PROVIDENCE — Carbon monoxide is known as the silent killer because the deadly gas emits no odor and people often don’t know they are being poisoned. Sometimes they can confuse the symptoms of poisoning — headaches, dizziness, nausea — with the flu or food poisoning.

On Sunday, seven people in Central Falls were reportedly poisoned from a generator running in a home.

Each year fire departments around the country respond to tens of thousands of carbon-monoxide incidents.

In 2005 alone, fire departments responded to more than 61,100 calls in which higher-than-normal levels of the gas were found, according to the National Fire Protection Association.

The majority of those calls occur in the colder months, when people are using their furnaces or other heating systems and the exhaust of those systems is improperly vented. But the problem also occurs when the exhaust of cars, gasoline-powered leaf blowers and fuel-burning generators leak into living areas of homes.

The federal Centers for Disease Control and Prevention estimates that about 480 people died annually between 2002 and 2004 from carbon-monoxide exposure. The gas poisons about 1,500 people a year.

In 2002, Rhode Island became one of the first states to require that each home have at least one carbon-monoxide detector before the house can be sold. Massachusetts has since passed a similar law.

The Rhode Island law came about after a North Providence family nearly died because of carbon-monoxide poisoning. The family believed they had all been stricken by the flu when they came down with headaches and dizziness one evening about a week before Christmas in 1997. First responders rescued them. Investigators discovered a seam had popped on the exhaust pipe on the family’s natural-gas furnace. Later, the gas company said the house contained carbon monoxide at 1,400 parts per million. Detectors sound at 35 parts per million.

Carbon monoxide is undetectable without a monitoring device. They can be purchased for about $25.

To prevent a buildup of carbon monoxide inside your home, fire officials advise:

•Make sure heating systems are installed by licensed professionals with the proper permits from the local permitting authorities.

•Never use an oven or grill to heat your home.

•Don’t run your car, lawn mower or leaf blower in a garage that is attached to your house.

•Clear out the snow from around the vents of dryers, furnaces, stoves and fireplaces.

•Know the difference between the sounds of a smoke detector and a carbon-monoxide alarm. A carbon-monoxide alarm will sound four short beeps followed by a pause and then another four short beeps. A smoke detector will sound a more constant tone.

•Remember: a smoke detector is no substitute for a carbon-monoxide detector.

•Call your local fire department’s non-emergency number if you have questions about carbon monoxide. In Providence, you can call the department’s fire prevention hot-line number: (401) 243-6034.

tmooney@projo.com