diagnosed their symptoms as the flu, according to a hospital spokesman. All eight were given oxygen treatment and released.

De Jarnette and Edmunds, who live directly above the building's boiler room, were admitted to the hospital overnight and were listed in stable condition.

Arlington Chief Fire Marshal Frank Kaye said construction debris, possibly from chimney work when the apartments were converted to condominiums a year ago, had blocked the chimney and that, plus a lack of proper ventilation, apparently caused carbon monoxide to build up in the boiler room. The gas then seeped into nearby apartments.

Kaye said he was unsure why the problem had not occurred earlier, speculating that some of the debris might have become dislodged recently and increased blockage in the chimney.

Asked what might have happened if no one from the apartments had alerted authorities, Fire Marshal Kaye said: "Oh Lord, I don't even want to think about it."

Residents from the affected units were allowed to return home and instructed to leave their windows slightly open this evening, a condominium spokesman said.

Rescue squad personnel took Tom -- feeling much better -- to an animal shelter for the night, De Jarnette said.