Air purifier CADR: the number the box leaves out

There is no federal “best air purifier,” and the EPA’s consumer guide is explicit that the agency does not certify or recommend brands. What those pages actually give a buyer is a room-size particle test called clean air delivery rate (CADR), a warning that the ENERGY STAR sticker is about watts, and a refusal to approve ozone machines for rooms people occupy. Those three sentences are the buy check. A square-foot claim on a carton is marketing until it survives them.

CADR is not a ranking. It is a cubic-feet-per-minute number for how fast a portable unit delivers filtered air for three particle sizes. It is typically measured at the unit’s highest speed. It does not rate gases. It does not rate a living room you measured from a listing photo. And it does not make a gadget a substitute for turning off the source.

CADR is three numbers, and you want the small one

Most portable air cleaners that carry an AHAM Verifide seal list three CADR figures: tobacco smoke, dust, and pollen. EPA treats those three as stand-ins for small, medium, and large particles, not as a claim that the machine is only for cigarettes or ragweed. The small number is the one that matters for the particles of greatest health concern. For PM2.5, EPA says to choose a high CADR for tobacco smoke, and to make sure that CADR is large enough for the room you will actually use.

AHAM’s public directory page says the same thing in carton language: the higher those three numbers, the faster the unit filters the air. The seal is usually on the back or the side of the box, not in the hero photo. If a listing shows a room size and no smoke CADR, you do not have the number EPA is asking you to read.

The rating is typically taken at the highest fan speed. Run the unit quieter and you are no longer buying the tested CADR. EPA’s operational line is blunt: higher fan speeds and longer run times move more air through the filter. A machine that is too loud to leave on is a machine you sized from a number you will not use.

Size the room, then look at the smoke CADR

EPA’s sizing method is length times width, in square feet, compared with the maximum recommended room size on the packaging. An open floor plan is the whole space the cleaner would serve, not the rectangle of one rug. Ceilings above eight feet need a unit sized for a larger area. A space larger than any one unit will serve can take more than one cleaner.

EPA’s estimation table, calculated for an eight-foot ceiling, pairs room area with a minimum CADR in cubic feet per minute: 100 square feet with 65, 200 with 130, 300 with 195, 400 with 260, 500 with 325, and 600 with 390. That is an estimate, not a brand ranking, and not a measurement of your joist height.

AHAM’s 2/3 rule is the same idea in a sentence: tobacco-smoke CADR should be at least two-thirds of the room’s area. Their worked example is a 10-by-12 room — 120 square feet — needing a smoke CADR of at least 80. A smaller room just gets cleaned faster. Ceilings higher than eight feet need a unit rated for a larger room. AHAM’s filtration-standards page adds a wildfire line that is stricter than the everyday 2/3 rule: for wildfire smoke, they recommend a smoke CADR equal to the room’s area in square feet. EPA’s wildfire filtration page stays with two-thirds. Those are two primary pages, not a conflict we will paper over. If the carton is being sold for smoke, read both and take the larger number.

EPA’s wildfire page also says to run a portable cleaner continuously on the highest fan setting you will actually leave on, keep airflow unobstructed, and — when it is safe — close the doors and windows of that room. Place it where people spend the time. That is operation, not a model name.

Gases are a different filter, and there is no CADR for them

Most filters are built for particles or for gases, not both. CADR is a particle rating. EPA is direct that there is no widely used performance rating for portable cleaners designed to remove gases. If the problem is volatile organic compounds, the same guide says to look for an activated-carbon or other absorbent filter with a large amount of material — thicker is the word they use — not a second CADR that does not exist. A high CADR plus a real carbon bed can address both. A HEPA-only box will not become a gas cleaner because the marketing paragraph mentioned “odors.”

EPA also says research has not yet shown that plasma, photocatalytic oxidation, or ultraviolet light can remove gases effectively in portable residential cleaners. Those are technologies on a carton, not a substitute for the carbon spec.

