Air Purifiers and Indoor Air Quality: What Actually Works
Air purifiers have become popular home purchases, marketed with claims ranging from reasonable to wildly exaggerated. This guide covers what the technology actually does, what the evidence shows works, and how to choose a unit that will genuinely improve air quality rather than just running quietly in the corner.
The short answer
HEPA filtration is effective for particles — it captures 99.97% of particles 0.3 microns and larger, including fine dust, pollen, mould spores, pet dander, and airborne synthetic fibres. Activated carbon removes some VOCs and gases but has limited total capacity. Ozone generators and ionisers that produce ozone are harmful — do not buy them. Ventilation is more effective than any purifier for VOC reduction. Source reduction — choosing low-VOC materials, fragrance-free products, natural fibre furnishings — is the most durable solution. A purifier is a useful supplement, not a primary strategy.
What different technologies do
HEPA filtration
HEPA stands for High-Efficiency Particulate Air. A true HEPA filter captures 99.97% of particles at 0.3 microns (the most penetrating particle size). This covers:
- Fine dust and particulate matter (PM2.5 and larger)
- Pollen, mould spores, fungal fragments
- Pet dander
- Airborne synthetic microfibres
- Bacteria and larger virus clusters (viruses themselves are typically smaller than 0.3 microns, but they often travel on carrier particles that HEPA captures)
HEPA is effective and well-evidenced. Note: "HEPA-type" or "HEPA-style" filters are not HEPA — they use a lower-efficiency filter and typically capture only 85–99% of larger particles. Only buy units with "True HEPA" or "H13 HEPA" (European standard) if particle filtration is the goal.
Activated carbon
Activated carbon is a highly porous carbon material that adsorbs VOC molecules on its surface. This is the right technology for chemical gases — HEPA does not capture gases at all. However:
- Activated carbon has finite capacity — once all the surface sites are occupied, it stops working (and can release previously captured compounds)
- It works best at lower VOC concentrations
- Thin "activated carbon layers" in budget purifiers contain too little carbon to be meaningful — look for units with substantial carbon weight (300g+ for room-sized units)
- It does not capture formaldehyde efficiently — formaldehyde requires specialist impregnated carbon
An activated carbon filter helps in a room with moderate VOC levels where ventilation is limited. It is not a substitute for addressing VOC sources or ventilation.
Ionisers and ozone generators
Do not use these. Ionisers that emit negative ions cause airborne particles to deposit on surfaces (temporarily removing them from breathing air) but do not capture them — they end up on walls and floors. More importantly, many ionisers produce ozone as a byproduct. Ozone is a lung irritant at low concentrations and a respiratory hazard at higher concentrations. Ozone generators marketed as "air purifiers" or "sanitisers" produce ozone intentionally — this is harmful in occupied spaces. The California Air Resources Board has restrictions on ionising air purifiers for this reason.
UV-C light
UV-C units in purifiers kill bacteria and viruses in the airstream. They work, but only on organisms moving through the UV-C chamber — the exposure time is brief and efficacy varies by unit design. UV-C is a useful addition for infection control applications but not a priority for general home air quality improvement.
The CADR number: what it means and how to use it
CADR (Clean Air Delivery Rate) measures how quickly a purifier cleans a room's air — expressed in cubic metres per hour (m³/h) or cubic feet per minute (CFM). A unit with a CADR of 300 m³/h will clean the air in a 30m² room with 2.4m ceilings (72m³) approximately 4 times per hour — which is the recommended minimum for allergy management.
The general rule: choose a purifier with a CADR of at least 4–5× the room volume in m³ per hour. For a 15m² bedroom (36m³), you want at least 150 m³/h CADR. For a 30m² living room (72m³), at least 300 m³/h.
Manufacturers sometimes list CADR at maximum fan speed, which is typically too loud for use. Check what the CADR is at medium speed.
Where to prioritise in the home
- Bedroom: 8 hours of continuous exposure during sleep. Most impactful placement. Run on low/medium overnight.
- Nursery: For the same reason, with higher sensitivity. Run continuously on low when the baby is sleeping.
- Main living area: If you have significant VOC sources (new furniture, carpet) or if family members have respiratory conditions.
What ventilation does that a purifier cannot
Opening windows replaces indoor air with outdoor air — this is the only way to reduce the concentration of gases like formaldehyde and VOCs by dilution. A purifier recirculates indoor air and removes particles and some gases; it does not bring in fresh air. In a room with significant off-gassing, ventilation reduces concentrations far more effectively than any purifier. Use both together: ventilate when outdoor air quality permits, purify when it does not.