The Air You Breathe

TL;DR. You inhale roughly 11,000 litres of air a day, which is far more than you eat or drink by volume, and the lungs have no acid, no liver first-pass, and a surface area the size of a tennis court. Air pollution is the largest environmental cause of death worldwide, contributing to millions of deaths a year. Most people worry about the outdoor air and spend 90 percent of their time indoors, where concentrations of several pollutants are higher. The three biggest indoor sources are cooking, combustion (candles, wood burners, gas hobs), and, in some regions, radon.

1. What you are inhaling

Why particle size decides everything: PM10 deposits in the upper airway. PM2.5 reaches the alveoli, where gas exchange happens and where there is no mucociliary clearance. Ultrafine particles cross into the bloodstream and have been found in the brain, placenta, and other organs. Smaller means deeper and more systemic.

2. What it does

The scale. WHO estimates that outdoor and household air pollution together contribute to roughly 6.7 to 7 million deaths a year, which makes it the largest environmental health risk in the world and comparable to tobacco. Around 99 percent of the world's population breathes air exceeding WHO guideline levels.

Mechanisms: particles cause airway and systemic inflammation, oxidative stress, endothelial dysfunction, and increased blood coagulability. This is why the effects extend far beyond the lungs.

Established associations:

There appears to be no safe threshold. WHO revised its guidelines downward in 2021, to an annual mean of 5 µg/m³ for PM2.5 and 10 µg/m³ for NO₂, on the basis that harm is measurable well below previous limits.

A landmark legal moment: in 2020, a UK coroner ruled that air pollution contributed to the death of nine-year-old Ella Adoo-Kissi-Debrah, the first time air pollution was recorded as a cause of death on a death certificate anywhere.

3. Indoor air: where you actually are

People in industrialised countries spend around 90 percent of their time indoors, and indoor concentrations of several pollutants routinely exceed outdoor ones. This is the part of the problem you can actually change.

Cooking

The largest single indoor source in most homes, and consistently underestimated.

• Frying and grilling generate PM2.5 at levels that can exceed heavily polluted outdoor air by a wide margin, sometimes reaching several hundred µg/m³ during a single meal.
• Gas hobs emit NO₂ directly. Studies have found gas cooking without adequate extraction can push indoor NO₂ above outdoor guideline levels, and a widely discussed 2022 analysis attributed a meaningful share of childhood asthma in the US to gas stove use. The methodology has been criticised and the direction of the association is supported by multiple studies.
• Gas hobs also emit benzene, a Group 1 carcinogen, and leak small amounts of methane and other compounds even when off.
• Burnt fat produces acrolein, an acute respiratory irritant (Chapter 91).

What to do: use an extractor hood that vents outside (recirculating hoods with carbon filters remove odours and not NO₂ or fine particles), use the back burners where the hood is more effective, open a window, run the extractor for 10 to 15 minutes after cooking, and prefer induction or electric where possible.

Combustion indoors

Wood-burning stoves and open fires are among the largest PM2.5 sources in many countries. In the UK, domestic wood and coal burning is now the single largest source of PM2.5 emissions nationally, exceeding road transport. Even well-sealed modern stoves leak particles indoors during refuelling, and the outdoor emissions affect neighbours.

Candles, especially scented and paraffin ones, produce measurable PM2.5, VOCs, and soot. Enjoyable, and not a "clean air" product.

Incense produces very high particulate levels and has been associated with respiratory disease in regular users.

Carbon monoxide deserves separate emphasis because it is the acute killer.

Carbon monoxide is colourless, odourless, and binds haemoglobin with about 200 times the affinity of oxygen. Symptoms (headache, nausea, dizziness, confusion) mimic flu and are frequently missed, and people fall unconscious before recognising it. Sources: faulty boilers, gas fires, blocked flues, generators, barbecues brought indoors or into tents, and running engines in garages.

A CO alarm costs very little and saves lives. Every home with any fuel-burning appliance should have one, tested regularly. A clue worth knowing: symptoms that improve when you leave the house and return when you come back.

VOCs

Formaldehyde from particleboard, MDF, insulation, and some fabrics; benzene from fuel, smoking, and gas appliances; toluene and xylene from paints and adhesives; plus limonene and pinene from cleaning products and air fresheners, which react with ozone indoors to produce formaldehyde and ultrafine particles.

"Air fresheners" do not clean air. They mask odour with fragrance and add VOCs and, in plug-ins and sprays, phthalates (Chapter 93).

New furniture, flooring, and paint off-gas most in the first weeks. Ventilate aggressively after DIY or new furnishings.

Radon

A radioactive gas from uranium decay in rock and soil, which accumulates in buildings, particularly in basements and ground floors and in geologies rich in granite.

It is the second leading cause of lung cancer after smoking, and the leading cause in non-smokers, causing an estimated 3 to 14 percent of lung cancers depending on the country. The risk multiplies with smoking, far more than additively.

It is invisible, odourless, and easily measured. Testing kits are cheap and available from national radon services. If you live in a known radon area and have never tested, this is one of the highest-value things in this chapter: mitigation, usually a sub-floor sump and fan, is effective and relatively inexpensive.

Damp, mould, and dust mites

Damp housing is a health issue, not a cosmetic one. Mould exposure is associated with asthma, respiratory infection, and allergic disease. The 2020 death of two-year-old Awaab Ishak in Rochdale from prolonged mould exposure led to legal changes in England requiring landlords to fix damp and mould within set timescales.

Dust mites thrive in warm humid bedding and are a major allergen. Keeping relative humidity between 40 and 60 percent controls both mites and mould, and is the single most useful intervention.

