Carbon Monoxide and Smoking: What It Does to Your Blood

Quick answer: Cigarette smoke contains carbon monoxide (CO), which binds haemoglobin about 245 times more strongly than oxygen, forming carboxyhaemoglobin (COHb). The UK Health Security Agency puts COHb at around 1–2% in non-smokers and 5–10% in smokers — oxygen-carrying capacity the blood loses. After you quit, CO leaves with a half-time of about 320 minutes: the NHS says it has halved by 8 hours, and it reaches a non-smoker's level within 12 hours to several days.

Most people know smoking is harmful, but the carbon monoxide effect is one of the most concrete — and one of the fastest to recover.

What Carbon Monoxide Is

Carbon monoxide is a colourless, odourless gas produced when things burn incompletely. It's the same gas that carbon monoxide alarms warn about and that makes faulty boilers and running engines in closed garages so dangerous.

In the EU and UK, the law caps cigarettes at 10 mg of carbon monoxide per cigarette as measured by a smoking machine — and, as the National Cancer Institute explains for tar, a machine cannot predict how much a smoker actually takes in.

The smoker doesn't get acute CO poisoning because the smoke is diluted and inhaled intermittently. But the effect on blood chemistry is constant.

How CO Blocks Oxygen

Red blood cells carry oxygen on haemoglobin, which picks oxygen up in the lungs and releases it to muscles, organs and the brain.

Carbon monoxide gets in the way:

CO binds to haemoglobin at the same site as oxygen
The UK Health Security Agency puts haemoglobin's affinity for CO at 245 times its affinity for oxygen
The resulting molecule — carboxyhaemoglobin (COHb) — cannot carry oxygen
CO also makes the remaining haemoglobin hold on to its oxygen more tightly, so less is released where tissues need it

The UKHSA names this — "carboxyhaemoglobin induced hypoxia" — as the main way CO does harm.

What This Means for Smokers' Blood

The UKHSA puts baseline COHb at around 1–2% in non-smokers and around 5–10% in smokers. How high it runs for you depends on how much and how deeply you smoke; we could not find a reliable table by cigarettes per day, so we don't print one.

The same page notes that in healthy people, COHb below 6% has no reliably demonstrated harm beyond a reduction in maximum exercise capacity — so the effect you are most likely to notice is in exertion. For people with heart disease the picture is different: the heart has to work harder with less oxygen.

In pregnancy: CO crosses the placenta, and the UKHSA says its half-life in the unborn baby is up to 4 to 5 times longer than in the mother. The NHS says CO is "very harmful to a developing baby in the womb". If you are pregnant, stopping smoking is one of the most important things you can do, and your midwife can refer you for support.

The Immediate Good News: How Fast CO Clears

The UKHSA gives the elimination half-time of CO as 320 minutes when breathing air — just over five hours; a pharmacology review gives 4–6 hours for COHb. So:

8 hours: the NHS says your oxygen levels are recovering and the carbon monoxide in your blood has halved
12 hours to several days: CO reaches a non-smoker's level — WHO says 12 hours, the NHS 48 hours, the CDC "several days"

You may have read that "within 24 hours your heart-attack risk decreases" because of this. The health bodies disagree about when heart-attack risk falls, and none measured it at a day — the CDC puts the sharp drop at one to two years — so we don't repeat it.

For comparison, the UKHSA gives the half-time as 80 minutes on 100% oxygen and 23 minutes with hyperbaric oxygen, which is why hospitals give oxygen for CO poisoning.

CO and Cardiovascular Risk

With less oxygen in the blood, the heart has to work harder for the same effort. The pharmacology review suggests CO may also dilate blood vessels, partly offsetting nicotine's narrowing. The larger picture comes from the 2020 Surgeon General's report: quitting reduces markers of inflammation and of the blood's tendency to clot, and excess coronary heart disease risk halves somewhere between one year (NHS, WHO) and three to six years (CDC).

Measuring Your Own CO Levels

Stop Smoking Services measure the carbon monoxide in your breath at appointments — the NHS says you'll have your carbon monoxide level measured as part of the support. Handheld breath monitors are also sold to consumers.

In one study of people quitting, exhaled CO was measurably lower from the seventh day. Many people find seeing the reading fall motivating.

Burnout tracks your smoke-free time from your first day — CO clearance is one of the first changes that happens invisibly in your body.

Sources

UK Health Security Agency. "Carbon monoxide: toxicological overview", 2022. gov.uk
Ernst A, Zibrak JD. "Carbon monoxide poisoning." New England Journal of Medicine, 1998. PubMed
Benowitz NL, Hukkanen J, Jacob P. "Nicotine chemistry, metabolism, kinetics and biomarkers." Handbook of Experimental Pharmacology, 2009. PMC
Ambrose JA, Barua RS. "The pathophysiology of cigarette smoking and cardiovascular disease: an update." Journal of the American College of Cardiology, 2004. PubMed

Frequently Asked Questions

How much carbon monoxide is in cigarette smoke?

In the EU and UK, cigarettes may yield no more than 10 mg of carbon monoxide each on the standard machine test. What you actually inhale depends on how you smoke — deeper, longer and more frequent puffs raise it — which is why blood levels vary so much between smokers (the UKHSA puts smokers' COHb at around 5–10%).

Does carbon monoxide cause lung cancer?

The National Cancer Institute lists carbon monoxide among the smoke's toxic chemicals, not among the cancer-causing ones. CO's harm is mainly to oxygen delivery and the cardiovascular system.

How long until breathing improves after quitting?

CO leaves your blood within hours to days. The NHS says breathing may feel easier from 72 hours as your bronchial tubes relax, and that lung function increases by up to 10% between 3 and 9 months. See Breathing Improves After Quitting.

Do low-tar cigarettes have less carbon monoxide?

Not reliably in practice. The National Cancer Institute explains that people smoking "light" cigarettes take deeper, longer or more frequent puffs, or block the filter's ventilation holes, so machine-measured yields understate real exposure.