Most public discussion of air pollution focuses on what happens outside: smog, traffic, industrial emissions. But WHO research shows indoor air can carry higher concentrations of some pollutants than outdoor air, and most people spend the majority of their time indoors. This post breaks down what each environment contributes, how they interact, and what that means for your health. For the wider editorial context behind this focus, see why indoorair.guide exists.
The basics: two environments, shared air
Outdoor and indoor air are not separate systems. Air moves between them continuously, through windows, doors, ventilation gaps, and HVAC systems. This means outdoor pollutants regularly enter homes, offices, and schools. At the same time, many significant pollutants are generated entirely indoors, from cooking, cleaning, furniture, and combustion appliances.
The result is that indoor air quality is shaped by two distinct sets of sources: what comes in from outside, and what is produced inside. Understanding both matters for decisions about ventilation, filtration, and daily habits.
WHO estimate
Indoor air pollution from solid fuel burning was estimated to be responsible for approximately 1.6 million excess deaths annually and around 3% of the global burden of disease. More recent estimates (WHO, 2019; GBD 2021) put household air pollution deaths at 3–4 million annually.
Where each type of pollution comes from
Outdoor Sources
- • Vehicle exhaust (PM2.5, NOₓ, CO)
- • Industrial emissions
- • Coal and diesel combustion
- • Crop and forest burning
- • Road and construction dust
- • Photochemical reactions (ozone)
- • Natural sources: pollen, sea spray, volcanic particles
Indoor Sources
- • Cooking (gas stoves, frying, high-heat burning)
- • Heating appliances (kerosene, wood, coal)
- • Tobacco and incense smoke
- • Paints, solvents, cleaning products (VOCs)
- • Furniture and building materials (formaldehyde, benzene)
- • Radon from soil and building foundations
- • Mould, dust mites, pet dander
Which pollutants appear indoors, outdoors, or both
Some pollutants are predominantly outdoor problems that infiltrate indoors. Others originate almost entirely inside buildings. Many appear in both environments but at different concentrations depending on sources and ventilation.
| Pollutant | Primary environment | Common indoor source |
|---|---|---|
| PM2.5 / PM10 | Both | Cooking, candles, incense, outdoor infiltration |
| Nitrogen dioxide (NO₂) | Both | Gas stoves, kerosene heaters |
| Carbon monoxide (CO) | Both | Incomplete combustion (faulty heaters, gas stoves); vehicle exhaust outdoors |
| Formaldehyde | Indoor | Pressed wood furniture, adhesives, paints |
| Benzene | Both | Tobacco smoke, paints, solvents, attached garages |
| Radon | Indoor | Soil and building materials beneath the structure |
| VOCs (general) | Indoor | Cleaning products, air fresheners, new materials |
| Ozone (O₃) | Outdoor | Photochemical reactions; some appliances also emit it |
| Sulfur dioxide (SO₂) | Outdoor | Primarily coal combustion and industry |
| PAHs | Both | High-heat cooking, incense, tobacco smoke, crop burning |
| Biological pollutants | Indoor | Damp surfaces, bedding, carpets (mould, dust mites) |
On particulate matter: WHO research notes that indoor levels of PM2.5 and PM10, in the presence of indoor combustion sources, are typically higher than outdoor PM levels. The same health guidelines that apply to outdoor PM are considered directly applicable to indoor spaces: there is no meaningful difference in the health impact of a particle based on whether it originated indoors or outdoors.
How outdoor pollution gets indoors
Buildings are not sealed enclosures. Outdoor pollutants enter continuously through gaps, windows, and ventilation systems. How much gets through depends on building construction and whether incoming air is filtered.
Particles (PM2.5, PM10): Infiltrate through all openings. Newer, airtight buildings limit entry but can trap indoor-generated particles if ventilation is poor. Older buildings with poor seals let more outdoor PM in, but indoor pollutants also escape more readily. Nearby renovation or roadwork is a common indoor-PM driver in Indian cities, covered in our construction-dust guide.
Gases (NO₂, CO): Pass through gaps and ventilation readily. Indoor concentrations tend to track outdoor levels unless strong indoor sources are also present. SO₂ is partially removed by reaction with indoor surfaces, so indoor levels are typically lower than outdoor readings even without filtration.
