Breathing and lung function tests

Exhaled carbon monoxide test

What is carbon monoxide?

Carbon monoxide (CO) is a poisonous gas that you can’t see, smell or taste.

Carbon monoxide is present in:

  • cigarette smoke
  • exhaust fumes
  • faulty gas boilers

Carbon monoxide is harmful because it stops oxygen from reaching your vital organs. If you’re pregnant, it can stop your baby from getting enough oxygen to grow.

What is an exhaled carbon monoxide test?

The exhaled carbon monoxide test measures how much carbon monoxide is in your body.

It’s a simple and easy test to do.

What’s it used for?

Most people with high levels of carbon monoxide are smokers. The exhaled carbon monoxide test is a useful tool to monitor smoking and help people to quit. The test can also show if you’re being exposed to dangerous levels of carbon monoxide in second-hand smoke, even if you don’t smoke yourself.

Because carbon monoxide is dangerous for unborn babies, this test is offered to pregnant women as a routine part of their antenatal care.

Carbon monoxide exposure is especially risky if you are pregnant because it affects a growing baby’s access to oxygen. Your baby needs oxygen to grow and develop.

How should I prepare for the test?

No special preparation is needed.

What happens during an exhaled carbon monoxide test?

You breathe into a cardboard tube attached to a handheld monitor. The monitor then shows the reading on its screen.

You will be asked to hold your breath for as long as possible, ideally 15 seconds. Then you will breathe out slowly into the mouthpiece, aiming to empty your lungs completely.

What will the results look like?

The results will be in parts per million (ppm) of carbon monoxide in your breath.

Reading Suggests
10 ppm and over recent exposure to a high level of carbon monoxide. This is a typical reading for smokers
5–9 ppm

recent exposure to a moderate level of carbon monoxide. This may mean that you’re a smoker, or that you’ve been exposed to second-hand smoke

1–4 ppm recent exposure to a low level of carbon monoxide. It’s normal to have a small amount of carbon monoxide in your breath even if you’re not a smoker

Help to stop smoking

Stopping smoking is hard, but there’s lots of help and support available.

Check out our information and resources on how to stop smoking.

If your reading shows a high level of carbon monoxide but you’re not a smoker, it’s possible that you have a faulty appliance in your home. Contact the National Gas Emergency Service on 0800 111 999.

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Last medically reviewed: January 2020. Due for review: January 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.