Dr. Carson Harris is always a little suspicious this time of year when patients come in with symptoms of the flu: headache, nausea, fatigue.

They might think it's swine flu, but it could be carbon monoxide poisoning.

"The symptoms are so similar," said Harris, an emergency room physician at Regions Hospital in St. Paul. "It can look just like the flu."

This year, with the H1N1 flu pandemic on everyone's mind, even doctors may confuse the two, Harris said. That's one reason the hospital and St. Paul Fire Department have joined forces to issue a public alert this week on how to distinguish between the two problems. It can, they say, "mean the difference between life and death."

With carbon monoxide poisoning, several people in the home may wake up with the same symptoms, including blurred vision, disorientation, shortness of breath and vomiting. But unlike the flu, these symptoms disappear when they leave the home and breathe fresh air.

The danger goes up during the winter, because the gas can be released by malfunctioning gas furnaces, wood-burning stoves or fireplaces and propane heaters. It's especially a danger in ice fishing houses and garages, where carbon monoxide can build up in enclosed spaces, Harris said. The gas itself is colorless, tasteless and odorless, so people may be unaware of the risk until the symptoms occur.

Tina Northcutt, of Circle Pines, said she thought it might be the flu when she was too tired to get out of bed the Sunday after Thanksgiving. But she didn't worry until two days later, when both her teenage children complained of the same thing. Then she called her clinic. "It's very rare that our whole house is sick at all, let alone at the same time," she said. "Things just weren't adding up right."

A nurse asked a few questions and, suspecting it might be carbon monoxide, told her to get the whole family to the emergency room.

All three had elevated levels of carbon monoxide, though only Tina, who had the highest level, needed treatment with oxygen.

Afterward, a gas company technician met them at home but could not find the cause. By that time, the readings were normal throughout the house, Northcutt said. While that's unnerving, she says, she now has carbon monoxide detectors outside every bedroom. "Now we're covered." All three quickly recovered, she added. The take-home message, she said, is "When in doubt, call."

It's important to diagnose the problem early, Harris said, because the longer you're exposed to carbon monoxide, the greater the health risk, including death. If caught soon enough, carbon monoxide poisoning can be treated with oxygen or hyperbaric chambers.

Pets can also be affected. "Sometimes small animals tend to be kind of haggard or feel a little drugged out, so to speak," Harris said. "They tend to be affected a little more quickly [than people]."

St. Paul fire officials recommend using carbon monoxide detectors and other precautions:

Don't use a gas stove or oven for heating; don't use kerosene space heaters indoors; don't warm up your car in the garage; only use gas or charcoal grills outdoors. Have your furnace or boiler inspected each year. State law requires that homes and apartments have carbon-monoxide detectors within 10 feet of every bedroom.

Maura Lerner • 612-673-7384