Gas-burning furnaces filled two homes on Long Island and in Connecticut with carbon monoxide in the last few days, poisoning the occupants during a lull in a cold snap. The Long Island residents escaped. But in Connecticut, three people — including two women who had arrived to help the family — were killed.

In a gray Victorian home in East Lyme, Conn., David and Kelly Dunn and their two daughters felt as if they had been stricken with the flu. On Saturday, the family decided to stay inside. Ms. Dunn’s mother and a family friend stayed with the Dunns to help them recuperate, the Connecticut State Police said.

On Sunday night, another relative found the family lying on the first floor of the house. Relatives and neighbors soon realized that the flulike symptoms had been an early sign of carbon monoxide poisoning, the police said.

Ms. Dunn’s mother, Aralia Cameron, and Kathleen Dumais, the women who had come to help, were dead at the scene. Mr. Dunn died yesterday after being flown to a hospital in Rhode Island. His wife and daughters, Chelsea, 8, and Elissa, 3, were in critical condition last night at Hartford Hospital, the police said.

Ronald Pringle, chief of the Niantic Fire Department, said carbon monoxide had built up in the home because of a plugged flue pipe attached to a propane heater in the basement. The family had also sealed several windows with plastic, a move that kept the cold at bay but also trapped the gas, officials said.

“It’s just ironic that this house was making them sick,” said Sgt. Michael Collins of the Connecticut State Police. “It was actually carbon monoxide that was killing them. It’s just devastating.”

About 2,500 people die each year of carbon monoxide poisoning, about 80 percent of those intentional deaths like suicides, according to health officials. They said the number of accidental poisonings increases in the winter, as families turn up heat in their homes and shut their windows against the cold.

“We typically see them when we have these cold snaps,” said Dr. J. Christopher DiGiacomo, associate director of trauma at the Nassau University Medical Center’s Wound Care and Hyperbaric Center, which treats carbon monoxide victims. “Temperatures go down into the single digits, and people are going to start cranking the kerosene heaters.”

Carbon monoxide, an odorless and colorless gas that is a byproduct of burning fossil fuels, bonds to blood cells and typically nauseates people or makes them dizzy or lightheaded in small doses. In large amounts, it causes severe headaches and death, said James McDonnell, president of Professional Laboratories, which makes home safety equipment.

In the Long Island case, a Lakeview woman woke about 4 a.m. yesterday to hear her son whimpering, the Nassau County police said. The woman could not wake her husband or daughter and called 911 before passing out, the police said. Police officers pulled five adults and three children from the first floor and basement of the house, at 727 Jennings Avenue. All eight residents — five of whom were unconscious — were taken to the hospital, as were three police officers who suffered minor inhalation sickness, the police said.

They said the carbon monoxide leak stemmed from a faulty gas heater. The residents and the officers would recover, they said.

Both the Connecticut and the Long Island poisonings appeared to be accidental, the police said. In the Dunns’ quiet neighborhood, people were stricken by news of the deaths and waited for updates on the conditions of Ms. Dunn and her daughters.

A relative found the family inside the home about 8:30 p.m. Sunday. The relative, whom the police did not identify, had been talking to the Dunns about their flulike illness all weekend but grew frantic when she tried to call the family on Sunday and no one answered. The relative, who neighbors said lives in New York, drove to the house and discovered the six people lying unconscious on the first floor, fire officials said. She called 911 and dragged Chelsea, the 8-year-old, to a neighbor’s home, and a neighbor pulled Ms. Dunn from the house.