If your carbon monoxide detector is sounding a continuous alarm (four beeps in a row, repeating), get everyone out of the house immediately and call 911 from outside. Do not waste time opening windows or trying to find the source of the leak. Every minute you spend inside a home with elevated carbon monoxide levels increases your risk of poisoning.
Step by Step: What to Do Right Now
Get every person and pet out of the home. Go to a neighbor’s house or at least move well away from the building into fresh air. Once you’re outside, call 911 or your local fire department. Do not go back inside for any reason until emergency responders have inspected the home and told you it’s safe to re-enter.
If someone in the home is unconscious or unable to move on their own, open the nearest window or door to outside air while you help them out. But for everyone who can walk, the priority is simply getting out fast.
There’s one exception to the “get outside” rule. In extreme cold or severe weather where going outside is genuinely dangerous, you can shelter in a single room that has a window or door opening directly to the outside. Open all the windows in that room, close any doors leading to the rest of the house, and turn on an exhaust fan if one is available. Make sure no fuel-burning appliances (stoves, space heaters, generators) are running in that room and no vehicles are idling in an attached garage.
First, Make Sure It’s Actually an Alarm
Carbon monoxide detectors make different sounds for different situations, and not all of them mean there’s a dangerous leak. Understanding the pattern can save you from either panicking unnecessarily or ignoring a real emergency.
- Four beeps in a repeating pattern: This is the actual carbon monoxide alarm. Evacuate immediately.
- A single chirp every 30 to 60 seconds: This usually means the battery is low or the unit has reached the end of its lifespan and needs to be replaced. It’s not an emergency, but you should address it soon.
- A chirp every 30 seconds with “ERR” or “END” on the display: The detector has expired. CO alarms last about seven years, and all units made after 2009 are required to alert you when they’ve reached the end of their useful life.
If you’re hearing a single chirp rather than four loud beeps, you don’t need to evacuate. Replace the batteries or the entire unit.
Why Carbon Monoxide Is So Dangerous
Carbon monoxide is colorless and odorless. You cannot smell it, see it, or taste it. It works by binding to your red blood cells far more effectively than oxygen does, gradually starving your organs of the oxygen they need. At low concentrations, this happens slowly over hours. At high concentrations (400 parts per million and above), it can knock you unconscious in minutes.
The symptoms mimic the flu: headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion. That resemblance is part of what makes CO poisoning so deadly. People assume they’re getting sick and lie down to rest, breathing in more of the gas instead of leaving the house. If multiple people in the household develop “flu-like” symptoms at the same time, especially without a fever, carbon monoxide should be the first suspect.
Brain function starts to decline when blood levels of carbon monoxide reach just 2.5%. At 25 to 30%, people begin losing consciousness. Above 60%, death follows. The World Health Organization considers concentrations above about 9 ppm unsafe for exposures lasting longer than eight hours, and above roughly 86 ppm unsafe for even 15 minutes.
How Your Detector Decides to Sound the Alarm
CO detectors are designed with built-in delays so they don’t alarm over brief, harmless spikes. Under the safety standard used in the U.S., a detector exposed to 70 ppm won’t sound the alarm for at least 60 minutes but must trigger before 240 minutes. At 150 ppm, the alarm must go off within 10 minutes. At 400 ppm, it triggers within 4 to 15 minutes.
This means that by the time your alarm is actually sounding, the CO level has been elevated long enough or high enough to pose a real health risk. A CO alarm is not like a smoke detector that sometimes trips from burnt toast. If it’s giving you four beeps, take it seriously.
What Happens After You Call 911
Fire department responders carry portable CO monitors that give real-time readings throughout your home. They’ll measure the concentration in different rooms, identify which area has the highest levels, and begin narrowing down the source. They’ll tell you when it’s safe to go back inside, or whether you need to stay out until a repair is made.
If anyone in the household has symptoms like headache, dizziness, confusion, or nausea, tell the paramedics. Treatment for CO poisoning involves breathing high-concentration oxygen, which clears the carbon monoxide from your blood much faster than normal air would. This typically takes four to five hours. In severe cases, where someone lost consciousness, has heart involvement, or is pregnant, a hospital may use a pressurized oxygen chamber to accelerate the process further.
Common Sources of Carbon Monoxide Leaks
The most frequent culprits are fuel-burning appliances that are either poorly maintained or improperly vented. Furnaces, water heaters, gas stoves, fireplaces, and clothes dryers all produce carbon monoxide as a normal byproduct of combustion. Under normal conditions, that CO is routed safely outside through flues and vents. Problems arise when something blocks or breaks that pathway.
Specific things that cause CO to build up inside a home:
- Cracked or corroded heat exchangers in furnaces, which allow combustion gases to leak into the air your system circulates through the house
- Blocked vents or flues, especially after heavy snowfall covers exterior vent openings
- Poorly ventilated fireplaces with damaged or obstructed chimneys
- Gas ovens with blocked airflow, sometimes caused by lining the bottom with aluminum foil
- Cars or generators running in an attached garage, even with the garage door open
- Portable generators, grills, or camp stoves used indoors during power outages
After a winter storm, check that all exterior vents for your furnace, dryer, stove, and fireplace are clear of snow and ice. Blocked vents are one of the most common triggers for CO alarms during cold weather.
Getting a Professional Inspection
After the fire department clears your home, you’ll likely need an HVAC technician to pinpoint and fix the problem. A technician will drill a small hole in the flue pipe of your furnace or water heater and insert a combustion analyzer probe to measure CO levels directly in the exhaust stream. In some cases, they’ll also test the ductwork itself.
They’ll inspect the furnace and water heater for corroded burners, cracked heat exchangers, soot buildup, melted wiring, and moisture dripping from flue pipes. These are all signs that combustion gases aren’t being properly contained and vented. They’ll also check that horizontal vent pipes angle slightly upward as they exit the house, since a flat or downward-sloping vent can allow gases to pool and flow backward into the home. Any leaky vent connections get sealed or replaced.
Don’t skip this step. The fire department can confirm whether CO is present and ventilate your home, but they’re not there to diagnose a failing furnace. Until the source is identified and repaired, the problem will come back.
Keeping Your Detectors Working
Install CO detectors on every level of your home, including near sleeping areas. Place them at roughly breathing height or on the ceiling, and keep them away from fuel-burning appliances (which can cause nuisance readings at close range).
Replace the batteries at least once a year, and replace the entire unit every seven years. The sensor inside a CO detector degrades over time and eventually stops responding reliably. If your detector is chirping every 30 seconds or showing “ERR” or “END” on its display, it has reached the end of its life and needs a new unit, not just new batteries.
Annual maintenance on your furnace, water heater, and any other fuel-burning appliances is the single most effective way to prevent a CO emergency in the first place. A technician can catch a cracked heat exchanger or corroded flue long before it becomes dangerous.