CHRISTINE DEMPSEY | The Hartford Courant

Four people remained in critical condition today from carbon monoxide poisoning, a fire official said.

An investigation showed that a deteriorating chimney caused the gas to back up, making them ill, said Vincent Palmeri, assistant fire chief. The home, which houses four apartments, has been declared uninhabitable, he said.

Firefighters were called at 3:28 p.m. Monday to 15 Crosby St., where five adults and an infant had been exposed to high levels of the deadly gas, Palmeri said. One of the female victims was partially conscious, he said.

All six people were transported to Hartford Hospital. Palmeri said he could not identify the victims because of privacy laws; he didn't know how many people live in the four apartments.

Firefighters measured a carbon monoxide level of 485 parts per million ? a life-threatening amount for someone exposed over a period of time, he said. By comparison, he said, someone exposed to carbon monoxide level of 400 parts per million for three hours is in immediate risk of death.

When firefighters arrived at the scene, Palmeri said, "There was a rigorous effort in the first few minutes to search the home thoroughly to find other victims, because the levels were high enough to immediately render even a healthy victim unconscious."

The 911 caller, who was not one of the patients, reported that people in the house had flu-like symptoms, he said. Emergency service workers suspected carbon monoxide poisoning, in part because more people are relying on fireplaces or heaters because of the recent cold weather.

"We're very careful to look for carbon monoxide, especially when you have multiple patients with the same symptoms," Palmeri said.

An investigation by building and fire officials showed that a blocked chimney prevented the gas from venting properly, Palmeri said. Debris from the chimney's deterioration is what led to the blockage, he said. Bonnie Nichols, the town's director of inspections and permits, said it was a boiler chimney.

The mishap prompted Palmeri to urge people to monitor carbon monoxide levels in their homes with a CO detector and to get their heating equipment inspected on a regular basis. The deadly gas is odorless.

In November 1993, carbon monoxide killed three children in a Suffield house, and in January of 2004, the gas killed two family members and a family friend in East Lyme.