Air pollution is the highest-burden entry on the register, and it is also the one where the standard advice points at the wrong source. Almost everything written for the public is about traffic. In the United Kingdom, the largest single source is now something people install deliberately, in their own living rooms, and light for pleasure.
1. Why PM2.5 specifically
Particulate matter under 2.5 micrometres is small enough to reach the alveoli and cross into the bloodstream. From there the mechanism is not primarily respiratory:
- Systemic inflammation and endothelial dysfunction, then accelerated atherosclerosis — the same pathway as everything in the heart topic. Cardiovascular death is the largest share of the burden, not lung disease.
- Outdoor particulate pollution is classified by IARC as a Group 1 carcinogen — the same category as tobacco smoke and asbestos, which describes the strength of the evidence rather than the size of the risk.
- No safe threshold has been identified. The exposure–response curve is steepest at low concentrations, which is unusual and important: reductions at the clean end still buy something.
- Also implicated in dementia, diabetes, adverse birth outcomes and childhood asthma, with evidence strength varying by outcome.
2. Where the UK actually is
| Figure | |
|---|---|
| WHO guideline, annual mean PM2.5 | 5 µg/m³ |
| England's statutory target | 10 µg/m³ by 2040, with an interim target of 12 by 2028, plus a 35% cut in population exposure against 2018. Double the WHO figure, twenty years out |
| Progress | Average exposure in England fell about 54% between 2003 and 2023, and almost everywhere is now below the 2040 target — while still above the WHO guideline. Two-fifths of that fall happened in 2020 |
| The gap that has not closed | The most deprived fifth of areas had about 8% higher PM2.5 than the least deprived in 2023, with no clear trend over two decades. This is an exposure that tracks income |
Two things follow from that table. The first is that the UK has genuinely improved and the improvement is under-reported. The second is that a legally compliant level is not a safe level — the statutory target is twice what the WHO recommends and does not arrive until 2040, so “within limits” and “fine” are different claims.
3. The wood burner problem
The most effective thing most people can do about air pollution is in their own house
- A modern stove is not a clean stove. Ecodesign and Defra-exempt labels mean a stove emits less than an older one or an open fire. They do not mean it emits little. The comparison is against a very dirty baseline.
- Burning wood indoors raises indoor particulate as well as outdoor. Every door opening to refuel releases particles into the room, and measured indoor spikes during lighting and refuelling are substantial.
- The trend is the part that matters, and it needs stating accurately. Domestic combustion emissions rose steeply through the 2010s — on the order of a doubling between 2011 and 2021 — and have fallen back somewhat since 2020. The honest version is that it is the only major source substantially higher than it was a decade ago, not that it is currently rising every year.
- It is almost never the household's main heat source. Survey work suggests only a small minority burn solid fuel for all their heating — for most it is supplementary and aesthetic, which is what makes this an unusually available choice compared with, say, moving house or changing job.
- It falls on neighbours as well. Unlike most items on this register, this exposure is substantially imposed on other people — the street, the terrace, the flat above.
- If you are going to burn anyway: dry seasoned wood or approved fuel only, never wet wood, never treated or painted timber, never household waste or coal in a smoke control area. A properly hot fire emits far less than a smouldering one, and a swept chimney matters. But the honest ranking is: not burning beats burning well.
This page is blunter about this than about anything else on the register, and the reason is the arithmetic rather than any view about how people should live. It is one of the two largest sources, it is the only one substantially higher than it was a decade ago, it is discretionary for most owners, and the exposure lands on neighbours who did not choose it.
