Lab result explainedBlood count (CBC)Last updated November 11, 2026

Carboxyhemoglobin (COHb): normal range, levels in smokers and what a high result means

Carboxyhemoglobin (COHb) · CO-oximetry (arterial or venous blood gas with co-oximetry) · reported in percent of total hemoglobin (% COHb)

Carboxyhemoglobin (COHb) is hemoglobin that has carbon monoxide bound to it instead of oxygen, and the carboxyhemoglobin test reports what percentage of your hemoglobin is in that state. It is the blood test for carbon monoxide poisoning. Non-smokers normally run under 2 to 3 percent, smokers 3 to 10 percent, and a level above 3 to 4 percent in a non-smoker or above 10 percent in a smoker, in someone with symptoms, confirms carbon monoxide exposure. Levels above 20 to 25 percent are serious, and levels above 50 percent are often fatal.

Carbon monoxide binds hemoglobin about 240 times more tightly than oxygen, so even a small concentration in the air (a few hundred parts per million from a faulty furnace, a generator in a garage or a charcoal grill indoors) can load a large fraction of your hemoglobin within an hour or two. The bound hemoglobin cannot carry oxygen, and the rest of the hemoglobin holds on to its oxygen more tightly, so tissues are starved even though a standard pulse oximeter reads a reassuring 97 to 100 percent. The brain and heart suffer first, which is why headache, dizziness, nausea, confusion and chest pain are the typical complaints.

The number has to be read with the clock in mind. Carbon monoxide leaves the blood with a half-life of about 4 to 5 hours in room air, around 60 to 90 minutes on 100 percent oxygen by mask and 20 to 30 minutes in a hyperbaric chamber, so a sample drawn two hours after someone was pulled from a smoke-filled room, having been on oxygen in the ambulance, can be far lower than the peak. For that reason the level confirms the diagnosis but correlates poorly with how ill someone is or how they will do; the symptoms, any loss of consciousness, the pH, lactate and troponin matter more.

COHb at a glance

What it is: the percentage of hemoglobin bound to carbon monoxide, measured by co-oximetry on a blood sample; a standard pulse oximeter cannot detect it.
Normal: under 2–3% in non-smokers (usually 0.5–1.5%); 3–10% in smokers, occasionally up to 15% in heavy smokers; newborns and people with hemolysis run slightly higher.
Carbon monoxide poisoning: above 3–4% in a non-smoker or above 10% in a smoker with a compatible history; 10–20% headache, nausea, dizziness; 20–40% confusion, chest pain, fainting; above 40–50% seizures, coma, death.
The level falls fast once out of the exposure (half-life about 4–5 hours in air, 60–90 minutes on 100% oxygen), so a late or post-oxygen sample underestimates the peak.
Treatment is 100% oxygen immediately; hyperbaric oxygen is considered for loss of consciousness, heart involvement, pregnancy or levels above 25%.
Low COHb is normal; there is no low level of concern.

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What COHb measures

Carbon monoxide is a colorless, odorless gas produced whenever carbon-based fuel burns incompletely: gas furnaces and water heaters, car exhaust, portable generators, charcoal, wood fires, kerosene heaters, tobacco and hookah smoke. Inhaled CO crosses into the blood and binds the iron in hemoglobin to form carboxyhemoglobin, displacing oxygen. The body also makes a small amount of CO itself when heme is broken down, which is why nobody's COHb is zero: endogenous production gives about 0.4 to 1 percent, more in hemolytic anemia and in newborns.

The test is run on a co-oximeter, a spectrophotometer that measures light absorption at several wavelengths and separates oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin. It is usually part of an arterial or venous blood gas; venous blood is just as accurate for COHb and is what most emergency departments draw. Non-invasive pulse CO-oximeters (fingertip devices with extra wavelengths) exist and are used by fire services for screening, but they can be off by several percentage points in either direction and a blood level is needed to confirm. A conventional two-wavelength pulse oximeter reads COHb as oxyhemoglobin, so saturation looks normal in a poisoned patient.

COHb reflects the balance between recent exposure and elimination. After a brief, intense exposure the level peaks within an hour or so and then declines with a half-life of roughly 320 minutes breathing room air, 74 minutes on high-flow 100 percent oxygen, and about 20 to 30 minutes at 3 atmospheres in a hyperbaric chamber. In pregnancy, fetal hemoglobin binds CO more tightly and clears it more slowly, so the fetus's COHb runs 10 to 15 percent higher than the mother's and lags behind it, which is why pregnant women are treated more aggressively at lower levels. Fetal hemoglobin in newborns can also cause some co-oximeters to read COHb falsely high by a few percent.

