Particulate matter (PM) is the microscopic solid and liquid particles suspended in air. It's classified by size: PM10 (up to 10 micrometres), PM2.5 (up to 2.5 µm) and ultrafine particles. The smaller the particle, the deeper it penetrates the lungs and the more harmful it is — PM2.5 reaches deep into the lungs and enters the bloodstream, which is why it's the focus of health guidance.
Where indoor particulates come from
Indoor PM2.5 has both indoor and outdoor origins, and indoor activities can spike it far above outdoor levels:
- Cooking — especially frying, searing and high-heat cooking, which can briefly raise kitchen PM2.5 to very high levels; the single biggest indoor source in many homes.
- Wood-burning stoves and open fires — a major particulate source, both directly indoors and from neighbourhood smoke drawn back in.
- Candles and incense — significant, often-overlooked sources.
- Outdoor air — traffic and urban pollution infiltrating, particularly near busy roads; a key reason filtered supply air matters in cities like London.
- Smoking, dust and resuspension from activity.
The health case and guideline levels
PM2.5 is linked to respiratory and cardiovascular disease and premature death; there is no threshold below which it's considered entirely safe, so the aim is to keep exposure as low as reasonably possible. The World Health Organization tightened its guidelines in 2021:
| Averaging period | WHO guideline | Note |
|---|---|---|
| Annual mean | 5 µg/m³ | Long-term exposure target |
| 24-hour mean | 15 µg/m³ | Short-term exposure target |
Controlling particulates — source, ventilation, filtration
- Source control first — use an effective, externally-ducted cooker hood when cooking; reconsider wood-burning; limit candles and incense. Removing the source is always the most effective step.
- Ventilation — extract cooking particulates at source and ventilate to dilute what remains; a CO₂-led ventilation strategy generally improves particulate dilution too.
- Filtration — MVHR with a fine supply filter (F7 / ePM1 grade) removes a large fraction of incoming fine particles, which is especially valuable in polluted urban areas; portable HEPA purifiers can supplement in key rooms.
Filter grades explained — ePM1, F7, MERV and HEPA
Filtration performance is graded, and knowing the labels helps you specify and maintain a system properly. The current European standard, ISO 16890, rates filters by the fraction of particles they capture in defined size bands — ePM1, ePM2.5 and ePM10 — so an 'ePM1 60%' filter captures 60% of the finest, most harmful particles. This replaced the older EN 779 'F7/F9' grades, though F7 is still widely quoted and corresponds roughly to ePM1 50–65%, which is the practical minimum worth fitting on an MVHR supply in an urban area. The American MERV scale is an equivalent system (MERV 13 being broadly comparable to F7). HEPA is a separate, much higher class, capturing 99.95%+ of fine particles, used in portable room purifiers and specialist applications rather than as a standard whole-house supply filter, because its high resistance demands more fan power. For most London homes, an F7/ePM1 supply filter on the MVHR, kept clean, is the sensible standard, with a HEPA room purifier as a supplement where someone is particularly vulnerable.
| Grade | System | Typical use |
|---|---|---|
| ePM10 / coarse (G4) | ISO 16890 | Pre-filter; protects the unit, little health benefit |
| F7 ≈ ePM1 50–65% (MERV 13) | ISO 16890 / ASHRAE | Standard MVHR supply filter — urban minimum |
| F9 ≈ ePM1 80%+ | ISO 16890 | Enhanced filtration, higher resistance |
| HEPA (H13/H14) | EN 1822 | Room purifiers; 99.95%+ of fines |
Wood burning — the avoidable urban source
Domestic wood and solid-fuel burning deserves particular attention because it has become a leading source of urban fine-particle pollution, both inside the burning home and across whole neighbourhoods. A stove leaks particulates indoors every time it is opened to refuel, and the smoke from chimneys raises outdoor PM2.5 across a street, which then infiltrates every nearby home — so even non-burning neighbours breathe it. Open fires are worse than closed stoves, and burning damp or unseasoned wood worse still. In Smoke Control Areas, only exempt appliances and authorised fuels are permitted, and the public-health direction of travel is firmly against routine domestic burning in cities. For a household serious about indoor air quality, reconsidering an open fire or a frequently-used stove is one of the highest-impact source-control decisions available — far more effective than any filter at reducing the particulates generated in the first place.
