An air purifier can only remove allergens while they are airborne. Cat and dog allergens ride partly on small particles that stay in the air for hours, so filtration is relevant for them. Pollen is large but enters constantly during the season, so a purifier helps keep indoor levels down between sources. Dust mite allergen sits on heavy particles that settle within minutes and lives in bedding and textiles, so a purifier helps least there. In every case, source control does more than filtration — and a purifier is not a medical treatment.
Why particle size decides everything
How long a particle stays in the air depends mostly on its size. Larger particles settle quickly under gravity; smaller particles are carried by room air currents for hours. A purifier can only capture what reaches its intake, so an allergen that spends most of its life on the floor or in a pillow is largely out of reach.
| Particle diameter | Approximate settling speed in still air | Time to fall 2 m |
|---|---|---|
| 1 µm | ≈ 0.004 cm/s | Many hours — effectively stays airborne |
| 5 µm | ≈ 0.08 cm/s | About 40 minutes |
| 10 µm | ≈ 0.3 cm/s | About 10–11 minutes |
| 30 µm | ≈ 2.7 cm/s | Just over a minute |
Values are approximate for particles of unit density in still air; real rooms have air currents and activity that keep particles suspended longer and re-suspend settled dust.
Allergen by allergen
| Allergen | Main carrier particle size | Behaviour | How relevant is a purifier? |
|---|---|---|---|
| Cat allergen | A significant share on particles below ~5 µm | Stays airborne for hours; sticks to clothing and surfaces | Relevant — continuous filtration lowers airborne levels |
| Dog allergen | Broad range, including small particles | Similar to cat allergen, generally at lower airborne levels | Relevant, alongside cleaning and keeping pets out of bedrooms |
| Pollen | Whole grains typically 10–100 µm; fragments can be much smaller | Settles fairly quickly but enters continuously via windows, doors, clothes and hair | Moderately relevant during the season; captures incoming and resuspended pollen |
| Mould spores | Typically ≈ 2–10 µm | Can stay airborne for tens of minutes to hours | Relevant for airborne spores — but the moisture source must be fixed |
| Dust mite allergen | Mainly on particles ≈ 10–40 µm | Airborne briefly after disturbance (making beds, vacuuming), settles within minutes | Limited — source control matters much more |
What the evidence says
The U.S. EPA's consumer guide to air cleaners summarises the research carefully: several studies of portable HEPA air cleaners found improvements in one or more allergy or asthma symptoms, but results have been mixed and not every study showed a benefit. Effects are most consistent where airborne allergens are the dominant exposure and where the purifier is adequately sized and actually used. A purifier should therefore be seen as one part of an allergen-reduction plan and never as a replacement for medical advice or prescribed treatment.
Setting up a purifier for allergy relief
- Start with the bedroom. It is where most people spend the longest uninterrupted time. Close the door so the purifier treats one room, not the whole home.
- Size generously. Aim for at least 5, and preferably 6–8, clean air changes per hour at a setting quiet enough to run all night. See how to size an air purifier and noise levels for sleep.
- Run it continuously during the season or whenever pets are present, not only when symptoms appear. Allergens are replenished all the time.
- Place it to serve the bed area without blowing directly at the sleeper (see placement guide).
- Change filters on time; a loaded filter delivers less clean air.
Source control: where the biggest gains are
Dust mites
- Wash bedding weekly at 60 °C or higher, which is hot enough to kill mites.
- Use allergen-impermeable covers on mattresses and pillows.
- Keep indoor relative humidity below about 50% where practical; mites depend on humid conditions.
- Reduce textile reservoirs in the bedroom: heavy curtains, rugs, soft toys.
- Vacuum with a sealed vacuum cleaner fitted with a HEPA exhaust filter.
Pollen
- Keep windows closed on high-pollen days, especially on warm, windy afternoons; ventilate in short bursts or when counts are lower.
- Change clothes and wash hair after spending time outdoors in the season.
- Dry laundry indoors during peak season.
Pets
- Keep pets out of the bedroom.
- Clean hard floors and wash pet bedding regularly.
- Groom pets outdoors where possible.
Mould
- Find and fix the moisture source: leaks, condensation, poor extraction in bathrooms and kitchens.
- Professional remediation is needed for larger areas; a purifier only captures spores that become airborne.
What to look for in a purifier for allergens
- Particle CADR suitable for the room — dust or smoke CADR is more informative than pollen CADR.
- A true HEPA (H13 or U.S. HEPA) or well-sealed high-grade filter — see HEPA H13 explained.
- Low noise at the setting you will use overnight.
- No ozone-producing features, which can irritate airways — see ionizers, UV and ozone.
Key takeaways
- Purifiers only capture allergens that are airborne; particle size determines how long that is.
- Most relevant for cat and dog allergen and airborne mould spores; moderately for pollen; least for dust mites.
- Bedroom first, door closed, sized for 5–8 clean ACH at a quiet setting, running continuously.
- Source control — bedding, humidity, pets, windows — delivers the biggest reductions. Follow medical advice for symptoms.
Frequently asked questions
Will an air purifier stop my allergies?
It may reduce exposure to allergens that stay airborne, and the EPA notes that several studies found some symptom improvements with portable HEPA cleaners, but results vary and a purifier is not a treatment. Combine it with source control and follow your doctor's advice.
Where should I put an air purifier for allergies?
In the room where you spend the most continuous time — usually the bedroom — with the door closed and the unit sized for at least 5 clean air changes per hour on a setting quiet enough to run all night.
Does an air purifier help with dust mites?
Only a little. Dust mite allergen is carried mainly on relatively heavy particles that settle within minutes and live in bedding, carpets and upholstery. Washing bedding hot, using allergen-proof covers and vacuuming with a sealed HEPA vacuum matter more.
Sources and further reading
- U.S. EPA — Guide to Air Cleaners in the Home
- U.S. EPA — Air Cleaners and Air Filters in the Home (incl. Residential Air Cleaners: A Technical Summary)