PM2.5 — particulate matter under 2.5 microns — is the indoor pollutant with the most variable readings and the most consequential effects. Here is how to read what your monitor is telling you.
The WHO 2021 reference levels.
- Annual mean: below 5 µg/m³.
- 24-hour mean: below 15 µg/m³.
These are stricter than most national air quality regulations and reflect updated evidence on long-term health effects.
Indoor reference bands.
- Below 12 µg/m³. Clean indoor air, often cleaner than outdoor (particularly in busy urban areas).
- 12–35 µg/m³. Elevated. Common during low-intensity cooking, candles, or winter evenings with windows shut.
- 35–150 µg/m³. High. A clear cooking event, often peaking and falling over 30–60 minutes.
- 150–500 µg/m³. Very high. Aggressive frying or poorly-extracted gas cooking.
- Above 500 µg/m³. Acute. Open windows, switch off the hob, or leave the room.
Reading the curve, not just the value.
A single PM2.5 number is less useful than the shape of the curve over 24 hours. Watch for:
- The morning peak. Toaster, hob, candle?
- The evening surge. Dinner cooking and how long the kitchen takes to clear.
- The overnight floor. What baseline does the room return to?
- The seasonal pattern. Winter PM2.5 is consistently higher than summer in most UK homes.
The kitchen-extractor test.
Run the extractor hood from before you start cooking until 10 minutes after. Watch the PM2.5 peak. If the dashboard shows a 200+ µg/m³ peak that takes more than 30 minutes to clear with the extractor running, the extractor either has a clogged filter, recirculates rather than vents outside, or is underpowered for the kitchen.
When PM2.5 is not cooking.
Outdoor pollution events (Saharan dust, urban smog, bonfires) raise indoor PM2.5 over 4–8 hours depending on how leaky the building is. The signature is a slow rise everywhere in the home (not just the kitchen), correlated with the outdoor air quality forecast for your area.