Air Purifier for Cats: CADR Sizing and the Ozone Line

A purifier removes what is already floating; it does not stop a cat from producing more. Buy the smallest machine that gives 5 air changes an hour in the room the cat actually uses, keep it 30-50 cm off the wall, and switch off any ozone or ion feature when nobody is in the room.

Those 4 rules cover most of the decision. The rest of this page is the arithmetic behind them and the one number — 0.05 ppm — that decides whether a unit is safe to run around a cat at all.

What it can and cannot do in a cat household

Cat allergen (Fel d 1) rides on particles from 1 to 10 microns, and a HEPA filter catches those at over 99% per pass. What it cannot do is reach the source: a cat sleeping on the bed sheds into the mattress, and a purifier moving 300 m³/h will not pull allergen out of a 20 cm thick duvet. Wash bedding at 60 C every 2 weeks and the machine has 50-70% less work to do.

Odour is a different problem. Ammonia from litter is a gas at 17 g/mol and passes straight through HEPA; only 2-5 kg of activated carbon does anything, and it saturates in 3-6 months in a small flat. If the litter box smells at 1 m, fix the litter and the scooping interval before buying a bigger filter.

CADR is the only sizing number that means anything

CADR is clean air delivered in m³/h, measured separately for dust, smoke and pollen. Use the smoke figure, which is the lowest of the 3 on almost every unit. The requirement is simple: CADR ≈ room volume × air changes per hour, and 4-6 ACH is the working range for allergy households, 3 ACH is the floor for a healthy cat.

A 20 m² room with a 2.5 m ceiling is 50 m³, so 5 ACH asks for 250 m³/h. Chinese labelling (GB/T 18801) converts CADR to coverage at 0.07-0.12 m² per m³/h, which puts a 250 m³/h unit at 17-30 m² — the same answer from the other direction. US practice is CADR in cfm at two-thirds of the floor area in square feet: 215 sq ft needs 143 cfm, which is 243 m³/h.

Room size to CADR, in one table

Room (m²)	Volume at 2.5 m (m³)	CADR for 5 ACH (m³/h)	Filter life (months)
10	25	125	12-18
15	38	190	9-12
20	50	250	6-12
30	75	375	6-9
45	113	565	4-6

Filter life in the last column assumes 12-24 hours a day of running and one cat. Two cats, or a long-haired breed, cut it by 30-40%. A unit rated 400 m³/h running on medium (roughly 60% of maximum) delivers about 240 m³/h, so buy 1.5-2 times the calculated CADR if you intend to run it below maximum for noise.

Filter grade: the 0.3 micron number

HEPA is defined at the worst-case particle size, 0.3 microns: H13 removes 99.95%, H12 99.5%, H11 95%, and an ungraded HEPA-type filter may be 60-85%. Cat allergen clusters at 1-10 microns, where even H11 does well, so pay for H13 only if someone in the house has asthma.

Higher grade costs airflow. The same fan pulls 20-30% fewer m³/h through H13 than through H11, which shows up as noise at the same CADR. A 2-stage unit (washable prefilter plus H13) keeps its rated CADR for 3-4 times longer than a single-layer filter, because the prefilter takes the 10-50 micron cat hair that would otherwise blind the media in 4-8 weeks.

Placement and noise: 30-50 cm, 28-34 dB

Keep 30-50 cm clear behind and 30 cm at the sides. Pushed into a corner, recirculation drops delivery by 20-30% and the wall collects a grey halo within 2 months. Put it where the cat sleeps, not where the box is: the clean-air zone is roughly 2-3 m around the outlet, and a machine in the hallway cleans the hallway.

Cats hear from 48 Hz to 64 kHz, with peak sensitivity at 2-8 kHz, so a fan that reads 45 dB at 1 m is audible to them at 8-10 m. Night mode at 28-34 dB is the practical target; if the cat leaves the room when it starts, the pitch is the problem, not the airflow. Run it 1-2 hours before the cat's nap window rather than all night at maximum.

The ozone red line: 0.05 ppm, and ionizers off when alone

Ozone is the one feature that can hurt a cat. US regulators (CARB, and the FDA device guidance) cap consumer air cleaners at 0.05 ppm ozone, which is about 100 micrograms per m³; the Chinese standard tests the same emission and reports it per unit. Any device advertising ozone sanitising or active oxygen output in mg/h is not an air purifier and should not run in a room with a cat, a kitten or an asthmatic person.

Ion generators and UV lamps are the usual source. Ozone has a half-life of 20-30 minutes indoors and reacts with dust and fur, so concentration is highest at 0.3-1 m from the vent — exactly where a cat sits. Cats with asthma or chronic rhinitis show it first: coughing 2-3 times a day, or 30-40 breaths a minute at rest against a normal 20-30. Switch ion and UV off, or buy a unit without them, and never run either in an empty room.

Running schedule and filter changes

24 hours a day at low speed beats 8 hours at maximum: 24 x 90 m³/h delivers 2,160 m³ a day, while 8 x 300 m³/h delivers 2,400 m³ but leaves the room dirty for 16 hours, including the 6-8 hours the cat is most active at night. Most units draw 5-20 W on low, so 24/7 costs 44-175 kWh a year.

Vacuum the prefilter every 2-4 weeks. Replace the HEPA when CADR has visibly dropped or after 2,000-4,000 hours, not on a calendar date; a loaded filter can lose 30-50% of rated airflow while sounding quieter, which reads as working fine. Write the install date on the filter frame with a marker.

Do this today

Measure the room: length x width x ceiling, then multiply by 5 for the CADR you need.
Check the label for 3 CADR values; the lowest one is the real number.
Move the unit 30-50 cm off the wall, on the side where the cat sleeps.
Switch ion and UV functions off, or leave them off when the room is empty.
Wash bedding at 60 C every 2 weeks — cheaper than 200 m³/h of extra airflow.

If you are still deciding where the money goes, the running-cost calculator compares filter and electricity spend over 5 years, and water fountains matter more than filtration for a cat that drinks too little. Long-haired cats add a fur load that shortens filter life; the coat schedule in the long-hair care calendar is the other half of the same problem, and brushing 3 times a week at 5 minutes takes 20-30% off the prefilter's work.

Frequently asked

What CADR do I need for one cat?

Room volume times 5: a 20 m² room with a 2.5 m ceiling is 50 m³, so look for 250 m³/h on the smoke CADR figure. Allergy households go to 6 air changes, and if you plan to run on medium rather than maximum, buy 1.5-2 times the calculated number because a 400 m³/h unit delivers about 240 m³/h at 60% fan speed.

Are ionizers and UV lamps safe around cats?

Only if they produce no measurable ozone. Consumer cleaners are capped at 0.05 ppm ozone, about 100 micrograms per m³, and any device advertising ozone output in mg/h exceeds that by design. Ozone peaks 0.3-1 m from the vent, exactly where a cat sits, and cats with asthma or chronic rhinitis show it as coughing 2-3 times a day.

Should the purifier run all night?

Run it 24 hours at low speed rather than 8 hours at maximum: 24 x 90 m³/h gives 2,160 m³ a day against 2,400 m³ for 8 x 300 m³/h, but the room stays clean through the 6-8 hours the cat is most active. Low speed draws 5-20 W, and 28-34 dB keeps it below the range that drives cats out of the room.