We spend most of the day indoors, and indoor air is not automatically cleaner than outdoor air. On top of outdoor pollution come cooking, candles, dust, pollen, mould spores and fumes from furniture and cleaning products. An air purifier with a good filter can reliably reduce some of these. It is not a universal fix, though, and it does nothing at all for stale, exhaled air.
How to read the numbers: Target = the value we aim for · Fact = a finding from research or biology
value for PM2.5 (2021)
particles by an H13 filter
as a sensible target
What we actually breathe indoors
The most studied pollutant is fine particulate matter, PM2.5 – particles smaller than 2.5 micrometres. They penetrate deep into the lungs, and the WHO ranks them among the most important environmental health risks. At home they come from cooking, burning candles and incense or smoking, and they also drift in from outside, typically in winter when solid fuels are burned for heating. Larger particles include pollen, mould spores and allergens from dust mites and pets.
Gases are a different group: volatile organic compounds (VOCs) from paints, glues, new furniture or perfumes, nitrogen dioxide from gas cooking and the carbon dioxide (CO₂) we exhale. Particles and gases need different filters.
True HEPA versus “HEPA-type”
The core of any worthwhile purifier is a mechanical HEPA filter. In Europe filter classes are defined by the EN 1822 standard: H13 captures at least 99.95 % and H14 at least 99.995 % of particles at the most penetrating size (around 0.1–0.3 µm). For a home, H13 is entirely sufficient. Labels such as “HEPA-type” or “HEPA-like” are not a standard class and may mean a much weaker filter. A tight seal matters too – if air bypasses the filter through gaps in the housing, the filter class is irrelevant.
Activated carbon: odours and VOCs
A HEPA filter does not capture gases. For odours and some VOCs you need activated carbon, and the amount is what counts: a thin carbon-coated mesh saturates quickly. If you are dealing with odours or fumes after a renovation, look for a filter with a stated carbon weight, ideally in the hundreds of grams. Even so, removing the source and ventilating remain the most effective steps.
Neither a HEPA filter nor ordinary activated carbon removes carbon dioxide. “Stuffy” air in the bedroom is solved only by bringing in fresh air – opening windows or mechanical ventilation with heat recovery. Purifying and ventilating complement each other; one does not replace the other.
Output: CADR and room size
The most useful figure is CADR (Clean Air Delivery Rate), the volume of cleaned air per hour in m³/h. Divide CADR by the room volume and you get air changes per hour. A 15 m² bedroom with a 2.5 m ceiling has 37.5 m³, so a purifier with a CADR of 180 m³/h changes its air about five times an hour. A claim such as “for rooms up to 60 m²” is often based on one or two air changes at maximum speed. Choose a model with headroom and check the CADR at the quiet setting.
Noise and filter costs
A purifier you switch off because of the noise cleans nothing. Around 25–35 dB is suitable for sleeping, while the top speed often reaches 50–60 dB. Check the price of replacement filters too: a HEPA filter is usually changed every 6–12 months, and over a few years can cost more than the device itself.
Ionisers, ozone and UV-C
Ionisers charge particles, which then settle on walls and furniture, and some of them generate ozone. Avoid ozone generators sold as “purifiers” altogether: ozone irritates the airways, worsens asthma and can react with other substances in the air to form unwanted by-products such as formaldehyde. UV-C lamps in portable purifiers make limited sense – air passes them in a fraction of a second and there is little independent evidence of benefit at home. You should be able to switch off both ionisation and UV-C.
A good air purifier is essentially a fan and a dense filter in a well-sealed housing. The more extra “technologies” it promises, the more carefully you should check that it does not produce ozone.
What is supported and what is marketing
It is well documented that HEPA purifiers reduce PM2.5 in a room. The health benefits are more modest: short-term studies, mainly in areas with polluted outdoor air, show a small drop in blood pressure, and some studies in allergy and asthma have found a slight improvement in symptoms.
- Strong evidence HEPA purifiers with adequate CADR substantially reduce indoor concentrations of PM2.5 and pollen.
- Moderate evidence A small, short-term reduction in blood pressure, particularly in areas with polluted air.
- Weak evidence Slight relief of allergy and asthma symptoms – results vary between studies.
- No evidence “Detoxing” the body, boosting immunity, “oxygenating” the air or lowering CO₂ with a purifier.
Placement, humidity and cheap alternatives
The best place is the bedroom. Stand the purifier in open space rather than in a corner, and keep the door closed while it runs. Continuous operation at a low setting works better than short bursts at maximum. Keep humidity at 40–60 % – above that range mould and dust mites thrive. A purifier captures airborne spores, but it will not fix a damp wall.
Cheap steps: airing the room briskly when outdoor air is clean, a cooker hood vented outside, no smoking indoors, fewer candles and a CO₂ meter that tells you when to ventilate. Readings persistently above 1,000 ppm indicate a lack of fresh air.
Who should be cautious
- People with asthma or respiratory conditions – avoid ionisers and ozone generators.
- People allergic to mould – a purifier is a supplement; the source of damp has to be removed.
- People with chronic heart or lung disease – a purifier is no substitute for treatment. Do not change your treatment; talk to your doctor.
- Households with small children and pets – place the device securely and clean pet hair off the pre-filter regularly.
What to look for when buying
- A True HEPA filter of class H13 or H14 under EN 1822, not merely “HEPA-type”.
- Independently measured CADR in m³/h and at least 4–5 air changes per hour for your room.
- Noise in dB at each speed, especially in night mode.
- Activated carbon with a stated weight if you need to tackle odours or VOCs.
- The price of replacement filters and how often they need changing.
- No ozone production, and the option to switch off ionisation and UV-C.
- Red flag: promises of “detox”, curing diseases or “removing 100 % of viruses”.
Conclusion
A purifier with a True HEPA filter and adequate output reliably reduces fine dust, pollen and spores, especially in the bedroom. The health benefits are rather modest, and it does nothing for CO₂ or humidity. The best results come from a combination: removing sources of pollution, ventilating according to a CO₂ meter, keeping humidity at 40–60 % and using an ozone-free purifier as a supplement, not a miracle device.