Intervention
Natural ventilation
Part of: Ventilation
Also: window ventilation, airing
Air exchange driven by wind and by temperature difference rather than by a fan — open windows, trickle vents and leakage through the building envelope. It is the cheapest way to remove what is already in the air, and the least controllable: the rate varies with weather, with the season and with whoever last closed the window.
Why it matters indoors
Ventilation is the only measure that works while a source is still emitting, and natural ventilation is the version available in almost every building without installing anything. Measurements in naturally ventilated rooms show what it does and what it does not: concentrations of what is generated indoors — carbon dioxide, cooking particles, formaldehyde — fall quickly when a window is opened, and concentrations of what comes from outdoors rise.
What it achieves, in numbers
Cross-ventilation with windows on two sides and the door open typically halves the excess carbon dioxide within 5–10 minutes; a single tilted window does far less in the same time. Work on cooking emissions shows the same pattern for particles: opening a window during and after cooking cuts the peak substantially rather than marginally. The quantity that describes all of this is the air exchange rate — a closed dwelling sits around 0.2–0.5 h⁻¹, a well-ventilated room reaches about 1 h⁻¹, an open window several times that.
What it does not do
It does not filter. When the outdoor air carries traffic particles, pollen or wildfire smoke, ventilation brings them in — the classroom studies that measure indoor and outdoor air together show indoor concentrations following the outdoor ones once the windows open. It also cannot be relied on in the season when it is least likely to happen: in winter the windows stay shut, which is exactly when relative humidity, carbon dioxide and emissions from heating accumulate. And it does not remove the source: ventilating against a product that is still emitting replaces the air but not the reason for replacing it, and it costs heat every time.
How to use it deliberately
Short and intense beats permanent and small: a few minutes of cross-ventilation exchanges more air, and loses less heat from the walls, than a tilted window left open all day. A carbon dioxide display makes the decision for the room rather than for the person — teachers and occupants ventilate when the number says so, not when the air feels stale, by which point it is already well past the threshold.
What the studies report
12 of 33 Findings — those with the strongest study design.
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Ventilation | mean 6.51 | — · room | field experiment in rooms with natural ventilation: ventilation rates measured by tracer gas decay under cross ventilation and single-sided ventilation at several window opening ratios, with and without the air conditioner running, and the infection risk assessed with the Wells-Riley equation | F-1439 Park Natural ventilation strategy and related issues to prevent c… (2021) | machine |
| Ventilation | mean 11.20 | — · room | field experiment in rooms with natural ventilation: ventilation rates measured by tracer gas decay under cross ventilation and single-sided ventilation at several window opening ratios, with and without the air conditioner running, and the infection risk assessed with the Wells-Riley equation | F-1440 Park Natural ventilation strategy and related issues to prevent c… (2021) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | difference 58.4 | — · dwelling | measurement study of natural ventilation scenarios during cooking in a dwelling: fine particulate matter and carbon dioxide logged in the living room with the kitchen and in the bedroom, and outdoors, with the doors and windows fully open, door open only and fully closed | F-1552 Natural Ventilation Reduces Cooking-Related PM2.5Peaks Indoo… (2026) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | difference 55.8 | — · dwelling | measurement study of natural ventilation scenarios during cooking in a dwelling: fine particulate matter and carbon dioxide logged in the living room with the kitchen and in the bedroom, and outdoors, with the doors and windows fully open, door open only and fully closed | F-1553 Natural Ventilation Reduces Cooking-Related PM2.5Peaks Indoo… (2026) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | other 39 | — · home | measurement study: indoor and outdoor PM2.5 and PM0.4 sampled at four fixed residential stations in Tehran over four seasons (August 2020 to July 2021), 6- to 12-hour samples, source shares estimated from the measurements | F-0091 Oroji Assessment of Indoor/outdoor Particulate Matter in Residenti… (2023) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | ratio 2.9 | — · home | measurement study: indoor UFP, PM10, PM2.5, black carbon and CO measured with handheld monitors in the living areas of five non-smoking households in Guildford using four stove types and four solid fuels during winter | F-0107 Particulate matter exposure from different heating stoves an… (2025) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | ratio 1.8 | — · home | measurement study: indoor UFP, PM10, PM2.5, black carbon and CO measured with handheld monitors in the living areas of five non-smoking households in Guildford using four stove types and four solid fuels during winter | F-0108 Particulate matter exposure from different heating stoves an… (2025) | machine |
| Formaldehyde (CH₂O) | difference 0.012 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0341 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | difference 3.17 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0342 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| Relative humidity | difference 3.61 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0343 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| Formaldehyde (CH₂O) | difference 0.008 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0344 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | difference 2.19 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0345 Window typology as a determinant of indoor air quality in na… (2026) | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
In the literature
- Fortenberry et al. Analysis of indoor particles and gases and their evolution with natural ventilation… (2019). Indoor air. doi:10.1111/ina.12584 CKB 0028
- Indoor air quality in naturally ventilated Italian classrooms (2015). Atmosphere. doi:10.3390/atmos6111652 CKB 0015
- Natural Ventilation Reduces Cooking-Related PM2.5Peaks Indoors (2026). ACS EST Air. doi:10.1021/acsestair.5c00427 CKB 1198
Studies in the Coridum Knowledge Base in which this term is used. The definition follows standard usage in WHO, ISO and textbook sources; it is not quoted from these papers.
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Natural ventilation. Coridum Knowledge G-0099, 16 September 2026. https://coridum.com/k/G-0099/