Carbon monoxide testing

When you smoke, carbon monoxide - a poisonous gas - replaces some of the oxygen in your blood and affects your baby.

Carbon monoxide (CO) is a colourless, odourless and tasteless poisonous gas. You can't see it or smell it but it is in cigarette smoke. CO is also present in faulty gas boilers and car exhaust fumes.

How does carbon monoxide affect my baby?

When you smoke a cigarette, CO replaces some of the oxygen going into your lungs and so CO gets into your bloodstream.

This prevents the red blood cells from carrying oxygen around your body and to your baby.

It is dangerous because it deprives the baby of oxygen, slows its growth and development, and increases the risk of miscarriage, stillbirth and sudden infant death.

Carbon monoxide breath testing

If you're a smoker, your midwife or stop smoking advisor can give you a carbon monoxide (CO) breath test to see how clear your lungs are. The test is simple and easy and will show how much of this gas you and your baby have been exposed to.

How the CO test works

  • You blow into a hand-held machine, called a CO monitor, which measures the level of CO in your body.
  • Some machines can also work out how much CO is getting to your baby.
  • The more CO you have inhaled, the higher your CO reading will be.
  • In other words, the more smoke you have inhaled, the more CO you will have in your body.

This test can also show whether you’re inhaling harmful amounts of secondhand smoke.

If you see the results of the CO test, the reading is measured in COppm, which means the number of CO molecules in one million parts of air. Women with a CO test reading of 3ppm or higher may be referred to stop smoking services. 

When you stop smoking, your CO readings will fall showing that you and your baby are becoming free from this poisonous gas.

If your reading is high and you do not smoke, you might be at risk of CO poisoning from a faulty appliance and you should call the Health and Safety Executive gas safety line.

Is there carbon monoxide in electronic cigarettes?

No. Electronic cigarettes do not contain carbon monoxide or many of the other harmful gases found in cigarettes. If you are using electronic cigarettes only you will not have a higher reading in a CO test.


  1. NHS Choices [accessed 28/04/2015] Smoking and pregnancy
  2. . NICE (2010) Quitting smoking in pregnancy and following childbirth 
  3. Farsalinos KE, Polosa R (2014) Safety evaluation and risk assessment of electronic cigarettes as tobacco cigarette substitutes: a systematic review.
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    Last reviewed on April 1st, 2015. Next review date April 1st, 2018.

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    Please note that these comments are monitored but not answered by Tommy’s. Please call your GP or maternity unit if you have concerns about your health or your baby’s health.

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