Normal range for COHb

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (percent of total hemoglobin (% COHb))	Notes
Non-smoking adults	Below 2–3%	Typically 0.5–1.5%; urban dwellers and people exposed to traffic may reach 2–3%. Labs differ in the cut-off they print.
Smokers	3–10%	Roughly 4–6% for a pack a day; heavy smokers and hookah users can reach 10–15%. Falls to non-smoker levels within a day of stopping.
Newborns and people with hemolytic anemia	Up to about 3–5%	Endogenous CO from heme breakdown; fetal hemoglobin can also interfere with some analyzers.
Carbon monoxide exposure, confirmed	Above 3–4% (non-smoker) or above 10% (smoker)	With symptoms and a plausible source, this establishes the diagnosis. The level is a snapshot and underestimates the peak if time or oxygen has passed.
Mild poisoning	About 10–20%	Headache, nausea, dizziness, fatigue; often mistaken for flu or food poisoning, particularly when several household members are affected.
Moderate poisoning	About 20–40%	Confusion, severe headache, vomiting, chest pain, fast heart rate, fainting; troponin and ECG changes are common.
Severe poisoning	Above 40%	Seizures, coma, cardiac arrhythmia, metabolic acidosis, respiratory failure; death is common above 50–60%.
Common hyperbaric oxygen thresholds	Above 25% (above 15–20% in pregnancy), or any loss of consciousness, cardiac ischemia or severe acidosis regardless of level	Criteria vary between centers; the decision rests on symptoms more than the number.

High carboxyhemoglobin

A raised carboxyhemoglobin means you have been breathing carbon monoxide. In a non-smoker, anything above about 3 percent is abnormal; in a smoker the threshold is around 10 percent, because smoking itself keeps the level in the 3 to 10 percent range. With a matching story, a faulty heater, a running engine in an enclosed space, a house fire, a generator used indoors after a storm, or several people in the same building with the same headache, a high level confirms carbon monoxide poisoning and treatment with 100 percent oxygen starts at once. The first and most important step is removing everyone, including pets, from the source.

The level gives a rough idea of the exposure but a poor idea of the damage, for two reasons. First, it falls quickly once the person is in fresh air or on oxygen, so a measured 12 percent in the emergency department may have been 30 percent an hour earlier. Second, carbon monoxide does more than block hemoglobin: it binds to myoglobin in heart muscle, to cytochrome oxidase in mitochondria and triggers inflammation in the brain, so people with modest levels can have significant heart or brain injury. Clinicians therefore grade severity by symptoms (loss of consciousness, confusion, seizures), by the ECG and troponin, and by the lactate and blood pH, and reserve hyperbaric oxygen for those with the most worrying features rather than a specific number.

The feared long-term consequence is delayed neurological sequelae: memory problems, personality change, difficulty concentrating, movement disorders or depression that appear 2 to 40 days after an apparent recovery, in roughly 10 to 30 percent of significantly poisoned people and more often in those over about 35, those who lost consciousness and those with longer exposures. Hyperbaric oxygen within 6 to 24 hours appears to reduce this risk in the best randomized trial, which is the main reason it is offered. Anyone treated for carbon monoxide poisoning should expect a follow-up visit at 4 to 6 weeks to check memory and mood.

