Key learning points

  • Carbon monoxide (CO) is a toxic gas inhaled when smoking tobacco
  • Those who smoke will have measurable CO in their exhaled breath 
  • CO monitors offer easy-to-use, portable, handheld tools to measure CO levels
  • Monitoring CO provides a visual motivational tool for patients trying to quit smoking


Carbon monoxide (CO) is a poisonous gas produced as a result of incomplete combustion of carbon-based materials. CO is emitted by faulty gas appliances and is found in car exhaust fumes. However, it is also one toxin found in tobacco smoke.1

CO binds with haemoglobin in red blood cells over 200 times more readily than oxygen.2 When inhaled CO sticks to red blood cells, it reduces their ability to carry oxygen. This leads to impaired breathing and causes blood to become thicker and stickier. In acute CO poisoning, this can rapidly lead to tissues being starved of oxygen and eventual death.2

For those who smoke tobacco throughout a typical day, a constant supply of carbon monoxide enters the body. The resulting oxygen deprivation can eventually lead to an increased risk of coronary heart disease (CHD) and other circulation problems.3 This is alongside the additional risks associated with smoking, including increased risk of respiratory disease such as COPD and various cancers.4 

CO monitors can provide smokers with visual proof of the harm of smoking, provide a way to monitor progress and act as a motivator during a quit attempt.

Why measure CO levels? 

Measuring CO levels with a breath monitor is an easy, non-invasive and inexpensive approach that provides more immediate results of CO poisoning from a single breath than waiting for blood test results.5 Within smoking cessation settings, measuring CO is a standard way to biochemically establish a person’s smoking status, measure their progress, and provide a visual aid to encourage a smoker to persevere with their quit attempt.6,7 Indeed, the addition of a CO breath monitor to a structured plan has been shown to increase the likelihood that a patient will quit.7

Seeing a reduction in carbon monoxide levels is often a rewarding experience for a smoker who has been resisting cravings to smoke, as they are able to see the progress they have made. Patients liken it to trying to lose weight and getting on the scales after a week of healthy eating.

Measuring CO levels

CO levels can be measured with a CO monitor, which are handheld, portable devices that are easy to use. The patient takes a deep breath in and holds for 15 seconds while the monitor counts down. They subsequently exhale and try to empty their lungs completely. They should not talk during the test or take additional breaths. The monitor will instantly display two values: a CO reading (in parts per million), and the percentage of carboxyhaemoglobin (COhb) in the expired air.

How to use a CO monitor

  • Ask the patient to empty their lungs of air
  • Tell the patient to take a deep breath in and hold
  • Apply a fresh mouthpiece to the monitor, and then hand the monitor to the patient while they continue to hold their breath
  • Keep talking to the patient during the 15 second countdown while gas exchange takes place in the lungs
  • Once the countdown has completed, instruct the patient to exhale completely into the monitor

Interpreting readings

CO breath monitors measure levels of CO as a percentage of the blood’s oxygen-carrying capacity. CO has a short half-life of around 4 hours.1 Therefore, CO readings in smoking cessation settings indicate the number of cigarettes smoked in the previous 24 hours. Even if the patient has not smoked for several days, a relapse in the last 24 hours would be indicated by the CO reading. Likewise, someone who usually smokes heavily but has abstained for 24 hours could present with a reading similar to a non-smoker.

CO readings are given in parts per million (ppm), with 9 ppm considered the maximum acceptable level in someone who is not smoking.8 Normally, the CO concentration in exhaled air of a non-smoker does not exceed 4 ppm. The recommended cut-off value to separate smoker and non-smoker is 9 ppm so a reading of 10 ppm or more signifies smoking.7

Professional judgement is needed to assess the likely cause of unexpected readings. In smokers, morning readings are typically lower as CO accumulates in the body throughout the day.9 Other factors can sometimes affect CO readings, including ambient CO conditions (for example due to weather or air pollution) and more rarely lactose intolerance.10

Those who claim not to have smoked or to have recently been exposed to smoke or fumes should be investigated further and advised to have any gas or solid fuel appliances checked.11


The widespread availability and ease of use of CO monitors offers a key tool to support and monitor smokers. CO monitoring provides instant feedback to patients and may act as a motivational factor during a quit attempt.

Lynne Hughes, smoking cessation nurse, Liverpool, UK