CO, the chemical name for carbon monoxide, is a deadly gas created when fossil fuels burn incompletely – like in your gas-fired hot water heater or in your gas-, coal- or oil-fired furnace. And even though you may have CO monitors installed and working, the U.S. Environmental Protection Agency tells us that these detectors are not a replacement for proper use and maintenance of your fuel-burning appliances.

Don’t let CO monitors, says the EPA, lull you into a false sense of security. They are not as reliable as smoke detectors, where you can easily detect the cause for an alarm; CO is invisible, tasteless, and odorless and so it’s harder to tell if a CO alarm is false or genuine.

Play it safe, then, and know the symptoms. Low levels of CO can cause shortness of breath, mild headache and mild nausea. Moderate levels can cause severe headache, dizziness, mental confusion, nausea or fainting.

At high levels, it can kill a person in minutes. The effects of CO exposure can vary greatly from person to person, says the EPA, depending on age, overall health, and the concentration and length of exposure.

While the symptoms of CO poisoning may suggest flu, food poisoning or something else to you, don’t take a chance. Get fresh air immediately by opening windows and doors and turn off combustion devices. Get out of the house and get to an emergency room.

Long-term, of course, you want to prevent CO poisoning by having your fuel-burning appliances inspected, cleaned and tuned up annually by professionals. Be sure that flues, vents and chimneys are fully connected, not leaking, in good condition and not blocked to the outside. Preventing CO from becoming a problem in your home, says the EPA, is better than relying on an alarm.