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If your monitor is showing PM2.5 of 12 — that's good. Above 25? Worth investigating. Above 55? Open a window or run your purifier on max. But context matters, and the right answer depends on whether you're cooking, sleeping or just sitting in the lounge.
Here's a plain-English UK guide to what a 'good' indoor PM2.5 reading actually looks like, what's normal during cooking spikes, and exactly what to do at each level.
Where these numbers come from
The WHO 2021 global air quality guidelines set a 24-hour PM2.5 guideline of 15 µg/m³ and an annual guideline of 5 µg/m³ — the reference most indoor targets are built on. See the WHO global air quality guidelines.
For UK context, Defra's UK-AIR network publishes live and historical PM2.5 by monitoring site, so you can compare your indoor reading against the outdoor air on your street before deciding whether to ventilate or filter — see Defra UK-AIR.
Not sure your placement is helping? Our air purifier placement guide explains how to test the same room in two positions.
Key Factors to Consider
What a Good Indoor PM2.5 Reading Looks Like (UK Bands)
Roughly aligned with WHO 2021 guidance and real UK home patterns we've measured:
• 0–9 µg/m³ — Very good. Cleaner than typical UK outdoor air. Achievable with a HEPA purifier running 24/7.
• 10–19 µg/m³ — Good. Normal for a tidy, well-ventilated UK home.
• 20–34 µg/m³ — Moderate. Common during cooking, vacuuming or in busy households. Sensitive people may notice symptoms.
• 35–54 µg/m³ — Unhealthy for sensitive groups. Allergy sufferers, asthmatics, children, elderly should reduce exposure.
• 55–149 µg/m³ — Unhealthy. Outdoor pollution flooding in, or active indoor source.
• 150+ µg/m³ — Very unhealthy. Treat as an event — kill the source, run purifier on max, ventilate carefully.
The WHO 24-hour guideline is 15 µg/m³. UK outdoor annual legal limit is 20 µg/m³ — but most homes can easily stay under 10 with a purifier. Full breakdown in our what do PM2.5 levels mean guide.
What's a Good Reading During Cooking?
Cooking will spike PM2.5 — frying, gas hobs and toasting routinely push readings to 100–300+ µg/m³. That's normal. What matters is the recovery time.
A good kitchen with extractor + nearby HEPA purifier should drop back to under 20 µg/m³ within 30–60 minutes. If yours sits at 80+ µg/m³ for hours after cooking, your ventilation isn't keeping up.
What's a Good Reading at Night in the Bedroom?
Aim for under 10 µg/m³ overnight. This is the period you're breathing most deeply, for the longest time. Sustained 15+ µg/m³ overnight is associated with poorer sleep quality and morning congestion, especially in allergy sufferers.
If your bedroom PM2.5 climbs overnight, common causes are: open windows (outdoor PM2.5 infiltration), pets, or dusty soft furnishings. A HEPA H13 purifier from our bedroom picks typically holds bedrooms under 5 µg/m³ all night.
What to Do at Each PM2.5 Level
• Below 10 µg/m³: nothing — you're winning.
• 10–20 µg/m³: normal baseline. Keep purifier running, keep cleaning routine consistent.
• 20–35 µg/m³: identify the source — cooking just finished? Hoovering? Candle burning? — and ramp the purifier up.
• 35–55 µg/m³: close windows if outdoor air is the cause, run purifier on max, leave the room if you're allergy-prone.
• 55+ µg/m³: treat as an event. Find and remove the source. Ventilate if outdoor is cleaner, filter hard if not.
What If My Monitor Reads 0 µg/m³ Constantly?
Either you live in genuinely exceptional air (rare), or your sensor isn't working. Even cleanest UK homes sit at 2–5 µg/m³ baseline. A constant 0 means the sensor is faulty, blocked, or it's a cheap unit that bottoms out below a threshold. Try cooking — if it still reads 0, replace it.
Real UK Home PM2.5 Patterns
• London flat near busy road: 15–25 µg/m³ baseline indoors, drops to under 10 with HEPA running.
• Sealed new-build bedroom: 3–8 µg/m³ baseline, spikes overnight from dust if no purifier.
• Family kitchen-diner, gas hob: 100–250 µg/m³ during cooking, 15 µg/m³ within 40 minutes with extractor + purifier.
• Pet household, fitted carpet: 12–20 µg/m³ baseline, spikes to 40+ when pet shakes or vacuum runs.
• Older terrace with open fire: 30–80 µg/m³ when fire is lit; consider a purifier on max during burns.
Indoor PM2.5 bands, what they mean and what to do (UK 2026)
| PM2.5 (µg/m³) | Rating | What it usually means | Action |
|---|---|---|---|
| 0–9 | Very good | Cleaner than typical UK outdoor air; HEPA running | Nothing — maintain |
| 10–19 | Good | Normal tidy, ventilated UK home | Keep purifier on auto/low |
| 20–34 | Moderate | Cooking, hoovering or a busy household | Find the source, raise fan speed |
| 35–54 | Unhealthy for sensitive groups | Outdoor infiltration or an active indoor source | Close windows, purifier on max |
| 55–149 | Unhealthy | Strong source: frying, open fire, smoke | Remove source, filter hard |
| 150+ | Very unhealthy | Acute event | Kill the source, ventilate carefully |
Bands aligned with WHO 2021 air quality guidelines (24-hour PM2.5 guideline: 15 µg/m³) and readings measured in UK homes.
Frequently Asked Questions
Conclusion
A 'good' indoor PM2.5 reading is under 10 µg/m³. Up to 20 is normal. Above 35 needs action. The single biggest lever for keeping indoor PM2.5 low is a HEPA H13 purifier running 24/7 in rooms you actually use.
If your readings are higher than you'd like, our best air purifier UK picks fix it fastest. If you don't have a monitor yet, see our best air quality monitor UK shortlist.
Don't Forget Your Filters
Air purifiers only work if the filters are replaced regularly. Most people forget — and end up breathing dirty air.
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