Washington, DC – Firefighters in D.C. saved a man who was sickened by carbon monoxide Thursday morning.

Fire crews were called to the 3300 block of 20th Street NE for a report of an unconscious person who may have had a heart attack. When firefighters entered the home, their carbon monoxide detectors went off, and they realized the man was not having a heart attack.

Firefighters revived the man and he was transported to the hospital. Two other people inside the home were also taken to the hospital.

Their conditions are not known at this time.

Fire officials say carbon monoxide levels in the home were incredibly dangerous.

“We were getting over 500 parts per million. We just went ahead and cut the gas off to the whole building and ventilated until the CO was back to an acceptable under 35 parts per million,” Battalion Fire Chief Timothy Jeffrey said.

Jeffrey said there shouldn’t be any carbon monoxide in a home, but carbon monoxide levels start to become dangerous at 35 parts per million.

Fire officials say the home’s furnace appears to be the source of the carbon monoxide leak.

The carbon monoxide detectors in the home were not working.

Carbon monoxide is odorless and colorless. According to the National Fire Protection Association (NFPA), symptoms of carbon monoxide poisoning can be confused with “shortness of breath, nausea, dizziness, light headiness or headaches.”

The NFPA suggests the following carbon monoxide safety tips:

Use generators in a well-ventilated location that is outdoors and away from windows, doors and vent openings.

Install CO alarms, especially in each sleeping areas.

Test CO alarms at least once a month.

If your CO alarm goes off, move to a location outdoors or by an open window or door.

If you need to warm a vehicle, remove it from the garage immediately after starting it. Do not run a vehicle or other fueled engine or motor indoors, even if garage doors are open.

Make sure the exhaust pipe of a running vehicle is not covered.