The role of carbon monoxide detectors

Installing a carbon monoxide detector in your home may help you feel secure, as it can alert you to danger before symptoms are noticed. However, it's important to realize that these detectors are your second line of defense, after you've followed the necessary maintenance and safety precautions listed above.

If your carbon monoxide detector alarms…

and someone in your house is experiencing symptoms of carbon monoxide poisoning, have everyone leave the house immediately. Call 911 or the fire department from a neighbour's telephone or cell phone.
and no one is suffering symptoms of carbon monoxide poisoning, remain calm. Open your windows and doors for fresh air and take immediate steps to determine the reason for the alarm. It's possible the detector reacted to:

a low/dead battery
exhaust fumes from a nearby vehicle
prolonged humidity, if installed near a bathroom
chemicals, cleaning products or cigarette smoke
accumulated low levels of carbon monoxide

If you can't determine why the detector alarmed, call a licensed gas contractor to inspect your appliances and check for other possible sources of carbon monoxide.