Health outcome · marker · measured in ppb
Fractional exhaled nitric oxide (FeNO)
Also: FeNO, exhaled nitric oxide, eNO, exhaled NO
The concentration of nitric oxide in a standardised exhaled breath, measured in parts per billion. It rises with eosinophilic inflammation of the airway, which makes it a non-invasive marker of the response an exposure has provoked.
What it is
The airway lining produces nitric oxide, and produces more of it when a particular kind of inflammation — the eosinophilic kind that characterises allergic asthma — is active. Measuring it takes a single slow breath into a handheld analyser, which is why it can be repeated on schoolchildren week after week where spirometry and blood sampling cannot.
That repeatability is what makes it valuable in indoor research. Where lung function changes slowly and asthma diagnoses accrue over years, this marker moves within days of an exposure changing, so a filtration trial or a school with a new source can be studied within a term.
Two limits belong with any value. It is raised by allergic inflammation specifically, so it misses the neutrophilic response that irritants and combustion products can produce; and it is affected by atopy, smoking, diet and inhaled steroid treatment, all of which a study must account for.
Indoor factors with evidence
| Factor | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Particulate matter (PM₂.₅ and PM₁₀) | SMD -0.16 (-0.28–-0.04) | — | systematic review and meta-analysis of 27 randomised controlled trials of personal-level protective measures against air pollution (air purifiers, respirators and similar), GRADE-assessed | F-0238 Effectiveness of Personal‑Level Protective Measures Against … (2026) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | OR 1.30 (1.10–1.91) | — · school | cross-sectional study of school children: indoor pollutants measured in suburban and urban schools and related to fractional exhaled nitric oxide in two-level hierarchical logistic regression models | F-1611 Isa et al. Evaluation of the Relationship between Fractional Exhaled Ni… (2022) | machine |
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Cite this page
Coridum Research. Fractional exhaled nitric oxide (FeNO). Coridum Knowledge G-0177, 16 September 2026. https://coridum.com/k/G-0177/