What Is Carbon Monoxide Poisoning? Symptoms, Causes and How It's Treated
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer
Carbon monoxide poisoning is a condition in which inhaled CO crowds out oxygen in red blood cells, starving the brain and heart of oxygen. [4] Symptoms progress from headache and nausea to chest pain, confusion, fainting and loss of coordination. [4] A blood test confirms it, and treatment is 100% oxygen by mask. [4] It can't be treated at home. [4]
Key takeaways
Carbon monoxide is an invisible, colorless, odorless gas created when fuel burns, [4] and it builds up to dangerous levels without any warning you can sense.
CO molecules enter red blood cells faster than oxygen does, crowding out the oxygen your brain and heart depend on. [4]
Early warning signs include a mild headache, nausea, and shortness of breath; more serious symptoms include chest pain, confusion, fainting, and loss of muscle coordination. [4]
High concentrations can be fatal in fewer than five minutes; even lower concentrations can cause poisoning within one to two hours. [4]
More than 400 people in the United States die from accidental CO poisoning each year, and cases peak in winter. [1] [4]
Treatment requires pure oxygen by mask under medical supervision; there is no safe home treatment. [4]
A battery-operated or battery-backed CO detector, tested twice a year and replaced every five years, [4] is the best protection.
What the research says
Winter, young children, and older adults are most affected. U.S. surveillance for 2005–2018 found about 56.5 emergency department visits and 3.3 deaths per million people each year, with the highest ED rates in children aged 0–9 (70.5 per million), the highest death rate in adults over 80 (9.9 per million), and a peak in December and January; headache, nausea, and dizziness were the most reported symptoms (Shin et al., Ann Emerg Med, 2023) [1].
Problems can appear after the poisoning seems over. A systematic review of treatments reports that roughly 10% to 30% of people develop delayed encephalopathy after acute CO poisoning, and that no specific treatment is available (Xu et al., Medicine, 2019) [2].
Evidence for hyperbaric oxygen is mixed. In a meta-analysis of 6 randomized trials comparing hyperbaric with normal-pressure oxygen, fewer people reported memory impairment with hyperbaric oxygen (18.2% vs 23.8%), but the difference was not statistically significant (relative risk 0.80, 95% CI 0.43–1.49) (Lin et al., Medicine, 2018) [3].
What Is Carbon Monoxide Poisoning?
Carbon monoxide poisoning is a serious, potentially life-threatening illness that occurs after breathing in fumes containing carbon monoxide. This gas is produced whenever fuel burns, such as gasoline, natural gas, propane, kerosene, charcoal, or wood. [4] At high levels, carbon monoxide can kill in a matter of minutes, [4] which is why suspected exposure should always be treated as an emergency.
The most common early symptoms are a mild headache and shortness of breath, along with nausea. [4] Because these symptoms can resemble the flu or food poisoning, CO poisoning is often mistaken for something less serious — which is part of what makes it so dangerous.
Carbon monoxide is an invisible, colorless, odorless gas. [4] You cannot see it, smell it, or taste it. It is produced during combustion — the process by which fuels like gasoline, oil, natural gas, and wood burn — and it builds up quickly in enclosed spaces. [4] It can reach dangerous levels at high concentrations before anyone notices.
In the United States, accidental carbon monoxide poisoning kills more than 400 people every year. [4] Surveillance data for 2005–2018 show about 56.5 emergency department visits per million people per year in the 17 states studied (Shin et al., 2023). [1]
Carbon Monoxide Poisoning Symptoms
Symptoms depend on how much CO you inhale and for how long. Lower concentrations cause gradual, flulike symptoms, while high concentrations can knock someone out within minutes. [4]
Exposure level | Symptoms |
Early warning (low levels) | Mild headache, nausea, shortness of breath |
Moderate to high exposure | Severe headache, chest pain, dizziness and weakness, loss of muscle coordination, mental confusion, upset stomach with nausea and vomiting, fainting or loss of consciousness |
Even low-level fumes are toxic. Breathing them in for longer periods can still cause serious harm, and delayed symptoms may appear after low-level exposure ends. [4]
If you notice early warning signs, go outside immediately and contact emergency services (911 or your local emergency number). Do not stay inside to make the call. If you cannot get out, open a window or door and stay near it while calling for help. [4]
What Causes Carbon Monoxide Poisoning?
Breathing in carbon monoxide fumes is what causes poisoning. The gas interferes with the way red blood cells carry oxygen to the body's tissues and organs.
