Indoor Air Quality at Home: Sources, Ventilation and What Actually Helps

Healthy home · Published 2026-10-24

An open wooden casement window looking out over green fields — An open window airing a room.

We spend most of our time indoors, and much of it at home. The US Environmental Protection Agency notes that indoor levels of some pollutants can be two to five times higher than typical outdoor levels. The good news: the most effective fixes are simple and cheap.

The main sources of indoor pollution

Cooking

Frying, grilling and roasting produce fine particles (PM2.5) and other pollutants, whatever the fuel. Gas hobs and ovens also produce nitrogen dioxide (NO2), which can irritate the airways and is associated with asthma symptoms, particularly in children. Using an extractor hood that vents outdoors, on a higher setting and using the back burners where possible, reduces exposure.

Burning: wood stoves, open fires, candles and incense

Solid fuel burning releases particles and gases into the home and the neighbourhood. The UK government's emission statistics identify domestic wood burning as one of the largest sources of PM2.5 emissions in the country. Globally, the WHO estimates that household air pollution from cooking with polluting fuels such as wood, coal and kerosene causes around 3.2 million deaths a year (2020), mostly in low- and middle-income countries. Candles and incense also add particles indoors.

Volatile organic compounds (VOCs)

VOCs are released by paints, varnishes, glues, new furniture and flooring, cleaning and personal care products, air fresheners and some hobbies. Levels are highest after renovation or when new furnishings are installed. Some VOCs, such as formaldehyde and benzene, are harmful at sufficient exposure. Ventilating well during and after decorating, choosing low-VOC products and using fewer scented products reduce exposure.

Radon

Radon is a natural radioactive gas that seeps from the ground into buildings, mostly in certain geological areas. You cannot see or smell it. The WHO identifies radon as a leading cause of lung cancer after smoking, and the risk is much higher in smokers. The only way to know a home's level is to test it, usually with a passive detector left in place for three months. Reference and action levels differ: the WHO proposes a reference level of 100 Bq/m³ (and no higher than 300 Bq/m³ where that is not achievable); the UK action level is 200 Bq/m³; the US EPA recommends fixing homes at 4 pCi/L (about 148 Bq/m³) or more. Public health agencies publish maps of affected areas.

Carbon monoxide

Carbon monoxide (CO) from faulty boilers, gas appliances, stoves and blocked chimneys can kill. Symptoms include headache, dizziness, nausea, breathlessness and confusion, often affecting several people in a home at once and improving when away from it. Every home with a fuel-burning appliance should have a working CO alarm, and appliances should be serviced by qualified engineers.

Moisture and mould

Damp air and mould affect respiratory health; see healthy humidity and mould.

What guidelines say

The WHO's 2021 global air quality guidelines set guideline levels for PM2.5 of 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours, and for NO2 of 10 µg/m³ annual and 25 µg/m³ over 24 hours. They apply to both outdoor and indoor air. In England, NICE guideline NG149 (2020) on indoor air quality at home advises using extractor fans when cooking and bathing, opening windows where possible, ensuring trickle vents are open, and avoiding blocking ventilation, along with advice for builders, landlords and health professionals.

Pollutant	Annual average	24-hour average
Fine particles (PM2.5)	5 µg/m³	15 µg/m³
Nitrogen dioxide (NO2)	10 µg/m³	25 µg/m³

Source: WHO global air quality guidelines, 2021 (apply to outdoor and indoor air).

Ventilation: the main fix

Ventilation dilutes pollutants generated indoors and removes moisture. Practical steps:

Use the cooker hood whenever you cook, and keep it on for a while afterwards. Hoods that recirculate air through a filter remove less than those vented outdoors.
Run the bathroom extractor during and after showers, or open a window.
Keep trickle vents in windows open, and do not block air bricks or vents.
Open windows wide for a few minutes several times a day, especially after cooking, cleaning or when many people are present. Short, wide openings exchange air quickly with less heat loss than leaving a window slightly ajar for hours.
When outdoor air is heavily polluted (near heavy traffic at rush hour, during wildfire smoke episodes), ventilate at quieter times or rely more on filtration.

CO2 monitors as a ventilation indicator

Carbon dioxide from breathing builds up in poorly ventilated rooms. CO2 at the levels found in homes is mainly a marker of how stale the air is, rather than a hazard in itself. Outdoor air is around 420 ppm. Guidance used in UK workplaces and schools treats consistently low readings (often cited as below about 800 ppm in occupied rooms) as a sign of good ventilation and sustained readings above around 1,500 ppm as a sign that ventilation should be improved. A home CO2 monitor (choose one with an NDIR sensor) can show when a bedroom or home office needs more fresh air.

CO2 reading	What it suggests
About 420 ppm	Outdoor air
Consistently below about 800 ppm (occupied room)	Good ventilation
Sustained above about 1,500 ppm	Ventilation should be improved

Thresholds as commonly cited in UK workplace and school ventilation guidance; CO2 here is a marker of stale air, not a hazard in itself.

Air purifiers

Portable air cleaners with HEPA filters, sized for the room, can reduce airborne particles such as smoke, dust and pollen; the US EPA describes them as a supplement to ventilation and source control, not a replacement. They do not remove gases such as CO or radon. Avoid devices that deliberately produce ozone, which is a lung irritant; the EPA and California regulators warn against ozone generators sold as air cleaners.

The houseplant myth

A widely quoted NASA study from 1989 tested plants in small sealed chambers. A 2020 review that converted results from many such chamber studies to real buildings concluded that achieving meaningful VOC removal would require roughly 10 to 1,000 plants per square metre of floor space, and that normal ventilation removes VOCs far faster (Cummings and Waring, Journal of Exposure Science and Environmental Epidemiology). Plants are pleasant to live with, but they are not an air purification strategy.

In short

Cooking, combustion, VOCs, radon, carbon monoxide and damp are the main indoor air issues at home.
Source control and ventilation (cooker hood, bathroom fan, open vents and windows) are the most effective, cheapest measures.
Test for radon in affected areas and fit a CO alarm if you have any fuel-burning appliance.
HEPA purifiers help with particles; ozone generators and houseplants are not solutions.

This page is general information. If you suspect carbon monoxide, leave the home, get fresh air and call your gas emergency service. If you have breathing symptoms, see a doctor.

Sources

US Environmental Protection Agency, Indoor air quality.
World Health Organization, WHO global air quality guidelines (2021); Household air pollution fact sheet; Radon and health.
NICE, Indoor air quality at home (NG149), 2020.
US EPA, Guide to air cleaners in the home.
Cummings BE, Waring MS. Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies. J Expo Sci Environ Epidemiol 2020;30:253–261.