Shines germicidal light on passing air, which gets far less exposure time than the claims suggest.
also: germicidal UV purifier, UVGI air cleaner, photocatalytic purifier
The basics
What to know about air purifiers
A fan and a filter
An air purifier is a fan and a filter inside a box. Nothing in it neutralizes a room the way a spray pretends to: air has to physically enter the machine, pass through media, and come back out cleaner. Whatever the fan never pulls in never gets cleaned, so performance here is a rate rather than a state. That rate is CADR, clean air delivery rate, measured in cubic feet per minute of perfectly clean air the machine effectively adds to the room. A unit rated 200 CFM for smoke acts like a hidden vent supplying 200 cubic feet of pollution free air every minute, working against everything that keeps arriving from cooking, pets, the street and your own footsteps.
How a filter actually catches
Fibers catch particles three ways, and the mix explains the one number everyone misreads. Coarse particles above roughly 1 micrometer, a micrometer being a thousandth of a millimeter, carry too much momentum to steer around a fiber and smack into it. Mid sized ones follow the air but graze fibers and cling. Anything under about 0.1 micrometers jitters randomly and hits fibers by accident, so it is caught easily. Hardest is the size in between, near 0.3 micrometers. This is exactly where HEPA gets graded: 99.97 percent capture at 0.3 micrometers under the US standard, or classes H13 at 99.95 percent and H14 at 99.995 percent under the European EN 1822 test. Read that as the worst case rather than the best. Ultrafine soot and virus carrying droplets are trapped more effectively than 0.3, so the common worry about tiny things slipping through has it backwards. Gas is the true gap. Odor molecules, solvent fumes and formaldehyde are thousands of times smaller than any particle and drift through HEPA media untouched. Catching those takes activated carbon, which holds molecules on 500 to 1,500 square meters of internal surface per gram until it fills up.
When one is worth having
Reach for one when a source you cannot remove is making the air worse. Pollen weeks, a cat, dust mite debris kicked up when you make the bed, a wildfire season that lasts a month, road soot 50 feet from the window, frying that fills a kitchen with fine grease particles, a new sofa or new flooring off gassing for months. Put it where you spend the most hours, which for most people is the bedroom, since seven closed door hours in one room is the cheapest air quality gain on offer. Know the boundaries too. Carbon monoxide needs an alarm, not a filter. Radon takes a soil vent. Dust already on the floor stays there until a vacuum with a sealed body lifts it out, the same sealing point that decides whether a vacuum helps an allergic household. Damp that grows mold gets fixed by drying the building, never by filtering the spores it sends up.
What extra airflow costs
Noise and money are what extra airflow costs. Fan sound climbs with roughly the fifth power of speed while airflow climbs in a straight line, so doubling the speed doubles clean air and adds about 15 decibels. A 10 decibel rise already sounds twice as loud. Top settings on room units often measure 55 to 65 decibels, which few people sleep through. Buying more capacity than you need and running speed two of four puts you near 30 to 35 decibels with the same daily result. Filters carry the real ownership cost. A HEPA cartridge runs 40 to 120 dollars and lasts 6 to 12 months, carbon needs changing sooner. Across five years consumables often pass what the machine cost. Power is trivial beside that, since 30 watts running around the clock for a year is about 260 kilowatt hours.
Judge it on verified CADR
Judge one on verified numbers and disregard coverage claims. Independently verified units publish three CADR figures, for smoke, dust and pollen; buy on smoke, because it covers the smallest particles and is always the lowest of the three. Size with the two thirds rule: at an 8 foot ceiling, smoke CADR in CFM should be at least two thirds of the floor area in square feet, so 300 square feet wants 200 CFM. That lands near five air changes per hour, which you can check with CADR times 60 divided by room volume. Some markets publish CADR in cubic meters per hour, where 1 CFM equals 1.7, and the underlying test differs by country, so compare only figures measured the same way. Then check the decibel rating at the speed that delivers your CADR rather than at speed one, feel for a gasket that seals the filter so air cannot slip around it, look for carbon quoted in grams instead of a passing mention of a coated mesh, and confirm replacement filters are still sold. A machine promising 1,500 square feet with no CADR printed anywhere is telling you what it does not have.
The part people skip
Upkeep is short, and the part people skip is airflow. Vacuum or rinse the pre-filter every two to four weeks, because a felt of lint across the intake strangles the fan long before the HEPA is spent, and plenty of owners buy a new cartridge when a two minute clean was the actual fix. Change the HEPA every 6 to 12 months of continuous running, sooner through a smoke season, and keep water away from it, since washing mats the fibers and warps the frame and a warped filter stops sealing. Replace carbon by the calendar rather than by smell, because a saturated bed gives back what it held once a room warms up. Clean the particle sensor window with a dry cotton swab every three months, or auto mode reads a dusty lens as clean air and idles at speed one. Filter timers count hours, not dirt, so believe the pre-filter you can see over the light on the panel.
Buyer checklist
Before you buy air purifiers
A major appliance must fit the space, the utilities and the service network. Running cost and repair access belong in the first comparison.
01
Measure the full installation
Include doors, handles, hoses, vents, clearance, service access and the delivery path, not only the cabinet opening.
02
Confirm every utility
Check voltage, circuit, plug, water, drain, gas and ventilation requirements against the exact model and local rules.