Red blood cells deliver oxygen throughout the body, and every organ needs oxygen to function. Carbon monoxide molecules enter red blood cells faster than oxygen molecules do, so the CO quickly crowds out the oxygen in the bloodstream. [4] As a result, the brain and heart receive far less oxygen than they need (a state doctors call hypoxemia when blood oxygen runs low). Carbon monoxide also combines with proteins in the body and damages cells and organs. [4] Heavy inhalation can cause unconsciousness and suffocation within minutes. [4]
The poisoning itself comes from fuel-burning appliances and equipment that are not properly ventilated. Common scenarios include:
Scenario | Why it's dangerous |
Idling a car or truck in a closed garage | Exhaust fumes build up fast and seep into attached homes |
A vehicle with a blocked exhaust, such as snow in the tailpipe | Fumes have nowhere to escape and recirculate |
Using a kerosene heater or gas grill indoors | Combustion fumes accumulate in the enclosed space |
Common sources of carbon monoxide include car and truck engines, small gasoline-powered engines, fuel-burning space heaters (not electric ones), gas stoves and ranges, grills, lanterns, home heating furnaces, and the burning of charcoal, kerosene, propane, or wood. [4]
How Quickly Does Carbon Monoxide Cause Poisoning?
The speed of poisoning depends on concentration. In an area with a high concentration of CO, poisoning can occur in fewer than five minutes. [4] In an area with a lower concentration, it may take one to two hours for symptoms to develop. [4] Because a person who is asleep may never wake up to feel the symptoms, a working CO detector is the only reliable alert while you sleep.
Who Is Most at Risk?
Everyone who breathes air contaminated with carbon monoxide is at risk, but some people are harmed by lower doses than others. [4]
Higher-risk group | Why the risk is greater |
People exposed in winter | Heating systems are running more often |
Developing fetuses | Fetal tissue is highly oxygen-dependent |
Infants and small children | Small body size means effects appear sooner |
Adults over 65 | Reduced ability to compensate for low oxygen |
People at high altitudes | Oxygen levels are already lower |
People with chronic heart disease | Heart tissue is sensitive to oxygen shortage |
People with anemia | Reduced capacity to carry oxygen |
People with respiratory conditions (such as asthma) | Lungs already work harder to get oxygen |
People who smoke tobacco | Baseline oxygen delivery is already compromised |
Pets and animals | Smaller bodies often show symptoms first — pets are usually the first in a household to get sick |
Occupational risk is higher for people who work in environments with combustion engines and fires, such as boiler rooms, warehouses, and petroleum refineries. Careers with elevated exposure include firefighters, mechanics, drivers, and welders. [4]
Carbon Monoxide Poisoning Complications
If exposure lasts long enough, carbon monoxide poisoning can be fatal. If it is caught in time, treatment can reverse the effects. [4] However, complications can linger even after successful treatment:
Breathing difficulties
Parkinson's disease
Memory loss
Heart abnormalities
Roughly 10% to 30% of people develop delayed encephalopathy after acute CO poisoning (Xu et al., 2019). [2] During pregnancy, carbon monoxide poisoning can lead to miscarriage or abnormal fetal development, because the developing fetus depends on the mother's blood supply for oxygen. [4]
How Is Carbon Monoxide Poisoning Diagnosed?
Diagnosis is straightforward once exposure is suspected. The main test is a blood test that measures carbon monoxide levels after exposure. [4] Providers may also check heart rate and oxygen levels using a small device placed on the fingertip. [4]
A complete medical history is part of the diagnosis. Providers ask about the length of exposure and the location where it occurred to estimate how much CO the person inhaled. [4]
Test | What it checks |
Blood test | Carbon monoxide level in the blood |
Fingertip oximeter | Heart rate and blood oxygen level |
Medical history | Exposure duration and location to estimate CO amount |
Carbon Monoxide Poisoning Treatment
The primary treatment is breathing pure oxygen through an oxygen mask. [4] Oxygen therapy offsets the carbon monoxide buildup in the blood and helps restore normal oxygen delivery to the brain and heart. In severe cases, some centers also use hyperbaric oxygen (oxygen delivered at higher-than-normal pressure), although trial evidence on its benefit is mixed (Lin et al., 2018). [3]
What to do immediately if you suspect exposure
If you suspect carbon monoxide exposure or poisoning, do not wait for symptoms to worsen. Contact emergency services right away and get outdoors for fresh air as you call. [4] If going outside is unsafe, open a door or window, stay close to it, and breathe fresh air while you call. [4]
If you discover CO in your home, go outside immediately. Do not re-enter until emergency services confirm it is safe, because fumes may linger. A qualified professional should then check your appliances and ventilation, and a CO detector should be installed. [4]
Treatment and recovery
Carbon monoxide poisoning cannot be treated at home. [4] You need medical supervision so a provider can monitor your oxygen levels and watch for long-term complications.
