Indoor PM2.5 & Particulates: Sources, Health & Filtration

PM2.5 and particulates indoors — sources, health and filtration.

Of all indoor pollutants, fine particulate matter (PM2.5) has the clearest link to serious health harm — and indoor levels are often driven by everyday activities like cooking and burning. This article explains what PM2.5 is, where it comes from indoors, and how ventilation, filtration and source control bring it under control.

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:

WHO 2021 air quality guidelines — PM2.5

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.

Common questions from clients.

Is cooking really a major source of indoor pollution?

Yes — high-heat cooking, especially frying and searing, can briefly raise kitchen PM2.5 to levels far above outdoor air. It's one of the biggest indoor particulate sources in most homes. An effective, externally-ducted cooker hood used while cooking, plus good ventilation, makes a substantial difference to the particulates you breathe.

Does making my house airtight trap pollutants?

Only if you make it tight without good ventilation. Airtightness combined with filtered mechanical ventilation (MVHR) actually lowers particulates, because incoming air is filtered (F7/ePM1) before delivery — cleaner than opening a window onto a busy street. The danger is airtight-without-ventilation, which traps cooking and combustion particles; 'build tight, ventilate right' avoids it.

Will MVHR filter out PM2.5?

A good MVHR system with a fine supply filter (F7 or ePM1 grade) removes a large fraction of fine particulates from the incoming air, which is particularly valuable near busy roads. It won't catch everything, and indoor sources like cooking still need extract and source control, but filtered supply air is one of the most effective ways to reduce the outdoor PM2.5 entering your home.