Myth vs fact · Air
The claim: “Indoor air is cleaner than outdoor air”
Is indoor air cleaner than outdoor air?
The ruling
It depends on the pollutant, and the split is instructive. For outdoor-origin particulate, indoor concentrations track outdoor ones closely unless you actively filter — homes are not sealed, and infiltration continues through gaps whether windows are open or not. For pollutants whose sources are indoors — nitrogen dioxide from gas cooking, formaldehyde from engineered wood, VOCs from cleaning products, cooking particulate — indoor concentrations are routinely well above outdoor ones.
How this was checked
Settled by the underlying physics
Indoor concentration is a balance between infiltration from outdoors, generation indoors, and removal by ventilation, deposition and filtration. Which term dominates depends entirely on the pollutant.
- 1Outdoor-origin particulate enters through gaps, doors and any ventilation. Without filtration, indoor concentrations settle at a fraction of outdoor — reduced by deposition on surfaces, not eliminated.
- 2Indoor-generated pollutants have no outdoor equivalent to dilute them. A gas hob produces NO₂ directly in the kitchen; the outdoor air is the low-concentration reservoir, not the source.
- 3WHO's indoor air quality guidelines exist as a separate body of work precisely because indoor concentrations of formaldehyde, benzene and several other pollutants are commonly higher than ambient.
- 4High-temperature cooking generates fine particulate directly, and in a kitchen without effective extraction concentrations rise far above whatever is outside.
- 5India's NAAQS are ambient standards and there is no statutory Indian indoor standard for homes, so the ambient values are the only reference point available for indoor comparison.
What is true in the claim
Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.
What it means for your purchase
- During an outdoor pollution episode, close windows and filter. That combination works and is the correct response.
- On a clean-air morning, ventilate. Fresh air dilutes indoor-generated pollutants that filtration does not touch.
- Fix indoor sources first: kitchen extraction that vents outdoors, low-emission board, ventilating after cleaning.
- Check outdoor AQI before deciding which way to lean on any given day. The right answer changes.
What this page does not establish
- The balance depends heavily on building construction, and Indian homes vary enormously in leakage. A sealed air-conditioned high-rise flat and an older naturally ventilated house behave very differently.
- This page reasons from mechanisms rather than from measurements in your home. A monitor placed indoors and outdoors simultaneously would answer it for your specific case.
Frequently asked questions
Does closing windows improve indoor air quality?
Is cooking really worse than outdoor pollution?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
WHO Global Air Quality Guidelines
World Health Organization · published 2021-09-22 · checked 2026-08-01
These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.