4. Indoors, where you spend 90% of your time
| Source | What helps |
|---|---|
| Cooking — the largest routine indoor source in most homes | Frying, grilling and anything that browns generates fine particles. Use the extractor every time and check it vents outside rather than recirculating through a filter, which many do not. Back burners with the hood over them. Open a window |
| Gas hobs | Produce nitrogen dioxide as well as particles, and have been associated with childhood asthma. Ventilation is the mitigation; induction removes the source |
| Candles, incense, open fires | Combustion in an occupied room. See the fragrance chapter |
| Outdoor air coming in | Keep traffic-side windows shut at rush hour and ventilate from the quieter side. Ventilation is still net positive nearly all the time — sealing a home is the wrong response |
| A HEPA air purifier | Genuinely effective at reducing indoor PM2.5, and one of the few consumer devices on this site that does what it claims. Size it to the room, run it continuously in a bedroom, and change filters. Ignore ionisers and ozone generators — ozone is itself a pollutant and reacts with fragrance to make more, as the cleaning chapter sets out |
5. Ventilation, and the problem modern homes created
| What accumulates | Why it matters |
|---|---|
| Moisture | The most consequential of the four. Breathing, cooking, showering and drying washing all add water. Where it condenses, mould grows — and damp and mould have better respiratory evidence behind them than most of the exposures on this register, particularly in children and in people with asthma |
| Carbon dioxide | Not toxic at indoor levels, but the best available proxy for whether a room is ventilated at all. Outdoor air is around 420 ppm; a well-ventilated room sits under about 800; above roughly 1,500 in an occupied bedroom or office means the air is barely being changed. Studies of cognitive performance at raised CO2 exist and are contested — treat the number as a ventilation gauge rather than a toxin |
| Everything else on this page | Cooking particles, NO2 from gas, VOCs from cleaning and fragrance, formaldehyde from furnishings, and radon in affected areas — all concentrate in proportion to how little the air moves |
What the options actually are
| Approach | Where it fits |
|---|---|
| Opening windows, and working extract fans | Free, and still the answer for most households. Trickle vents open, extractor used every time you cook or shower and left running afterwards, and a short sharp cross-ventilation once or twice a day rather than one window ajar all winter. Check the extractor actually vents outside — many recirculate |
| Positive input ventilation (PIV) | A filtered fan, usually in the loft, gently pressurising the house with fresh air and pushing stale moist air out through gaps. The common UK retrofit for persistent condensation and mould, cheap to run, and effective at the moisture problem it is sold for. It is ventilation, not filtration of what is already there — and in a home whose outdoor air is poor it brings that air in, filtered to whatever grade the unit carries |
| Mechanical ventilation with heat recovery (MVHR) | Balanced supply and extract through a heat exchanger, so air changes without losing the heat. The right answer for a genuinely airtight new build or deep retrofit, and largely wasted on a leaky house. Needs commissioning properly and filters changed — a neglected MVHR system performs worse than an open window |
| A HEPA purifier | Filtration, not ventilation — they solve different problems. A purifier removes particles from air already in the room; it does nothing for CO2, humidity or most gases. You may well want both, and neither substitutes for the other |
| Dehumidifier | Deals with moisture specifically where ventilation is impractical — a north-facing bedroom, a flat with one aspect, drying washing indoors in winter |
The tension worth naming. Advice to seal and insulate, and advice to ventilate, come from different directions and are usually given separately — so households follow the first and not the second, and end up warm, cheap to heat and mouldy. They are one instruction, not two: if you improve a home's airtightness, you have taken on the job of ventilating it deliberately. And if you rent, damp and mould carry legal duties on the landlord that were tightened after a child died from mould exposure — report it in writing rather than accepting it as a lifestyle problem.
6. Outdoors, and keeping it proportionate
- Do not stop exercising outdoors. The cardiovascular benefit of activity exceeds the harm of the pollution you breathe doing it, across almost every realistic UK exposure level. This is the most common way people get this subject wrong, and the error costs more than the pollution does.
- Choose the back street. Concentrations fall steeply within a few metres of a busy road, so a parallel quieter route cuts exposure substantially for no extra time.
- Inside a car is not cleaner — in-cabin concentrations in traffic are often higher than on the pavement beside it, because you sit in the exhaust plume of the vehicle ahead. Recirculate in traffic.
- Check the forecast on bad days if you have asthma, COPD or heart disease, and shift timing rather than cancelling.
- Masks are for specific circumstances — a genuine pollution episode, a vulnerable person — and a surgical mask does nothing for PM2.5. It needs to be a fitted filtering respirator to do anything at all.
And the honest limit of an individual page on a population problem. Air pollution is overwhelmingly determined by policy, geography and income, not by personal choice, and the people most exposed have the fewest options. Nothing above changes that. What is genuinely in an individual's hands is small but not nothing: the stove, the extractor fan, the purifier in the bedroom, and the route to work — and the first of those is larger than the rest combined for anyone who owns one.