What Are the Signs of Carbon Monoxide Poisoning?

The most common signs of carbon monoxide poisoning are flu-like symptoms, including the following:

Carbon monoxide—sometimes referred to as CO—is a colorless, odorless, flammable gas that’s produced by burning material containing carbon. It’s in the fumes produced when you burn fuel, so trace amounts of it are everywhere: cars, trucks, stoves, grills, fireplaces, gas ranges, and furnaces all produce some CO.

It’s deadly too. About 50,000 people in the United States visit the emergency room every year because of accidental carbon monoxide poisoning. And at least 430 people die annually from accidental poisoning.¹

The risk of unintentional carbon monoxide poisoning increases in the winter, when gas appliances are used most frequently.

CO poisoning happens when you start breathing in carbon monoxide. When there’s too much carbon monoxide in the air, the body replaces the oxygen in the red blood cells with carbon monoxide, which prevents oxygen from reaching the organs.² As carbon monoxide replaces the oxygen in the body, organs get less of the oxygen they need to work properly. If left untreated, victims may suffocate to death because of the lack of oxygen in their body.

High levels of carbon monoxide can start to affect the body in minutes, and without immediate treatment, people experiencing CO poisoning may lose consciousness, have a seizure, fall into a coma, or worse. People can also be exposed to carbon monoxide over a longer period of time with a slow leak. That’s when they’ll experience flu-like symptoms like headaches, mild nausea, and fatigue.

And if you’re in a high altitude, over 10,000 feet, carbon monoxide is especially dangerous. Those symptoms take effect much faster and have more severe effects at higher altitudes.

Damage from carbon monoxide varies. With prolonged exposure, there’s a risk of permanent brain damage, heart damage, miscarriage in pregnant people, and death. Rarely, carbon monoxide intoxication can also greatly increase the risk of Parkinson’s disease.

Exposure to carbon monoxide is especially dangerous for people with weakened or compromised immune systems: infants, children, pregnant people, older adults, and people with health problems like heart or lung disease.

Levels of carbon monoxide may be rising if headaches, mild nausea, and other flu-like symptoms clear up when you go outdoors. Also be on the lookout to see if several people in your home develop similar symptoms around the same time. Children and pets are usually the first ones affected.

These are some common early signs and symptoms of carbon monoxide poisoning:

• Headache
• Nausea
• Dizziness
• Fatigue

As carbon monoxide poisoning progresses, its symptoms worsen. Victims may experience the following:

• Confusion
• Shortness of breath
• Chest pain
• Vomiting
• Blurry or double vision
• Loss of muscle control
• Loss of consciousness

Some people may also experience a more severe headache, dizziness, weariness, and mental changes.

The severity of symptoms depends on your health and how much carbon monoxide is in the air. On average, a concentration of 35 parts of CO per million causes a headache and dizziness within six to eight hours of constant carbon monoxide exposure. In more elevated cases, 12,800 parts per million of carbon monoxide can render a person unconscious after two to three breaths. In less than three minutes, the poisoning becomes fatal.

Engines and fuel-burning appliances produce carbon monoxide in trace amounts that don’t pose any danger. But if you use the appliance or engine in an enclosed space—running your car with the garage door closed or using your charcoal grill in your living room, for example—carbon monoxide can rise to fatal levels.

These common appliances and tools can be a source of dangerous levels of carbon monoxide if not properly vented:

• Car and truck engines
• Gas cooking ranges and stove tops
• Fuel-burning space heaters
• Water heaters
• Furnaces
• Charcoal grills
• Fireplaces
• Portable generators
• Wood-burning stoves
• Water heaters
• Chimneys and flues

You may not be able to see or smell it, but carbon monoxide leaves certain signs of a leak in your house that you can identify. Here’s a good example: if there’s dripping water or heavy condensation on a window close to where an appliance is installed and you’ve taken steps to reduce moisture, it can be a good indicator of a carbon monoxide leak.

Here are some other signs to look out for:

• Sooty or brown/yellow stains around a leaking appliance
• Stale or stuffy air
• Soot, smoke, or fumes from a chimney or fireplace
• No upward draft in a chimney flue
• Fallen soot in fireplaces
• Solid fuel fires burning slower than usual
• A pilot light that frequently blows out
• The smell of exhaust or gas (often accompanies carbon monoxide)
• A yellow burner flame instead of a clear blue flame on gas stove tops and fireplaces

If you’re worried your furnace is leaking carbon monoxide, turn it off and contact a heating and cooling professional. They’ll work with you to identify where your system is leaking and replace parts as needed.

Carbon monoxide poisoning is often treated with high doses of oxygen applied through a face mask with an oxygen reserve bag. In the case of severe poisoning, a medical team may use hyperbaric oxygen therapy, which delivers super-high doses of oxygen.

It’s also important that anyone with carbon monoxide poisoning not return to the area of poisoning until the leak has been treated. A local fire department or public service company can help find the source of the carbon monoxide leak and let you know whether the building is safe. You can also call your regional poison center at 1 (800) 222-1222 for help.

The good news: carbon monoxide poisoning is very preventable.

If you have any generators, place them at least 25 feet away from any nearby dwellings. Never leave your car running in the garage or other enclosed space. And make sure propane heaters are used only outdoors.

Below are more steps you can take to detect carbon monoxide early, maintain your appliances, and keep areas well ventilated.

Much like smoke detectors and fire extinguishers are the first tools of fire safety, a solid carbon monoxide detector should be your first line of defense against potential carbon monoxide leaks.

Carbon monoxide detectors can be purchased separately or as part of a larger home security package. Most homes need a couple, and you should put one in the hallway near each sleeping area in your house. Check the batteries at least twice a year—whenever you check the batteries for your smoke alarm—and make sure to replace them as needed.

If your carbon monoxide detector sounds, evacuate the area immediately. If you’re in a large building where there are too many people to evacuate at once, like an office, open the windows to dilute the concentration of carbon monoxide as people filter out of the building.

From there, call the fire department and ask for medical assistance to be dispatched. Once you’ve relayed to the fire department that there’s a carbon monoxide threat, they’ll be able to locate the leak and turn off its source. Public service officials from utility companies can also correct gas leaks.

Routine maintenance will lower your risk of potential exhaust or carbon monoxide leaks. Heating systems, water heaters, and other appliances that burn gas, oil, or coal, should be serviced by a qualified technician every year.

You should also have a mechanic check out the exhaust system of your vehicle every year. A small leak in your car or truck’s exhaust system can lead to a deadly buildup of carbon monoxide in your car.

If you have a fireplace, make sure you have your chimneys and flues swept at least once a year to make sure there’s no blockage.

Be sure to use all gas appliances as recommended: gas ovens will produce too much carbon monoxide if they’re used to heat your home. Gas space heaters should only be used when someone is around to keep an eye on them and windows are open to provide fresh air.

It’s also important to maintain proper ventilation. Any fuel-burning appliance—gas heaters, furnaces, cooking ranges, fireplaces, and the like—should always be used in well-ventilated spaces. Save charcoal grills for outside.

That also goes for painting. Methylene chloride, a solvent found in paint thinner and varnish remover, can break down into carbon monoxide when inhaled. So make sure you keep the windows open when you’re doing a little home improvement.

Carbon monoxide poisoning is dangerous but also entirely preventable. By purchasing a carbon monoxide detector, taking steps to make sure your home is well ventilated, and maintaining your appliances, you can keep your family and your home safe from carbon monoxide poisoning.

1. CDC, “Carbon Monoxide (CO) Poisoning Prevention”
2. Mayo Clinic, “Carbon Monoxide Poisoning”