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Exhaled CO: what it measures and where it is used
- by Mohamed Al-Sabri & Hithesh K. Gatty
- 4 Feb 2026
What is exhaled CO and why does it matter?
Exhaled carbon monoxide, usually written as eCO, is the small amount of CO that leaves the body with every breath. Part of this comes from external sources such as tobacco smoke, and part is produced endogenously when heme is broken down by heme oxygenase1 during oxidative stress and inflammation. When the airways are irritated by smoke or other noxious exposures, expression of heme oxygenase1 increases, and this can lead to higher eCO levels. eCO is therefore a signal that integrates smoke exposure and inflammatory activity in the airways.
What is Fractional exhaled CO (FeCO) measurement?
Sampling the final part of a steady exhalation reduces the influence of dead-space air. In smokers, exhaled CO mainly reflects recent smoke exposure.
A signal with significant clinical value
One of the most robust clinical applications of eCO is as an objective marker of recent smoking. Healthy nonsmokers typically have very low eCO, whereas daily smokers show clearly higher values reflecting their recent smoke intake. This difference is strong enough that breath CO measurement has become a standard component of many smoking cessation services to verify abstinence and detect ongoing smoking.
In COPD and other smoking-related airway diseases, eCO can add further insight. People with COPD have higher mean eCO than healthy nonsmokers, and in several studies, smokers with COPD have shown higher eCO than smokers without COPD, consistent with greater airway inflammation and oxidative stress. Elevated eCO has been associated with worse airflow limitation and may track exacerbation-prone phenotypes in some cohorts, although findings are not fully uniform across all studies.
When eCO is followed over time, it can help to:
- Distinguish smokers from nonsmokers and validate self-reported abstinence in cessation programmes.
- Highlight individuals whose combination of smoking exposure and airway inflammation may put them at higher risk of developing COPD or of faster disease progression.
- Monitor how the lungs respond when someone stops smoking or initiates new therapies, as falling eCO generally reflects reduced smoke exposure and, in some settings, reduced airway inflammation.
Why eCO is becoming state of the art in COPD monitoring
As respiratory care shifts from hospital-centred models toward community, primary care and remote monitoring approaches, there is a growing demand for tools that are noninvasive, low-burden, and repeatable in everyday settings. Ideal tests for early COPD and risk stratification should be comfortable, sensitive to early changes, and easy to integrate into digital care pathways.
eCO matches this profile well. It is measured with a controlled exhalation into a handheld sensor, provides a result within seconds, and can be repeated frequently without discomfort. Because it reflects both recent smoke exposure and aspects of airway oxidative stress, eCO is increasingly used as:
- A point-of-care biomarker in smoking cessation services (with a 4week quit commonly validated at CO < 10 ppm in UK programmes).
- A support tool in COPD risk assessment, especially in current or recent smokers whose spirometry is borderline but whose inflammatory burden may be high.
Crucially, eCO complements rather than competes with spirometry. Spirometry captures the mechanical side of lung health, such as forced expiratory volume and vital capacity, while eCO reflects biochemical processes related to oxidative stress and inflammation that may precede irreversible structural damage. Used together, they provide a more forward-looking picture of lung health and risk.
Disclaimer: This article discusses scientific research and does not describe the intended purpose of any GattyInstruments product. Spiroluft is under development, is not CE-marked, and is intended only for use by healthcare professionals in smoking-cessation care and pulmonary function assessment where clinically indicated.
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