Secondhand and thirdhand smoke

Secondhand smoke causes lung cancer, heart disease, stroke, and, in children, asthma, respiratory infection, ear infection, and sudden infant death syndrome. There is no safe level, and ventilation and separate rooms do not eliminate it. Smoking bans in public places have been followed by measurable falls in hospital admissions for heart attack and childhood asthma.

Thirdhand smoke is residue deposited on surfaces, fabrics, and dust, which persists for months and can react to form nitrosamines. It is the reason a car or room that has been smoked in remains a genuine exposure for children.

4. Outdoor air, and what you can control

Traffic is the dominant urban source. Diesel produces disproportionate NO₂ and particles, and non-exhaust emissions from brakes and tyres are now a comparable or larger particle source than exhaust in many places, which means electric vehicles help less than expected for particles while helping substantially for NO₂ and CO₂.

What individuals can do:

• Check local air quality forecasts, and reduce strenuous outdoor exercise on high-pollution days, particularly with asthma or heart disease.
• Choose back streets. Pollutant concentrations fall steeply within tens of metres of a main road, and walking or cycling one street over can substantially cut exposure.
• Inside a car is often worse than outside it, because the cabin sits in the exhaust plume of the vehicle ahead. Use recirculation in traffic.
• Exercise is still worth it. Modelling consistently finds the benefits of walking and cycling outweigh the harms of inhaled pollution in all but the most extreme urban air quality.
• N95/FFP2 masks work for particulates when fitted properly. Cloth and surgical masks do not filter PM2.5 meaningfully.

5. Improving your indoor air

In order of effect:

1. Ventilate. Opening windows for 10 to 15 minutes a few times a day, or using mechanical ventilation with heat recovery, is the foundation. Modern airtight homes trap pollutants.
2. Extract at source when cooking, venting outside, and prefer induction to gas.
3. Stop indoor combustion: no smoking indoors, minimise candles, incense, and wood burning.
4. Fit a carbon monoxide alarm.
5. Test for radon if you are in a known area.
6. Control humidity at 40 to 60 percent to limit mould and dust mites, and fix damp at source rather than painting over it.
7. Use a HEPA air purifier in the room you spend most time in, correctly sized. HEPA genuinely removes PM2.5, allergens, and smoke; avoid ozone-generating and "ionising" purifiers, which produce a lung irritant.
8. Reduce VOC sources: fragrance-free cleaning products, no air fresheners or plug-ins, low-VOC paint, and ventilate after DIY.
9. Vacuum with a HEPA filter and damp-dust to avoid resuspending particles.
10. Take shoes off at the door, which measurably reduces tracked-in lead, pesticides, and dust.

Houseplants do not clean indoor air. The NASA study endlessly cited was conducted in sealed chambers, and the extrapolation to real rooms suggests you would need something like 10 to 1,000 plants per square metre to match ordinary ventilation. Have plants for the pleasure of them.

6. The bottom line

• You inhale roughly 11,000 litres of air a day, with no acid barrier and no liver first-pass. Air pollution is the largest environmental cause of death worldwide, contributing to several million deaths a year, and there appears to be no safe threshold.
• PM2.5 is the pollutant that matters most, because it reaches the alveoli and drives systemic inflammation. Cardiovascular disease, not lung disease, accounts for the largest share of deaths.
• You spend 90 percent of your time indoors, where you have the most control. Cooking, combustion, and damp are the main sources.
• Vent your extractor outside, use the back burners, and prefer induction to gas. Recirculating hoods do nothing for NO₂ or particles.
• Fit a carbon monoxide alarm, and test for radon if you live in an affected area. Radon is the leading cause of lung cancer in non-smokers and is cheaply measured and fixable.
• Keep indoor humidity at 40 to 60 percent, ventilate daily, use HEPA rather than ionising purifiers, and take your shoes off.
• Houseplants do not clean the air, and exercising outdoors is still worth it almost everywhere.

Sources and notes

Pollutant definitions, health effects, and the 2021 revised guideline values follow the WHO Global Air Quality Guidelines. Global mortality attribution follows WHO and the Global Burden of Disease air pollution analyses. IARC's 2013 classification of outdoor air pollution and particulate matter as Group 1 carcinogens is Monograph 109. Cardiovascular mechanisms follow Brook et al.'s American Heart Association scientific statement. Dementia associations follow Livingston et al.'s Lancet Commission on dementia prevention, which added air pollution as a modifiable risk factor. The Ella Adoo-Kissi-Debrah inquest verdict of 2020 is a matter of public record. Indoor concentrations and the 90 percent indoor time figure follow EPA and UK indoor air quality reviews. Gas stove NO2 and childhood asthma attribution follows Gruenwald et al., International Journal of Environmental Research and Public Health, 2023, with its methodological critiques; benzene emissions follow Lebel et al., Environmental Science and Technology, 2022. UK domestic wood burning as the largest PM2.5 source follows Defra's National Atmospheric Emissions Inventory. Carbon monoxide physiology and alarm recommendations follow UKHSA and CDC guidance. Radon as the second leading cause of lung cancer follows WHO's radon handbook and Darby et al., BMJ, 2005. Damp and mould health effects follow WHO guidelines; the Awaab Ishak inquest and the resulting English legislation are public record. Secondhand smoke effects and post-ban admission falls follow the Surgeon General's reports and Meyers et al.'s meta-analysis. HEPA versus ozone-generating purifiers follows EPA guidance. The NASA houseplant study's inapplicability to real rooms follows Cummings and Waring, Journal of Exposure Science and Environmental Epidemiology, 2020.

Open questions. How much of the gas stove asthma association is causal is disputed, and the attributable-fraction methodology has been criticised. Indoor air is far less regulated and far less measured than outdoor air, so exposure estimates are weaker.

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