Source control first; a purifier will not dry a wet wall

EPA’s order of operations is the part the gadget aisle skips. The most effective ways to improve indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air. Filtration can supplement that. It does not replace it. Portable cleaners and furnace filters can reduce indoor pollution; they cannot remove all of it. They do not solve mold. Mold is a moisture problem. EPA’s line is to get rid of the water and clean up the growth. A cleaner may take some particles, and in some cases some odor, and still leave the wet wall exactly where it was.

Health claims stay inside EPA’s “possibly.” Studies with portable HEPA units have shown small cardiovascular and respiratory improvements that are not always noticeable to the person in the room, and allergy or asthma studies do not always show major improvements or improvements in every symptom. That is the federal ceiling. It is not a cure, and it is not a ranking of SKUs.

CDC’s respiratory-virus page is useful for the same reason: cleaner air is a prevention step, not a product category. The household list is open windows and exhaust fans when you can, run the HVAC fan on “on” rather than “auto” when you have visitors, use pleated filters and change them on the manufacturer’s schedule or about every three months, and use a portable HEPA cleaner. The “five or more air changes per hour” line on that page is aimed at organizations, as a combination of ventilation, outdoor air, and added devices. It is not a federal CADR-to-room formula for a bedroom, and we will not invent one from it.

Ozone is the gadget to walk past

EPA’s ozone-generator page is the one sentence that should end a certain kind of sales pitch: no agency of the federal government has approved these devices for use in occupied spaces. Ozone is a lung irritant. Relatively low amounts can cause chest pain, coughing, shortness of breath, and throat irritation, and can worsen asthma. Vendors who call it “energized oxygen” or “pure air” are renaming a toxic gas. An EPA establishment number on the carton identifies a facility. It is not an endorsement, and it is not a finding that the device is safe or effective.

At concentrations that stay inside public-health standards, EPA says ozone has little potential to remove indoor contaminants, does not effectively remove carbon monoxide or formaldehyde, does not remove particles, and does not effectively remove viruses, bacteria, or mold. The chemistry that does occur can produce other irritants, including aldehydes. Some studies show a machine can exceed health standards even when the owner follows the manufacturer’s directions. That is the opposite of a buy.

The consumer guide adds the quieter version of the same warning. Avoid cleaners that intentionally produce ozone. Electrostatic precipitators, ionizers, UV lights without adequate lamp coatings, and plasma cleaners may emit ozone even when they are not sold as “ozone generators.” EPA points to lists maintained by the California Air Resources Board and by AHAM for units tested to emit little or no ozone. ENERGY STAR’s room-air-cleaner criteria separately cap measured ozone at 50 parts per billion under UL 867 for units that want the label. That is a safety ceiling on a wattage program, not a cleaning medal.

EPA’s coronavirus filtration page treats bipolar ionization the same way: emerging, less documented than filtration, and capable of generating ozone and other by-products unless the design is constrained. If someone is still shopping that category, EPA’s only process note is a device that meets UL 2998 for zero ozone emissions. Filtration is the established method. Ionization is the sales object.

ENERGY STAR is watts. MERV 13 is the other path.

A portable cleaner with an ENERGY STAR mark meets EPA energy-efficiency guidelines. The guide says the label does not refer to air-cleaning effectiveness in any way. The packaging disclaimer is the same idea in legal type: EPA does not endorse manufacturer claims about how much healthier the indoor air will be. ENERGY STAR’s own air-cleaner page measures efficiency as an Integrated Energy Factor — a CADR-to-watt ratio — and says certified units are more than 50 percent more energy-efficient than standard models, which it puts at about 394 kilowatt-hours a year if left running. The dollar range it prints is about $18 to $40 a year depending on size. That is a running-cost comparison, not a particle ranking, and not a reason to buy an undersized unit because it is efficient.

If the house has a forced-air system, EPA’s other path is the furnace filter. Choose at least MERV 13, or as high as the fan and filter slot can take, and have a technician confirm that before you stuff a thicker pleat into a slot that will starve the blower. Filters with MERV 13 and above have to show at least 50 percent removal of the smallest particles tested. The system only filters while it is running, which EPA puts at usually less than 25 percent of the time in heating and cooling seasons. More filtration means longer fan run time, more electricity, and, in cooling season, a possible hit to humidity control. Wildfire guidance is to flip the thermostat fan from “Auto” to “On” when you are using that path, and to make the filter fit so air cannot bypass it. Window units and ductless heads are temperature machines with a little lint screen. They are not whole-room filtration.

