Indoor air in a city apartment is not the outdoor air with dust added. It is a different mixture: cooking particulates, cleaning-product VOCs, off-gassing from furniture, radon in some buildings, plus infiltrated outdoor pollution. WHO estimates that most urban dwellers spend 90 percent of their time indoors, and indoor air is generally more polluted than the outdoor air on the same street. This guide covers the four indoor pollutants that matter, the free habits that reduce them most, and where a HEPA purifier is genuinely worth the money.
Everything below is specific to modern flats: sealed windows, gas or electric hob, one or two occupants, no wood stove or fireplace.
The four indoor pollutants that matter in a flat
- Cooking particulates (PM2.5 and PM10). The biggest indoor source in most flats. Frying, grilling, and searing all produce fine particles that stay airborne for 30 to 90 minutes.
- VOCs from cleaning products, sprays, candles. Volatile Organic Compounds off-gas continuously from many household products. Peak levels come from scented candles, air fresheners, and immediately after new furniture arrives.
- CO2 buildup. Not a health pollutant at typical indoor levels but a strong indicator of ventilation. Sleeping in a sealed bedroom will push CO2 above 2000 ppm by morning, where cognitive performance measurably drops.
- Outdoor infiltration. Traffic PM2.5, NO2, and ozone leak into flats through gaps, especially on busy-road frontages. Higher floors get less; ground-floor flats on arterial roads get the most.
Cooking, the biggest fix in most flats
A single hob-fried meal without extraction routinely pushes a flat’s PM2.5 to 40-80 ug/m3 for an hour. WHO 24-hour guidance is 15 ug/m3. Practical countermeasures:
- Use the extractor hood. Every time. Set it high before you start cooking, not after the smoke starts. If your hood is a recirculating type (no vent outside), replace the carbon filter every 6 months; those filters do work but only when clean.
- Open a nearby window while cooking. Crossflow with a partially open window on the other side of the flat clears cooking air in about 8 minutes.
- Cover pans. Reduces both cooking-particle emission and moisture load.
- Consider oven roasting instead of stovetop searing for pollutant-heavy meals. Ovens leak much less into the room air.
Renters cannot always install ducted extraction. Where you cannot, the window strategy above is the single biggest indoor air-quality move available for zero cost.
The free habit that beats most purifiers
Shock ventilation, from German building physics research (Stosslüften):
- Open all windows in a room fully for 3 to 5 minutes.
- Close them.
- Repeat twice a day, morning and evening, all year.
Shock ventilation exchanges the room’s air without cooling the walls (walls hold thermal mass; short openings do not chill them). It also outperforms cracked-window ventilation on both air quality and heating cost. Continuous cracked windows lose heat continuously; a 4-minute burst does not.
Related: Green Cleaning Products for the Apartment covers how to reduce VOC load at the cleaning-product source.
Where a HEPA purifier actually helps
A HEPA filter captures at least 99.97 percent of particles at 0.3 micrometres, which covers pollen, dust, most PM2.5, some bacteria. It does not capture gases (CO2, NO2, VOC gases). Activated carbon layers in some units do capture some VOCs but are limited by carbon volume.
Worth buying a HEPA unit if:
- You have hay fever or asthma triggered indoors.
- You live on a busy arterial road with clear traffic PM2.5 exposure.
- You cook a lot with methods that produce smoke (wok, grill, deep fry) and your extractor is weak or missing.
- You have pets and the dander is a constant issue.
Do not buy a HEPA unit if:
- You have not tried shock ventilation twice a day for a fortnight first.
- You expect it to fix mould, damp, or a specific chemical smell (it will not).
- The unit is a low-cost novelty model (under £40) with a small fan that will not actually cycle a room’s air in under an hour.
The 20 percent of purifier buying decisions people get wrong
The right-size question is Clean Air Delivery Rate (CADR), measured in m3/h. Room air changes per hour equals CADR divided by room volume. For a typical UK bedroom (14 m2, 2.4 m ceiling = ~34 m3), aim for a purifier rated 200 m3/h to achieve 6 air changes per hour. Most cheap purifiers rate at 100 m3/h and simply do not move enough air to matter.
Quick reference: air quality moves ranked by impact
| Move | Impact | Cost |
|---|---|---|
| Extractor hood on high, always | Cuts cooking PM2.5 exposure ~60-80% | Free |
| Shock ventilation twice a day | Halves CO2 buildup, reduces VOC accumulation | Free |
| Cover pans while cooking | Cuts moisture and particle emission ~30% | Free |
| Replace scented candles with unscented | Eliminates a routine VOC source | ~£0-5 |
| HEPA purifier (200+ m3/h) in bedroom | Reduces overnight PM2.5 exposure ~50-70% | £100-250 |
| CO2 monitor | Diagnostic, tells you when to ventilate | £30-80 |
The one-week experiment
If you want to know whether air quality is a genuine issue in your flat, borrow or buy a £30-80 CO2 monitor and log the reading morning, midday, evening, before bed, and on waking for a week. Anything consistently over 1500 ppm indicates ventilation is inadequate. Add shock ventilation for a week. Log again. You will see the numbers move.
Indoor air quality is not a mystery and it is not solved by an expensive gadget. Cook with the extractor on, open windows shortly twice a day, avoid burning scented things, and the flat’s air stays measurably cleaner than most of the street outside.
The CO2 monitor as diagnostic
If you buy just one piece of measurement equipment for indoor air quality, make it a CO2 monitor rather than a PM2.5 sensor. CO2 is a proxy for ventilation adequacy, easy to read (parts per million), and directly tells you when to open a window. PM2.5 sensors are useful for cooking events but the answer is always the same (turn on the extractor, open a window), so the diagnostic loop is less actionable. A £30 to £80 CO2 monitor (Aranet4, Airthings Wave Mini) sitting on the bedroom shelf builds an intuitive sense of when air is stale that lasts long after you stop looking at the display.
Sources and further reading
Frequently asked questions
For anyone with allergies or living on a busy road: probably yes, £80-200 for a small-room unit. For most other city renters: ventilation habits do more good for less money. Buy the purifier if you already open the windows properly and want the next step.
Volatile Organic Compounds are chemicals that evaporate at room temperature. Indoors they come from cleaning products, aerosol sprays, new furniture off-gassing, scented candles, and paint. Highest exposure is in the first six months after new flat-pack furniture or repainting.
PM2.5 is airborne particulate matter under 2.5 micrometres in diameter, small enough to reach the deep lung. Frying, grilling, and toasting bread all produce PM2.5. Cooking a single meal on a gas hob without extraction can push indoor PM2.5 above WHO daily-average guidance for an hour.
Two "shock ventilations" a day: open all windows in a room for 3-5 minutes, close, move on. This exchanges the air without letting the walls cool. Vastly more effective than a window cracked all day.
