If carbon monoxide is in your home, get everyone outside immediately. Do not stop to open windows or locate the source. Once you’re safely outside, call 911 and do not re-enter until emergency responders have inspected the building and given you the all-clear. Speed matters: at concentrations of 800 ppm, collapse can occur within two hours, and at 3,200 ppm, unconsciousness can set in within 10 to 15 minutes.
Get Out First, Ventilate Later
The natural instinct when a CO alarm sounds is to throw open windows and start hunting for the source. That instinct is wrong. The U.S. Consumer Product Safety Commission is explicit: do not take time to ventilate the home unless someone is unconscious or physically unable to leave. Every second spent inside a CO-filled space extends your exposure and increases the risk of confusion, collapse, or worse.
Take everyone, including pets, outside or to a neighbor’s home. Stay there. If extreme weather makes going outside dangerous and neighbors aren’t nearby, the CPSC recommends isolating in one room with a door or window open to the outside. Close all interior doors to the rest of the house, turn on any exhaust fan in that room, and make sure no fuel-burning appliances or idling vehicles are running nearby. This is a last resort, not a first choice.
How Carbon Monoxide Clears From a Building
Once emergency responders arrive and identify the source, they’ll shut it down and begin ventilating the structure. Carbon monoxide disperses relatively quickly with airflow because it’s slightly lighter than air and mixes freely with it. Opening windows and doors on opposite sides of the house creates cross-ventilation that pushes contaminated air out. Fans accelerate the process. In most cases, a well-ventilated home can clear to safe levels within a few hours, but responders will use handheld CO meters to confirm concentrations have dropped before letting you back inside.
You should not re-enter and try to ventilate on your own. CO is invisible and odorless, so you have no way to judge whether levels are still dangerous without a calibrated detector. A reading of just 100 ppm can cause headaches within two to three hours, and 400 ppm becomes life-threatening after three hours. Professional measurement is the only reliable way to know it’s safe.
Finding and Fixing the Source
Clearing the gas is only half the job. If you don’t fix what produced it, CO will build up again. The most common household culprits are incorrectly installed, poorly maintained, or badly ventilated appliances: furnaces, water heaters, gas stoves, and fireplaces.
Specific failures to look for include:
- Blocked flues and vents. A fireplace flue left closed, a dryer vent clogged with lint, or heating system exhaust pipes buried under snow after a winter storm can all trap combustion gases inside.
- Cracked heat exchangers. The heat exchanger in a furnace separates combustion gases from the air circulating through your home. When it cracks, CO leaks directly into your ductwork. This is typically caught during annual furnace inspections.
- Obstructed oven airflow. Covering the bottom of a natural gas or propane oven with aluminum foil blocks the air circulation the burner needs for complete combustion, causing CO to accumulate.
- Generators, grills, and vehicles. These should never run inside a garage, basement, or any enclosed space, even with the garage door open. Concentrations build faster than ventilation can clear them.
After any CO incident, have a qualified technician inspect every fuel-burning appliance in your home before you use it again. A visual check isn’t enough. Technicians test for proper draft, measure CO output at the appliance, and verify that exhaust systems are intact and unobstructed.
How Your Body Clears Carbon Monoxide
Carbon monoxide binds to hemoglobin in your blood roughly 200 times more tightly than oxygen does. Once bound, it forms carboxyhemoglobin, which prevents those red blood cells from carrying oxygen to your tissues. Your body clears it by gradually replacing the CO on hemoglobin with oxygen, but this takes time.
Breathing normal room air, the half-life of carboxyhemoglobin is roughly 4 to 5 hours. That means if your blood is 20% saturated with CO, it takes 4 to 5 hours to drop to 10%, and another 4 to 5 hours to reach 5%. With high-flow oxygen delivered through a mask, the half-life drops to about 74 to 80 minutes. In a hyperbaric oxygen chamber, where you breathe pure oxygen at higher-than-normal atmospheric pressure, the half-life shrinks further to 24 to 53 minutes.
This is why the first thing paramedics do for CO exposure is put you on high-concentration oxygen. It’s not just supportive care. It actively accelerates the removal of CO from your blood. For more severe poisoning, especially cases involving loss of consciousness, confusion, or concurrent smoke inhalation, hyperbaric oxygen therapy may be recommended.
Symptoms That Signal Dangerous Exposure
CO poisoning mimics common illnesses, which is part of what makes it so dangerous. At low concentrations (around 35 ppm), you might develop a headache and mild dizziness over six to eight hours of continuous exposure. At 200 ppm, headache, impaired judgment, and vision problems develop within two to three hours. At 400 ppm, nausea joins the headache and the situation becomes life-threatening after three hours.
The higher the concentration, the faster the timeline compresses. At 1,600 ppm, confusion and staggering appear within 20 minutes, with death possible in under two hours. At 6,400 ppm, convulsions and respiratory arrest can occur within one to two minutes.
If multiple people in the same household develop flu-like symptoms simultaneously, especially headache, nausea, and dizziness, and the symptoms improve when you leave the building, suspect CO. This pattern is one of the clearest real-world warning signs.
Delayed Symptoms After Exposure
Even after CO clears from your blood and you feel fine, neurological symptoms can appear days or weeks later. This condition, known as delayed neuropsychiatric sequelae, occurs in roughly 3% to 40% of adult survivors depending on the severity of exposure. Symptoms typically emerge within 2 to 40 days after the initial poisoning and can include memory problems, difficulty concentrating, mood changes, and impaired coordination.
The wide range in that estimate reflects the lack of standardized diagnostic criteria and differences in how studies define and track the condition. One study of 217 CO poisoning patients found that about 23% developed delayed neurological symptoms. The risk is real enough that anyone treated for significant CO exposure should be aware that new cognitive or mood symptoms in the following weeks warrant medical attention.
Preventing CO Buildup in the First Place
Carbon monoxide detectors are the single most important line of defense. Install one on every floor of your home, and place at least one near sleeping areas. The alarm needs to be loud enough to wake you, since CO exposure during sleep is particularly dangerous because you can’t recognize symptoms while unconscious. Follow the manufacturer’s placement instructions exactly, as mounting height and distance from appliances affect performance. Replace detectors according to their expiration date, typically every five to seven years, and test them monthly.
Beyond detectors, annual maintenance of all fuel-burning appliances is the most effective prevention. Have your furnace, water heater, and any gas appliances inspected before each heating season. Keep vents and chimneys clear year-round, and check them after heavy snow. Never use portable generators, charcoal grills, or camp stoves indoors or in enclosed spaces like garages and screened porches.