Breathing and lung function tests

Exhaled carbon monoxide test

What is it?

This is a test to see how much carbon monoxide – a poisonous gas in cigarette smoke – is in your body. Carbon monoxide from smoking reduces the ability of red blood cells to carry oxygen.

It’s a simple and easy test to do.

What’s it used for?

It’s used to monitor smoking and help people to quit. Public health guidance recommends using this test for all 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.

What happens during the 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 and that you are a smoker
5–9 ppm recent exposure to a moderate level of carbon monoxide and you are possibly smoking
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


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Last medically reviewed: January 2017. Due for review: January 2020

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.