03
Compare ongoing use
Use the local energy or water label and include filters, detergent, bags and other consumables.
04
Check parts and service
Look for local technicians, realistic warranty coverage and the availability and price of common replacement parts.
Set a target before you shop: at an 8 foot ceiling, smoke CADR in CFM should be at least two thirds of the room's floor area in square feet. So 180 square feet needs 120 CFM and 500 square feet needs 335 CFM. Multiply by 1.25 for a 10 foot ceiling.
Buy roughly double that number so the machine reaches your target at speed two of four. Noise rises with about the fifth power of fan speed while airflow rises in a straight line. So an oversized unit on medium is quieter than an exact fit on maximum.
Count rooms rather than square feet, because doors and walls stop air. One unit per closed room is the working rule, an open plan kitchen and living area counts as a single large room, and a purifier parked in a hallway cleans the hallway.
Leave 12 to 24 inches clear on the intake face and above the outlet. Bottom intake models lose airflow on thick carpet, and a unit shoved into a corner recycles its own clean output and idles because its sensor reads air it just filtered.
Measure the footprint before you commit: room units occupy a 10 by 10 to 20 by 20 inch patch of floor, stand 20 to 30 inches tall and weigh 10 to 25 pounds, which decides whether you will really carry it between bedroom and living room.
Portable units draw 5 to 100 watts from a standard outlet, so power is a non issue. A whole house cleaner is different. The cabinet needs 4 to 6 inches of clearance at the air handler, the blower must tolerate the added resistance. Any duct modification or new wiring is licensed trade work.
Care
Caring for air purifiers
Vacuum or rinse the pre-filter every two to four weeks, since a mat of lint and pet hair across the intake cuts airflow and CADR long before the HEPA behind it is used up.
Replace the HEPA cartridge every 6 to 12 months of continuous running, and never wash one unless it is sold as washable, because water flattens the fine fibers and warps the frame so air escapes around the pleats.
Change activated carbon every 3 to 6 months on a calendar rather than waiting for the smell to return, as a saturated bed releases some of what it held once the room warms up.
Wipe the particle sensor window and its small air channel with a dry cotton swab every three months, or dust on the lens reads as clean air and auto mode sits at speed one all day.
Wash the collector plates of an electrostatic unit every two to four weeks and let them dry completely before restoring power, because collection efficiency drops sharply as the plates load.
Treat the filter life indicator as a stopwatch, not a measurement, since it counts running hours and knows nothing about how dirty your air is. Recheck any schedule or auto setting after a firmware update restores factory behavior.
Questions
Common questions
How big a purifier does a 300 square foot living room need?
About 200 CFM of smoke CADR, which is two thirds of the floor area at an 8 foot ceiling and works out near five air changes per hour. Add 25 percent for a 10 foot ceiling and another 25 percent if the room opens into a hallway you cannot close. Buy above that so medium speed does the job.
Does HEPA really stop particles smaller than 0.3 micrometers?
Yes, and better than it stops 0.3 itself. That size is hardest to catch because it is too light to crash into fibers and too heavy to bounce around much. Below roughly 0.1 micrometers, random motion throws particles into the fibers constantly, so capture climbs again. A filter rated 99.97 percent at 0.3 performs above that on ultrafine soot.
Should I run it all the time, or only when the air seems bad?
Run it continuously on a low speed. Particles come back the moment you cook, walk across carpet or open a door. A purifier holds a room clean only while air keeps passing through it. Continuous low running is quieter and cheaper than bursts on maximum, and 30 watts around the clock costs about 260 kilowatt hours a year.
What does one actually cost to own?
Filters dominate. Budget 60 to 200 dollars a year for a HEPA cartridge plus carbon on a shorter cycle, against roughly 25 to 50 dollars of electricity for a mid size unit running all day. Across five years the consumables often exceed the purchase price, so price the replacement filter and check it is still stocked before you buy.
Will a purifier clear cooking smells and paint fumes?
Only one carrying real carbon, measured in hundreds of grams. Smells are gas molecules and a HEPA mat passes them without resistance, while a carbon coated mesh weighing 50 grams fades within a few weeks. For fresh paint or new flooring, opening two windows on opposite sides of the home for 15 minutes removes more than any filter.
Are ionizers and ozone models a shortcut worth taking?
No. Ions make particles stick to walls and floors instead of removing them, and walking through the room lifts some straight back into the air. Both can emit ozone, a lung irritant that worsens asthma, and ozone meeting citrus based cleaners forms formaldehyde. California caps air cleaner ozone at 0.05 parts per million and requires certification; other countries differ, so check locally.
Can I wash the HEPA filter to save the replacement cost?
No, unless the maker sells it as washable, and almost none do. Water mats the fine fibers, opens holes in the pleats and warps the frame. Air escapes around the damage and the rating stops meaning anything. Wash the pre-filter instead, every two to four weeks. That alone restores most of the airflow people assume they have lost.
Where should it sit, and does leaving the door open ruin it?
Give it 12 to 24 inches of clearance on the intake side, keep it out of corners, off thick rugs if the intake is underneath, and clear of curtains. An open door does not ruin it, it enlarges the room: a 200 square foot bedroom joined to a hallway behaves like 350 square feet, so close the door or size up.