Ozone: Infiltrates but reacts with indoor surfaces and compounds as it moves through. Indoor ozone is typically lower than outdoor levels, but when it reacts with VOCs already present, it generates secondary pollutants including formaldehyde and ultrafine particles.
Seasonal smoke (crop burning, wildfires): During events like stubble burning in Punjab/Haryana, smoke plumes travel hundreds of kilometres. Even homes with closed windows see elevated indoor particle and PAH levels during peak burning weeks.
Why indoor pollution has received less attention
Research and regulation have focused heavily on outdoor air. WHO has explicitly acknowledged this gap, not because indoor risks are smaller, but for practical reasons:
- •Outdoor pollution is easier to monitor at scale with fixed stations
- •Concentration-based standards are straightforward to set and enforce outdoors; inspecting private homes is not
- •Most large epidemiological studies were built around outdoor monitoring data
- •The largest indoor pollution burden, solid fuel combustion in lower-income countries, was long outside the frame of wealthier nations' policy discussions
- •Questions like "how would you enforce indoor air quality standards in a private home?" have delayed guideline development for decades
Health risks: what each environment contributes
| Environment | Primary pollutants | Health outcomes |
|---|---|---|
Outdoor | PM2.5, ozone, NO₂, SO₂ | Cardiovascular disease, respiratory illness, lung cancer, premature mortality |
Indoor | Radon, formaldehyde, CO, VOCs, biological pollutants, PM2.5 from cooking | Lung cancer (radon); carcinogenic risk (formaldehyde); acute CO poisoning; respiratory disease and allergies |
Combined exposures: People are never exposed to just one pollutant at a time. Indoor and outdoor pollution interact, both in the environment and in the body. Some combinations produce synergistic effects where the joint risk exceeds the sum of individual risks. Radon combined with tobacco smoke is a well-documented example.
What good indoor air quality management looks like
WHO guidelines point to four levers, in order of effectiveness:
Source control
Use low-emission materials, cleaner fuels, and avoid unnecessary indoor combustion. A source that does not exist produces nothing to dilute or filter. This is always the most effective intervention.
Ventilation
Bringing in outdoor air dilutes indoor-generated pollutants. Trade-off in high-pollution areas: ventilation also imports outdoor pollutants. The right balance depends on both indoor source activity and current outdoor AQI. Use our Ventilation Guide to check when outdoor AQI is low enough to safely open windows.
Filtration
HEPA air purifiers or filtered HVAC reduce particle concentrations (PM2.5, PM10, smoke, biological particles). Does not remove gas-phase pollutants (CO, NO₂, formaldehyde, VOCs) without activated carbon or other adsorption media.
Monitoring
Consumer sensors for PM2.5, CO₂, VOCs, and CO reveal when and where levels spike. Useful for identifying the impact of specific activities like cooking, cleaning, or heavy outdoor pollution days.
On standards: Outdoor air quality is backed by legally enforceable concentration standards in most countries, with regular public monitoring. Indoor air quality is governed primarily by building codes, product standards, and non-binding guidelines. In practice, individuals must manage their own indoor air quality through the levers above.
The key takeaway
Indoor and outdoor air pollution are connected, not separate problems. Outdoor pollution infiltrates buildings. Indoor sources add to it. The combination, in a poorly ventilated space with active indoor sources, can result in exposures that exceed outdoor air quality guidelines even on days when the outdoor AQI is acceptable.
For most people in urban areas, this means outdoor AQI is an incomplete picture of their actual air pollution exposure. What happens inside the home, what fuels are burned, what products are used, how well the space is ventilated and filtered, matters as much or more than the number on an outdoor air quality monitor.
Related reading: India AQI vs US AQI — why the outdoor index number itself depends on which country's scale your app is using, before you even start adjusting for indoor sources. For classroom and school-environment context, the CSE India explainer we summarised in the air-pollution primer for Indian schools is a useful outdoor-side companion to this piece.
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Questions or feedback? Write to us at indoorairguide@outlook.com
Sources
- 1.WHO Guidelines for Indoor Air Quality: Selected Pollutants. Geneva: World Health Organization; 2010 Source ↗
- 2.Srivastava (2004), Indoor radon study in dwellings across 50 locations in India and Bangladesh Source ↗
- 3.Rao et al. (2008), Indoor radon levels in urban Hyderabad Source ↗
- 4.WHO Household Air Pollution fact sheet (2023) Source ↗
All references collated on the Reference Papers page.