Common causes

Faulty furnaces, boilers, water heaters and gas appliances
The commonest cause of accidental poisoning in homes, peaking in winter when heating runs and windows are shut. Blocked flues and cracked heat exchangers leak CO into living spaces; a CO alarm on every level of the home is the only reliable warning.
Portable generators and engines in enclosed spaces
A generator in a garage, basement or near an open window can produce lethal concentrations within minutes; deaths cluster after hurricanes, ice storms and power outages. Cars idling in attached garages, boats and ice-resurfacing machines in rinks are related sources.
Charcoal grills, camp stoves and heaters used indoors
Burning charcoal, kerosene or propane in a tent, RV, car or unventilated room is a frequent cause of poisoning in campers and in households trying to heat or cook during outages.
House and vehicle fires
Smoke inhalation delivers CO together with cyanide and irritant gases; a COHb above 10 percent in a fire victim suggests cyanide exposure as well, and both are treated.
Cigarette smoking
Each cigarette delivers CO; typical smokers run 4 to 6 percent and heavy smokers up to 10 to 15 percent, enough to reduce exercise capacity and worsen angina. This is the reason smokers have a higher diagnostic threshold.
Hookah (waterpipe) smoking
A single 45 to 60 minute session with charcoal-heated tobacco raises COHb to 10 percent or more, far above cigarette levels, and hookah lounges have produced clusters of acute poisoning with fainting and seizures.
Methylene chloride (dichloromethane) paint strippers
Absorbed through the lungs and skin and metabolized by the liver into carbon monoxide, so COHb can keep rising for 8 to 12 hours after exposure stops and persists longer than with inhaled CO; several US deaths led to a consumer ban in 2019.
Occupational exposure
Firefighters, garage and warehouse workers around forklifts, miners, steel and foundry workers, and people using gas-powered tools in enclosed spaces are regularly exposed; the OSHA limit is 50 ppm averaged over 8 hours.
Hemolysis and other endogenous production
Breakdown of heme generates CO, so people with hemolytic anemia, large bruises or hematomas, or sickle cell crises can have COHb of 2 to 4 percent with no external exposure.
Deliberate exposure
Car exhaust and charcoal burning in enclosed spaces are used in suicide attempts; any unexplained high COHb in an adult should prompt a sensitive question about intent and appropriate support.

Symptoms that can go with it

Dull, persistent headache, often the first and sometimes the only symptom
Dizziness, weakness and unusual tiredness
Nausea and vomiting, which together with headache is easily mistaken for flu or food poisoning, especially when several people or pets are unwell at once and feel better when away from home
Confusion, poor concentration, drowsiness or blurred vision
Shortness of breath, fast heartbeat and chest pain, particularly in people with existing heart disease
Fainting, seizures or loss of consciousness with higher levels
The classic cherry-red skin color is rare and usually seen only after death; most poisoned people look pale or normal

Read it alongside

Arterial or venous blood gas with lactate — a lactate above 4 mmol/L or a metabolic acidosis marks a more severe poisoning; in fire victims a very high lactate suggests cyanide as well.
Troponin and ECG — the heart is injured in about a third of moderate to severe poisonings, and a raised troponin predicts long-term mortality.
Methemoglobin — measured on the same co-oximetry run; also raised in some smoke-inhalation and chemical exposures and also invisible to pulse oximetry.
Hemoglobin and hematocrit — anemia lowers the oxygen reserve, so the same COHb percentage produces worse symptoms; a high COHb with evidence of hemolysis points to an internal source.
Creatine kinase — rises with muscle breakdown after prolonged immobility or seizures during poisoning.
Glucose and electrolytes — hypoglycemia and electrolyte shifts can mimic or worsen the confusion.
Pregnancy test — a positive test lowers the threshold for hyperbaric treatment because the fetus accumulates more CO than the mother.
Cyanide level (if available) — in fire victims with a high COHb, low blood pressure and high lactate, empirical treatment for cyanide is usually given before the result returns.

What usually happens next

If carbon monoxide is suspected, leave the building, call emergency services and do not return until the fire department or gas company has measured the air; oxygen treatment starts before the blood result is back.
In the emergency department expect a venous or arterial blood gas with co-oximetry, lactate, ECG and troponin, a pregnancy test where relevant, and a neurological examination; the COHb is repeated after oxygen until it is under 3 to 5 percent and symptoms have resolved.
Hyperbaric oxygen is discussed for loss of consciousness, confusion or seizures, cardiac ischemia, pregnancy, severe acidosis or a COHb above 25 percent; the nearest chamber may be some distance away and transfer decisions are made quickly.
Before going home, make sure the source has been identified and fixed (furnace, water heater, generator, vehicle) and that working CO alarms are installed; a poisoning with no identified source is a reason to keep everyone out of the home.
Ask for a follow-up appointment 4 to 6 weeks later to screen for delayed neurological symptoms (memory, mood, concentration, movement), and report any new symptoms in that window promptly.
If the exposure was at work, it is reportable; if it was a smoking-related high level, this is a concrete reason and opportunity to discuss quitting.

Diet, lifestyle and natural levers with evidence

What can move COHb once the cause is known. Nothing here replaces finding the cause.

