Carbon Monoxide and Smoking: The Invisible Harm in Every Cigarette

Carbon monoxide (CO) is a colourless, odourless gas produced when organic material burns incompletely. Burning tobacco produces it, so every cigarette puts a dose of CO into your bloodstream — and unlike the carcinogens, whose harm takes years, CO cuts your blood's oxygen-carrying capacity from the first cigarette.

The Chemistry

CO does its main harm through one mechanism: it binds to haemoglobin — the oxygen-carrying protein in red blood cells — and the UK Health Security Agency puts haemoglobin's affinity for CO at 245 times its affinity for oxygen.

Haemoglobin carrying CO is called carboxyhaemoglobin (COHb), and it cannot carry oxygen. The UKHSA calls "carboxyhaemoglobin induced hypoxia" — too little oxygen reaching tissues — the main way CO does harm.

What CO Levels Look Like in Smokers

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

In healthy people, the UKHSA notes that levels below 6% have no reliably shown harm beyond a reduction in maximum exercise capacity. Above that, and in people with heart disease, the effects matter more.

Effects of Chronic CO Exposure

Heart and Circulation

CO adds to nicotine's own effects on the heart:

Less oxygen for the same work: with some haemoglobin carrying CO, the heart and muscles get less oxygen, which shows first as reduced exercise capacity.
More strain on a diseased heart: in people with coronary heart disease, less oxygen in the blood means less margin when the heart is working hard.
Part of the wider picture: smoking damages blood vessels and raises clotting through several routes. The 2020 Surgeon General's report concludes that quitting reduces markers of inflammation and of the blood's tendency to clot.

Pregnancy

CO is particularly harmful in pregnancy. It crosses the placenta, and because the baby's haemoglobin binds CO more strongly, the UKHSA says CO's half-life in the unborn baby is up to 4 to 5 times longer than in the mother, with levels that can run higher than hers. The NHS calls CO "very harmful to a developing baby in the womb".

This is one of the strongest arguments for stopping during pregnancy. If you are pregnant, your midwife can refer you to free stop-smoking support; the NHS says licensed NRT is safe to use in pregnancy, after a word with a healthcare professional.

Brain and Tissues

The brain uses a large share of the body's oxygen, and CO poisoning at high levels causes headache, dizziness and confusion. Whether the lower, steady levels in smokers affect thinking or sleep day to day hasn't been well measured, so we won't claim it. Slower wound healing in smokers is well known, and WHO advises stopping at least four weeks before surgery; see also smoking and skin ageing.

How Fast Does CO Clear After Quitting?

Fast. The UKHSA gives CO's elimination half-time as 320 minutes in room air — just over five hours.

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

The NHS also says your energy will be increasing from about 72 hours. See what happens when you quit smoking.

CO Testing in Stop Smoking Services

Stop Smoking Services measure the CO in your breath at appointments — the NHS says you'll have your carbon monoxide level measured. The test:

Shows recent smoking, because smokers' breath carries much more CO than non-smokers'
Confirms you've stopped — in one study of quitters, exhaled CO was measurably lower from the seventh day
Motivates — watching your own reading fall is concrete feedback

Cut-off values vary between meters and services, so ask your adviser what your reading means.

FAQ

How much carbon monoxide is in cigarette smoke?

In the EU and UK, cigarettes may yield at most 10 mg of carbon monoxide each on the standard machine test. What a smoker actually inhales depends on how they smoke, which is why blood levels vary — the UKHSA puts smokers at around 5–10% COHb against 1–2% in non-smokers.

How long does carbon monoxide from smoking stay in your blood?

Its half-time is about 320 minutes in room air. The NHS says it has halved by 8 hours; it reaches a non-smoker's level within 12 hours to several days, depending on which health body you read.

Can you tell if someone smokes by testing their breath CO?

Yes — Stop Smoking Services use exhaled CO readings routinely. The cut-off depends on the meter and the service, and the reading falls within days of stopping.