For mild cases, symptoms usually fade shortly after breathing fresh air or receiving pure oxygen. It can take up to 24 hours for carbon monoxide to fully leave the body, and some lingering symptoms may persist for up to two weeks. [4]
What's the Outlook?
Carbon monoxide poisoning can be fatal — it can kill within minutes of inhalation. Someone who is sleeping can die before experiencing any symptoms, [4] which is why working CO detectors are critical in every home.
The overall outlook depends on how much CO was inhaled and how long the exposure lasted. [4] When treatment happens quickly, the effects of CO poisoning can often be reversed. However, there is a risk of permanent brain and heart damage, [4] because both organs need large amounts of oxygen to recover.
How Can I Prevent Carbon Monoxide Poisoning?
Prevention focuses on controlling fuel-burning appliances and detecting gas before it builds up.
Prevention step | Details |
Annual appliance inspection | At the start of each heating season, have a trained professional inspect furnaces, water heaters, ranges, dryers, space heaters, fireplaces, wood stoves, and flues/chimneys for blockages or cracking |
Use appliances wisely | Choose appliances that vent fumes outside; buy equipment carrying a national testing agency seal; follow instructions; use the right fuel for unvented heaters and open doors with a cracked window; never use a gas oven or dryer as a heater; never sleep in a room with an unvented gas or kerosene heater |
Install a CO detector | Battery-operated or battery-backed unit; test it twice a year when you change the clocks; replace the detector every five years; change batteries every six months; place it where you can hear the alarm |
Ventilate workspaces | Wear safety equipment such as a respirator where CO levels run high; keep vents clear of debris and blockages |
Avoid high-risk behaviors | Never idle a car in a garage, even with the door open; never use a charcoal or gas grill indoors; never run small gasoline engines (lawnmowers, generators, chainsaws, snow blowers) in enclosed spaces |
Living With Carbon Monoxide Poisoning History
Install a carbon monoxide detector in your home — it looks similar to a smoke detector or can be combined with one. When the alarm sounds, do not try to find the source of the gas. Instead, move outside immediately, call 911 or your local emergency number, account for everyone in the house, and do not re-enter until responders say it is safe. [4]
If you received treatment for CO poisoning, useful questions to ask your provider include:
Is the damage permanent, or will the effects go away over time?
Will I need follow-up care?
How can I prevent CO poisoning going forward?
Do I need any long-term treatment?
Are there any effects on my reproductive system or fertility?
Carbon Monoxide vs. Carbon Dioxide
People often confuse carbon monoxide with carbon dioxide, but the two are very different.
Property | Carbon monoxide (CO) | Carbon dioxide (CO2) |
Composition | One carbon atom + one oxygen atom | One carbon atom + two oxygen atoms |
How it forms | Byproduct of combustion with low oxygen | Forms naturally; released when you breathe out |
Toxicity | Toxic; can cause death at high concentrations | Not considered toxic, but hazardous at very large concentrations |
Conclusion
Carbon monoxide poisoning is a fast-acting, invisible threat that strikes without warning. The gas crowds out oxygen in your blood within minutes, and because you can't see, smell, or taste it, the only early alert is often a headache, nausea, or shortness of breath that mimics the flu. Prompt action — fresh air, emergency services, and pure oxygen therapy — can reverse its effects, while delay can lead to permanent heart and brain damage or death. The single most important step you can take today is installing a working CO detector and testing it twice a year.
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Frequently Asked Questions
What is carbon monoxide poisoning?
Carbon monoxide poisoning is a life-threatening illness caused by breathing in fumes containing carbon monoxide, an invisible gas produced when fuel burns. High levels can be deadly within minutes. [4]
What does carbon monoxide smell like?
Nothing at all. Carbon monoxide is invisible, colorless, and odorless. You cannot see, smell, or taste it, [4] which is why detectors are the only reliable warning.
What are the first signs of carbon monoxide poisoning?
The earliest warning signs are a mild headache, nausea, and shortness of breath. [4] These often resemble the flu, which is why CO poisoning is frequently mistaken for something less serious.
How quickly can carbon monoxide kill you?
In areas with high concentrations, poisoning can occur in fewer than five minutes. At lower concentrations, it may take one to two hours. A sleeping person can die before ever feeling symptoms. [4]
Can you sleep through carbon monoxide poisoning?
Yes. Because CO is undetectable by the senses, someone asleep may never wake up to the symptoms. This is why working CO detectors are essential in every home. [4]
What is the difference between carbon monoxide and carbon dioxide?
Carbon monoxide (CO) has one oxygen atom and is a toxic combustion byproduct. Carbon dioxide (CO2) has two oxygen atoms, occurs naturally, and is released when you exhale. CO is poisonous; CO2 is not considered toxic in normal concentrations. [4]
How does carbon monoxide hurt the body?