Stop smoking (including hookah)

Strong evidence

Smoking is the only common lifestyle source of a raised COHb. Pack-a-day smokers average 4 to 6 percent and a single hookah session raises COHb to around 10 percent, about three to four times the rise from one cigarette, because the charcoal heating the tobacco produces large amounts of CO. Because the half-life is 4 to 5 hours, COHb falls to non-smoker levels within 24 hours of the last cigarette, which immediately restores 5 to 10 percent of oxygen-carrying capacity and is one reason exercise tolerance and angina improve within days of quitting.

Carbon monoxide alarms and appliance servicing

Strong evidence

Nothing dietary or herbal changes COHb; prevention is the lever. Battery or plug-in CO alarms meeting UL 2034 on every level of the home and near sleeping areas, tested monthly and replaced every 5 to 10 years, detect the gas long before symptoms appear; US states that mandated alarms saw fewer CO deaths, and the CDC estimates most of the roughly 400 accidental CO deaths a year in the US are preventable. Annual servicing of furnaces, water heaters and chimneys removes the commonest household source.

Generator, vehicle and heater safety

Strong evidence

Portable generators account for a large share of US CO deaths after storms; the CDC and CPSC advise running them outdoors at least 20 feet (6 m) from any door, window or vent, never in a garage even with the door open, and never using charcoal, gas grills or camp stoves indoors or in a tent. Not idling a car in an attached garage and keeping boat exhaust clear of swim platforms address two other recurring causes.

Timing of the blood test

Moderate evidence

The result depends heavily on when it was drawn. COHb halves roughly every 4 to 5 hours in room air and every 60 to 90 minutes on high-flow oxygen, so a test drawn several hours after leaving the exposure, or after oxygen in the ambulance, can be normal despite a significant poisoning. Tell the clinician exactly when the exposure ended and when oxygen was started, so the level can be interpreted; a normal late result does not rule out poisoning in someone with typical symptoms.

When to see a clinician promptly

Headache, dizziness, nausea or confusion in anyone who has been near a running engine, generator, gas appliance, charcoal fire or in a house fire, especially if more than one person or a pet is affected: leave the building and call emergency services immediately.
A CO alarm sounding, even if nobody feels unwell: get everyone out, call the fire department and do not go back in until the source is found.
Chest pain, fainting, seizure, severe confusion or difficulty breathing after any possible exposure: these are signs of moderate to severe poisoning that may need hyperbaric oxygen.
Any carbon monoxide exposure during pregnancy, even with mild symptoms, because the fetus is affected more than the mother and treatment thresholds are lower.
New memory problems, personality change, difficulty concentrating, tremor or depression in the 6 weeks after a carbon monoxide poisoning, which may be delayed neurological injury.
A COHb above 10 percent in a smoker, or any elevation in a non-smoker, found incidentally on a blood gas, to look for an unrecognized source at home or work.

Frequently asked questions

What is a normal carboxyhemoglobin level?

In a non-smoker, under 2 to 3 percent, with most people between 0.5 and 1.5 percent; a little of this comes from the body's own breakdown of heme. Smokers run 3 to 10 percent depending on how much they smoke, and heavy smokers or hookah users can reach 10 to 15 percent. Newborns and people with hemolytic anemia may run slightly higher. Laboratories print slightly different cut-offs, so check the range on your report.

What carboxyhemoglobin level is dangerous?

Symptoms usually begin around 10 percent (headache, nausea, dizziness), levels of 20 to 40 percent cause confusion, chest pain and fainting, and levels above 40 to 50 percent cause seizures, coma and death. Many centers consider hyperbaric oxygen above 25 percent (15 to 20 percent in pregnancy). The level is an unreliable guide to severity, though, because it falls fast once you are out of the exposure; someone with a measured 15 percent who lost consciousness earlier is treated as a severe case.

What carboxyhemoglobin level is normal for smokers?

Most cigarette smokers run between 3 and 8 percent, roughly 4 to 6 percent for a pack a day, and heavy smokers up to 10 to 15 percent. This is why the threshold for diagnosing carbon monoxide poisoning in a smoker is a level above 10 percent rather than above 3. A single hookah session can push COHb above 10 percent on its own. The level returns to the non-smoker range within about 24 hours of the last cigarette.

How is carbon monoxide poisoning tested?