What to require on the box

1. A smoke CADR large enough for the room you measured, using EPA’s table or AHAM’s 2/3 rule, and the larger wildfire number if smoke is the job. Length times width. Open plan is the whole space. Ceilings over eight feet size up.
2. The AHAM Verifide seal with all three CADRs, not a room-size headline with the smoke number missing. AHAM’s directory is the public check that a third-party lab verified the claim.
3. No intentional ozone, and no “energized oxygen” rename. EPA has not approved ozone generators for occupied rooms. If the carton is ionization or plasma, you are off the established filtration path.
4. A carbon bed if the problem is gases or VOCs, thick enough to be a real filter. CADR will not tell you that. There is no widely used gas rating to hide behind.
5. A noise rating you will live with at the speed you will actually run, because CADR was taken on high. EPA’s reference point is about 50 decibels, roughly a modern refrigerator. Unobstructed placement in the room where people spend the time.

Those five make two cartons comparable. Having spent eighteen years on the industry side of consumer marketing and lead generation, the pattern I would flag is the box that leads with a square-foot claim, hides the smoke CADR, and treats an ENERGY STAR mark as proof the air will be healthier. Watts are not dirt. EPA does not rank the aisle, and neither does this page.

Questions readers actually ask

What does CADR actually measure?

How fast a portable air cleaner delivers particle-filtered air, in cubic feet per minute, usually at its highest speed. AHAM reports it for tobacco smoke, dust, and pollen as stand-ins for small, medium, and large particles. EPA says to use the smoke CADR when the concern is PM2.5. CADR is not a gas rating, not a brand ranking, and not a measurement taken on the quiet overnight setting.

How do I match CADR to a room?

Measure length times width. EPA’s eight-foot-ceiling estimate is 65 cfm of CADR per 100 square feet (65, 130, 195, 260, 325, 390 as the room goes from 100 to 600 square feet). AHAM’s 2/3 rule is smoke CADR at least two-thirds of that area — 80 for a 10-by-12 room. Size up for tall ceilings and for the whole open plan. For wildfire smoke, AHAM’s standards page wants smoke CADR equal to the room area; EPA’s wildfire page stays at two-thirds. Take the larger number if smoke is why you are shopping.

Does ENERGY STAR mean it cleans better?

No. EPA’s guide says the label is energy efficiency and “does not refer to its air-cleaning effectiveness in any way.” The packaging disclaimer says EPA does not endorse manufacturer claims about healthier indoor air. Use CADR for particles and ENERGY STAR for the electric bill.

Will a purifier fix mold, odors, or viruses by itself?

Not mold: EPA says fix the moisture and clean the growth. Odors and VOCs need a gas filter, which CADR does not rate. On viruses, EPA’s filtration page says a right-sized unit with a high smoke CADR, a HEPA designation, or a claim that it filters the 0.1–1 µm range can reduce airborne particles, including particles that can carry viruses, and that filtration alone is not enough. CDC lists a portable HEPA cleaner as one cleaner-air step next to ventilation. Ozone generators are not the method: EPA says they are unapproved for occupied spaces and, at concentrations that stay inside health standards, do not effectively remove viruses, bacteria, or mold.

Which air purifier is the best?

EPA does not certify or recommend brands. This page does not either. The comparable facts on a carton are smoke CADR versus the room you measured, whether the AHAM numbers were independently verified, whether the unit produces ozone, and whether there is a real carbon bed if gases are the problem. A “best of” list that skips those is an advertisement.

Educational only. This is not medical, legal, or indoor-air advice. I am not an industrial hygienist, physician, or HVAC technician. EPA does not certify or recommend specific brands of air cleaners.

Written by

Keith Guirao

Founder & Editor, ConsumersWeek

18+ years in consumer marketing and lead generation across insurance, personal finance, and home services. ConsumersWeek explains how these products are priced and sold so you can evaluate them with the same information the industry has.

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