What is carbon monoxide?

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

Carbon monoxide is found in:

  • cigarette smoke
  • exhaust fumes
  • broken gas appliances, like boilers.

Carbon monoxide is harmful because it stops oxygen from reaching your organs.

What is an exhaled carbon monoxide test?

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

It’s simple and easy to do. You won’t need to prepare for the test. 

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 how much carbon dioxide is in your body. If you smoke, it can help you 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.

This test is also offered to pregnant people.  Carbon monoxide exposure is very risky if you are pregnant because it affects a growing baby’s access to oxygen. Your baby needs oxygen to grow and develop. Read more about how passive smoking affects children’s lungs.

What happens during an exhaled carbon monoxide test?

You breathe into a cardboard tube attached to a hand-held monitor. The monitor then shows you the reading.

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, trying to empty your lungs completely.

What do my results mean?

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

Reading on the screen This means that:
10ppm and over you have recently been exposed to a high level of carbon monoxide. This is common in smokers. 
5-9ppm you have recently been exposed to a mild level of carbon monoxide. This may mean that you’re a smoker, or you’ve been exposed to second-hand smoke. 
1-4ppm you have recently been exposed to a low level of carbon monoxide. It’s normal to have a small amount of CO in your breath, even if you’re not a smoker. 

Help to stop smoking

Stopping smoking isn’t always easy, but there’s lots of help and support available. Check out our information and resources on how to stop smoking.

Contact the National Gas Emergency service on 0800 111 999 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. 

Get support

Call our Helpline for support with your condition. Get advice on your medicines, symptoms or travelling with a lung condition, or just call us to say hello.

Did you find this information useful?

We use your comments to improve our information. We cannot reply to comments left on this form. If you have health concerns or need clinical advice, call our helpline on 0300 222 5800 between 9am and 5pm on a weekday or email them.

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