With a carboxyhemoglobin measurement on a blood sample, run through a co-oximeter as part of an arterial or venous blood gas. A standard fingertip pulse oximeter cannot detect it and will show a normal oxygen saturation. Fingertip pulse CO-oximeters with extra wavelengths are used by some emergency services for screening, but they can be off by several percentage points and a blood test is needed to confirm. Because the gas leaves the blood quickly, the test should be drawn as soon as possible after exposure.

Why is my pulse oximeter normal if I have carbon monoxide poisoning?

Ordinary pulse oximeters use two wavelengths of light and cannot tell carboxyhemoglobin from oxyhemoglobin, so they count the CO-bound hemoglobin as if it were carrying oxygen. A person with 30 percent COHb can read 98 percent on the finger probe while their true oxygen-carrying capacity is reduced by a third. The same problem affects methemoglobin. Only a co-oximeter on a blood sample, or a multi-wavelength pulse CO-oximeter, measures COHb.

How long does carbon monoxide stay in the blood?

The half-life of carboxyhemoglobin is about 4 to 5 hours breathing ordinary air, so a level of 20 percent falls to about 10 percent in 5 hours and to under 3 percent within a day. On 100 percent oxygen by a tight mask the half-life drops to around 60 to 90 minutes, and in a hyperbaric chamber at 3 atmospheres to about 20 to 30 minutes, which is why oxygen is started immediately. Methylene chloride exposure is the exception: it is converted to CO in the liver and levels can keep rising for hours.

Can carbon monoxide poisoning cause long-term damage?

Yes. Between about 10 and 30 percent of people with significant poisoning develop delayed neurological problems 2 to 40 days later: memory loss, difficulty concentrating, personality or mood change, movement disorders or incontinence. Risk is higher with loss of consciousness, longer exposure and age over about 35. Most improve over a year, but not all. The heart can also be injured; a raised troponin at the time of poisoning is linked to higher long-term mortality. Hyperbaric oxygen is offered partly to reduce the neurological risk.

Is a carboxyhemoglobin of 2, 5 or 8 percent high?

Two percent is normal for anyone. Five percent is normal for a smoker but abnormal for a non-smoker and, with symptoms, counts as carbon monoxide exposure; without symptoms it can also come from heavy traffic exposure or hemolysis. Eight percent is within the smoker range but high for a light smoker and clearly abnormal in a non-smoker; if you do not smoke and have headaches or nausea at home, a source such as a furnace, water heater or attached garage should be checked.

References

Carbon monoxide poisoning. Mayo Clinic (2024)View source
Carbon monoxide poisoning. Merck Manual Professional Version (2024)
About carbon monoxide poisoning. Centers for Disease Control and Prevention (2024)
Weaver LK. Carbon monoxide poisoning. New England Journal of Medicine (2009)
Hampson NB, Piantadosi CA, Thom SR, Weaver LK. Practice recommendations in the diagnosis, management, and prevention of carbon monoxide poisoning. American Journal of Respiratory and Critical Care Medicine (2012)
Rose JJ, Wang L, Xu Q, et al.. Carbon monoxide poisoning: pathogenesis, management, and future directions of therapy. American Journal of Respiratory and Critical Care Medicine (2017)
Weaver LK, Hopkins RO, Chan KJ, et al.. Hyperbaric oxygen for acute carbon monoxide poisoning. New England Journal of Medicine (2002)
Weaver LK, Howe S, Hopkins R, Chan KJ. Carboxyhemoglobin half-life in carbon monoxide-poisoned patients treated with 100% oxygen at atmospheric pressure. Chest (2000)
Hampson NB, Hauff NM. Carboxyhemoglobin levels in carbon monoxide poisoning: do they correlate with the clinical picture?. American Journal of Emergency Medicine (2008)
Touger M, Gallagher EJ, Tyrell J. Relationship between venous and arterial carboxyhemoglobin levels in patients with suspected carbon monoxide poisoning. Annals of Emergency Medicine (1995)
Eissenberg T, Shihadeh A. Waterpipe tobacco and cigarette smoking: direct comparison of toxicant exposure. American Journal of Preventive Medicine (2009)
Buckley NA, Juurlink DN, Isbister G, Bennett MH, Lavonas EJ. Hyperbaric oxygen for carbon monoxide poisoning. Cochrane Database of Systematic Reviews (2011)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.