CO molecules enter red blood cells faster than oxygen molecules do, crowding out the oxygen in your bloodstream. The brain and heart are starved of oxygen, and CO also damages cells and organs by combining with proteins. [4]
What appliances produce carbon monoxide?
Car engines, small gasoline engines, fuel-burning space heaters, gas stoves and ranges, grills, lanterns, home heating furnaces, and the burning of charcoal, kerosene, propane, or wood all produce CO. [4]
Is it safe to idle a car in the garage with the door open?
No. Fumes build up quickly and can seep into an attached home even with the garage door open. Never idle a vehicle inside a garage. [4]
Can using a generator indoors cause carbon monoxide poisoning?
Yes. Small gasoline-powered engines, including generators, lawnmowers, chainsaws, and snow blowers, must never be used in enclosed spaces. [4]
Who is at highest risk for carbon monoxide poisoning?
Everyone is at risk, but infants, adults over 65, developing fetuses, people with heart disease, anemia, or respiratory conditions, people who smoke, and people at high altitudes are harmed by lower doses. Pets often show symptoms first. [4]
Are carbon monoxide symptoms worse in winter?
Risk rises in winter because heating systems run constantly, and poorly ventilated fuel-burning heaters are used more often. [4]
What do moderate to high CO exposure symptoms look like?
Severe headache, chest pain, dizziness and weakness, loss of muscle coordination, mental confusion, upset stomach with vomiting, and fainting or loss of consciousness. [4]
Can carbon monoxide poisoning look like the flu?
Yes. Headache, nausea, and fatigue closely resemble flu symptoms. If multiple people or pets in the same home feel ill at once, suspect CO and get outside immediately.
What should I do first if I suspect carbon monoxide poisoning?
Move to fresh air outside immediately and call 911 or your local emergency number. Do not stay inside to make the call. If you cannot get out, open a window and stay near it. [4]
How is carbon monoxide poisoning diagnosed?
A blood test measures CO levels after exposure. Providers also check heart rate and oxygen levels with a fingertip device and take a full history of the exposure's duration and location. [4]
How is carbon monoxide poisoning treated?
Breathing pure oxygen through an oxygen mask offsets the CO buildup in the blood. Treatment must be supervised by a medical provider — it cannot be done at home. [4]
How long does carbon monoxide stay in your body?
It can take up to 24 hours for CO to fully leave the body. Mild symptoms usually fade quickly with fresh air and oxygen, but some lingering symptoms can last up to two weeks. [4]
Can carbon monoxide poisoning cause long-term damage?
Yes. Even after successful treatment, lingering complications can include breathing difficulties, Parkinson's disease, memory loss, and heart abnormalities. [4]
Is carbon monoxide poisoning dangerous during pregnancy?
Yes. It can lead to miscarriage or abnormal fetal development because the fetus depends on the mother's oxygen supply. [4]
What should I do when my CO detector alarm goes off?
Don't search for the source. Move outside immediately, call 911, account for everyone in the house, and don't re-enter until emergency responders confirm it's safe. [4]
How often should I test my carbon monoxide detector?
Test it twice a year when you change the clocks, change the batteries every six months, and replace the entire detector every five years. [4]
Related Reading
Learn more in our guide to Blood Oxygen Level: Normal Range, How It's Tested and When to Worry.
References
Shin M, Bronstein AC, Glidden E, et al. Morbidity and mortality of unintentional carbon monoxide poisoning: United States 2005 to 2018. Ann Emerg Med. 2023;81(3):309-317. PMID: 36585319. https://doi.org/10.1016/j.annemergmed.2022.10.011
Xu XM, Luo H, Rong BB, et al. Management of delayed encephalopathy after CO poisoning: an evidence-based narrative review. Medicine (Baltimore). 2019;98(49):e18199. PMID: 31804341. https://doi.org/10.1097/MD.0000000000018199
Lin CH, Su WH, Chen YC, et al. Treatment with normobaric or hyperbaric oxygen and its effect on neuropsychometric dysfunction after carbon monoxide poisoning: a systematic review and meta-analysis of randomized controlled trials. Medicine (Baltimore). 2018;97(39):e12456. PMID: 30278526. https://doi.org/10.1097/MD.0000000000012456
Cleveland Clinic. Carbon Monoxide Poisoning. Accessed October 15, 2026. https://my.clevelandclinic.org/health/diseases/15663-carbon-monoxide-poisoning
Medical disclaimer
This article is for general informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment decisions. Call emergency services immediately if you suspect carbon monoxide poisoning.
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