Air purifiers
A portable air purifier with a true HEPA filter is the one home air device with solid evidence that it cuts the fine particles you breathe: in a bedroom it can remove most of them overnight. Whether that makes you healthier is unproven — the best recent trial found no change in blood pressure — and devices that claim to destroy pollutants with ozone, plasma or ultraviolet light can add pollutants instead.
If you buy one, buy a true-HEPA unit for the bedroom, sized so its clean-air delivery rate in m³ per hour is at least about five times the room's volume in m³, and run it every night. Skip ionizer, plasma, UV-photocatalytic and ozone units, and don't count on houseplants.
What a HEPA filter does, and what it can't
A HEPA (high-efficiency particulate air) filter is a dense fibre mat rated to remove at least 99.97% of particles at 0.3 µm. That size is its worst case, the one that slips through most easily; particles both larger and smaller, including the ultrafine ones from traffic and gas cooking, are caught even better.[1]
The target is fine particulate matter (PM2.5), particles smaller than 2.5 µm, which are the clearest air-pollution harm. They cross from the lungs into the bloodstream, and long-term exposure raises the death rate and probably the risk of dementia; the evidence is on Environmental toxins. Indoor air matters because people in industrialised countries spend about 90% of their time indoors — in one large US activity survey, 87% in buildings and another 6% in vehicles.[2] Indoor PM2.5 can exceed outdoor levels once gas stoves, scented products and off-gassing furniture add to what drifts in from traffic and wildfire smoke.
HEPA does nothing for gases: volatile organic compounds (VOCs), ozone and nitrogen dioxide pass straight through. In a portable unit the only option for gases is a dense bed of activated carbon; the thin carbon-coated pre-filters in standard consumer units saturate quickly and give little protection.
It lowers exposure; health benefits are unproven
In a four-week double-blind crossover trial, 33 residents of a US senior housing facility beside a major highway slept with a real or a sham filter in the bedroom. The real filter dropped bedroom PM2.5 from 8.3 to 1.3 µg/m³ and cut 24-hour personal exposure by about 60%, from 12.4 to 5.1 µg/m³. Blood pressure did not move: home systolic pressure averaged 127.0 mmHg on the real filter against 126.5 on the sham, a difference well inside chance. There were only non-significant trends toward benefit in older, heavier participants who started with higher pressure.[3]
A 2025 review of 63 indoor-air-quality experiments is more encouraging, but only for intermediate markers: filtration improved heart-rate variability, blood pressure and lung function. No filtration trial has shown fewer heart attacks, strokes or respiratory hospital admissions, and the review's authors go no further than suggesting that sustained indoor improvements "may decrease incidence rates of respiratory pathologies."[4]
Devices that "destroy" pollutants can make the air worse
A 2024 chamber study at the US National Institute of Standards and Technology tested ten portable air cleaners across seven technologies. The oxidation-based ones were the worst: photocatalytic-oxidation and germicidal-UV units gave off the most aldehydes (202 and 243 µg per hour), and three of the ten units put out more VOCs than they removed.[5] Ozone-generating ionizers and plasma units share the problem.
For gases, source control and ventilation beat any device: drop air fresheners, plug-ins and scented candles, cook under an extractor fan vented outdoors (a gas flame is a substantial source of nitrogen dioxide), and open windows for cross-ventilation when the outside air is reasonably clean.
Houseplants are not air purifiers
The idea comes from a 1989 NASA study that used sealed chambers, which don't behave like real rooms.[6] Ordinary ventilation dilutes VOCs faster than plants absorb them. A single potted plant cleans a median of about 0.023 cubic metres of air an hour; matching what normal air exchange with the outdoors already does would take on the order of 10–1,000 plants per square metre of floor.[7]
What to do
- Put a true-HEPA unit in the bedroom and run it nightly.
- Size it by clean-air delivery rate (CADR): at least about five times the room's volume in m³ per hour, roughly 12 times the floor area in m² for an ordinary ceiling. This is the metric form of the US "two-thirds rule" (CADR in cubic feet per minute at least two-thirds of the floor area in square feet).
- Avoid plasma, ionizer, UV-photocatalytic and ozone "purifiers".
- Handle gases at the source: no synthetic fragrance, an extractor fan when cooking, open windows when outdoor air is clean.
- Keep expectations honest: the filter reliably lowers what you breathe, but the best trial found no change in blood pressure, and no trial has shown fewer heart attacks or strokes.
Further reading
- Brook RD et al. Bedroom Air Filtration and Home Blood Pressures of Older Adults Living in a Residential Facility. J Clin Hypertens (Greenwich) 2026.[8]
- Link MF et al. Quantification of Byproduct Formation from Portable Air Cleaners Using a Proposed Standard Test Method. Environ Sci Technol 2024.[9]
- Cummings BE, Waring MS. Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies. J Expo Sci Environ Epidemiol 2020.[10]