If carbon monoxide is detected in your house, get everyone outside immediately. Do not stop to open windows, grab belongings, or search for the source. Once you’re outside, call 911 and do not go back inside until emergency responders have inspected the home and told you it’s safe to re-enter.
Carbon monoxide is colorless and odorless, which means your detector is likely the only warning you’ll get. Knowing exactly what to do in those first few minutes, and what comes after, can prevent a life-threatening situation from getting worse.
Step One: Get Out
The U.S. Consumer Product Safety Commission is clear on this point: immediate evacuation is necessary when a CO alarm sounds. Move every person and pet outside or to a neighbor’s home. Do not take time to ventilate the house first, and do not re-enter until emergency personnel have authorized it.
If it’s dangerously cold outside and no neighbor is nearby, you can shelter in a single room with a door or window open to the outside. Close any doors leading to the rest of the house, turn on an exhaust fan if one is available, and make sure no fuel-burning appliances or idling vehicles are running anywhere near you. This is a last resort, not the default plan.
Step Two: Call 911 and Get Medical Attention
Call 911 from outside the home. If anyone is experiencing symptoms, tell the dispatcher. The most common signs of carbon monoxide poisoning are headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion. These symptoms are easy to mistake for the flu, especially in winter when furnaces are running and windows are sealed.
Severe exposure can cause shortness of breath, loss of consciousness, memory problems, and irregular heartbeat. At high enough concentrations, carbon monoxide is fatal. Even if symptoms seem mild, anyone who was in the home should be evaluated by a medical professional. Carbon monoxide binds to your red blood cells far more effectively than oxygen does, and the effects can linger or worsen even after you’ve left the contaminated air.
Who Is Most at Risk
Young children, pregnant women, elderly adults, and people with heart or lung conditions are especially vulnerable. Unborn babies face a particular danger: fetal hemoglobin absorbs carbon monoxide more readily than adult hemoglobin, and CO levels in the fetus can reach 10 to 15 percent higher than in the mother. Pets, especially small ones, may also show signs of poisoning before adults in the household do because of their faster breathing rates and smaller body size.
Know What Your Alarm Is Telling You
Not every beep from a carbon monoxide detector means an emergency. The alarm patterns are standardized:
- Four loud, continuous beeps (beep, beep, beep, beep): Carbon monoxide is present. Evacuate immediately and call 911.
- One chirp every 30 or 60 seconds: The battery is low and needs to be replaced.
- A chirp every 30 seconds or an “ERR” or “END” display: The detector itself has expired and needs to be replaced entirely.
CO detectors have a life expectancy of about seven years. Models made after August 2009 include an end-of-life warning, but if your alarm was installed in 2018 or earlier, it may be approaching the end of its useful life. Test your detectors monthly and replace them on schedule, not just when they start chirping.
Finding and Fixing the Source
Once the fire department has cleared your home and confirmed it’s safe to re-enter, the next step is identifying what produced the carbon monoxide. Any appliance that burns fuel can be the culprit:
- Furnaces and boilers: The most common source in winter. A cracked heat exchanger can leak CO directly into your ductwork.
- Gas water heaters
- Gas stoves and ovens
- Fireplaces (gas and wood-burning)
- Clothes dryers (gas)
- Attached garages: A car idling in an attached garage can push CO into living spaces even with the garage door open.
- Portable generators, grills, and gas-powered tools: These should never be used indoors or in enclosed spaces.
There are also visible warning signs that a CO problem has been building. Streaks of soot around a furnace or water heater, orange or yellow flames where the flame should be blue, rusting on flue pipes, excess condensation on walls and windows, and fallen soot in a fireplace all suggest combustion gases aren’t venting properly. A blocked or damaged chimney is another common cause. If you notice discolored or crumbling bricks at the top of your chimney, get it inspected before using any connected appliance.
Making the Home Safe Again
If your alarm went off but no one is experiencing symptoms, you can start by ventilating the home (opening windows and doors) and turning off all potential sources: the furnace, gas water heater, gas range and oven, gas dryer, and any space heaters. Then have a qualified technician inspect every fuel-burning appliance and your chimney to confirm everything is operating correctly and nothing is blocking exhaust from venting outside.
If anyone in the household had symptoms, medical evaluation comes first. The house stays off-limits until a professional has identified and repaired the source. Do not assume that airing out the house solves the problem. Carbon monoxide clears from the air relatively quickly once the source is removed and ventilation is adequate, but if the source is still active (a cracked furnace, a blocked flue), concentrations will climb right back up once you close the windows.
How Much Carbon Monoxide Is Dangerous
Workplace safety guidelines cap exposure at 25 to 50 parts per million over an eight-hour period, depending on the agency. The ceiling set by the National Institute for Occupational Safety and Health is 200 ppm, meaning you should never be exposed to that concentration even briefly. For context, most residential CO detectors are designed to alarm at around 70 ppm after one to four hours of sustained exposure, or more quickly at higher concentrations.
There is no safe level of carbon monoxide exposure over long periods. Even low-level, chronic exposure below what triggers an alarm can cause persistent headaches, fatigue, and difficulty concentrating. If multiple members of your household are experiencing vague, flu-like symptoms that improve when they leave the house and return when they come home, suspect carbon monoxide even if your alarm hasn’t sounded. Your detector may be expired, improperly placed, or the concentration may be just below its trigger threshold.
Preventing It From Happening Again
Have all fuel-burning appliances professionally inspected once a year, ideally before heating season starts. This includes your furnace, water heater, fireplace, and any gas appliances. Make sure CO detectors are installed on every level of your home and near sleeping areas. Replace them every seven years regardless of whether they appear to be working.
Never run a generator, grill, or gasoline-powered tool inside a garage, basement, or any enclosed or partially enclosed space. Never use a gas oven to heat your home. And if you have an attached garage, never let a vehicle idle inside it, even with the garage door open. Carbon monoxide accumulates faster than most people expect, and by the time you feel symptoms, your ability to think clearly and get yourself to safety is already compromised.