CADR, HEPA and Room Size: The Air Purifier Numbers That Actually Mean Something

Last updated: September 21, 2026

We may earn a commission when you buy through links on this page, at no extra cost to you. How we research and rate.

The short version

Air purifier marketing runs on three numbers, and only two of them are defined. HEPA has a precise technical meaning: a filter that removes at least 99.97% of particles at 0.3 microns. CADR is a measured rate of clean air delivery, and the EPA’s rule is simple — the higher it is, the more particles removed and the larger the area served.

The third number, the square footage on the front of the box, is the one to treat carefully. It is a calculation derived from CADR, and the assumptions behind it vary between manufacturers.

The EPA is also unusually direct about one category: avoid air cleaners that intentionally produce ozone. Its position is that ozone is a lung irritant and that no federal agency has approved such devices for use in occupied spaces.

What HEPA actually commits to

Why 0.3 microns, and why that is the clever part

The instinct is to assume the smallest particles are hardest to filter, and that smaller means it slips through. Filtration does not work that way.

Large particles are caught by simple interception and impaction — they cannot follow the air around a fiber. Very small particles move erratically through Brownian motion and collide with fibers constantly. The hardest size to capture sits between those two behaviors, around 0.3 microns, which is why the standard is written at exactly that point.

The EPA’s wording is worth keeping: a HEPA filter “can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and other airborne particles with a size of 0.3 microns,” and “particles that are larger or smaller are trapped with even higher efficiency.”

So a true HEPA filter is not “99.97% on average”. It is 99.97% at the worst case, and better on either side of it. That is a genuinely strong specification, and it explains why the term is worth protecting — and why terms that sound adjacent are worth reading carefully. “HEPA-type”, “HEPA-style” and “99% filtration” are not the same claim, and a listing that avoids the plain word usually has a reason.

Our guides to air purifiers for large rooms, allergies, pets, smoke and mold record which listings state true HEPA and which reach for an adjacent phrase.

CADR: the number that describes performance rather than parts

HEPA describes the filter. CADR describes the machine. A purifier with an excellent filter and a weak fan moves very little clean air, and CADR is the measure that catches that.

Clean Air Delivery Rate is expressed as an airflow figure, and it is usually published as three numbers, because particles of different sizes behave differently:

A listing quoting one CADR number without saying which is quoting the flattering one. If a single figure appears, it is almost always pollen. The smoke figure is the most informative for wildfire smoke, cooking particles and general fine particulate, which is what most people are actually buying a purifier for.

The EPA’s guidance on using it is refreshingly plain: “the higher the CADR, the more particles the air cleaner will remove and the larger the area it can serve”, and portable air cleaners “often achieve a high CADR by using a high-efficiency particulate air (HEPA) filter”.

Room size claims are a calculation, not a measurement

This is where most purchases go wrong, because the square footage on the box is the number shoppers compare and the least standardized thing on it.

The EPA’s sizing method is one you can do yourself: “multiply the length and width of the area in which you intend to operate the air cleaner” and compare that against the product’s specifications, using its sizing chart of minimum CADR values by square footage.

The variable that manufacturers exploit is air changes per hour. A given CADR cleans a small room many times an hour, or a large room fewer times. Quote the coverage at a low number of air changes and the same machine covers a far bigger space on paper. This is why two purifiers with similar CADR figures can claim wildly different coverage, and why our large room guide compares the CADR rather than the claimed area.

A practical rule that avoids the whole argument: for a bedroom or living room where you care about smoke and fine particles, compare the smoke CADR against the room’s square footage directly, and be skeptical of any machine claiming to cover a large open-plan space on a modest CADR.

Ozone: the one category the EPA tells you to avoid

Most agency guidance is carefully neutral. This is not.

The EPA’s advice is to “avoid portable air cleaners and furnace/HVAC filters that intentionally produce ozone”, noting that ozone is a lung irritant and that no federal government agency has approved these devices for use in occupied spaces.

The devices in question are sold under several descriptions — ozone generators, “activated oxygen”, “energized oxygen” — and some ionizers produce ozone as a byproduct even when that is not their selling point. If a listing markets ozone as a feature, the EPA’s position is unambiguous. If it markets ionization without mentioning ozone, the question worth asking is whether ozone output has been tested and published.

This matters most in the categories where people are most desperate for a solution: smoke, mold and odor. Our guides to smoke and mold purifiers treat ozone output as a disqualifying feature rather than a differentiator.

What an air purifier cannot do

The EPA frames filtration as the third priority rather than the first, and the ordering is worth taking seriously because it saves money.

Its position: “the most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air”, with air cleaners as a supplement. Portable units are “designed to filter the air in a single room or area” rather than a whole home, and they “cannot remove all pollutants from the air”.

Three consequences that contradict how these products are sold:

• Source control beats filtration. A gas hob used without extraction, a damp wall, a smoker indoors — a purifier is bailing water while the tap runs.
• One unit does not clean a house. Coverage claims are per room, and doors matter.
• Gases and odors are a different problem from particles. HEPA captures particles; volatile organic compounds need activated carbon, in a quantity most consumer machines do not carry.

Our explainer on air purifiers versus humidifiers covers a related confusion, since the two machines solve entirely different problems and are frequently bought in place of one another.

Filters: the running cost nobody calculates

The purchase price is the smaller number over a machine’s life, and the EPA’s maintenance point is the one most owners ignore: “all filters need regular replacement. If a filter is dirty and overloaded, it won’t work well.” Manufacturers typically recommend changing filters every 60 to 90 days.

That turns into real arithmetic. A machine with a cheap sticker price and expensive proprietary filters costs more over three years than a more expensive unit with reasonable consumables. Before buying, the number worth finding is the filter price multiplied by the changes per year, added to the purchase price and divided across three years.

Two details that change the sum:

• Pre-filters that can be washed extend the life of the expensive HEPA element behind them, because they catch hair and large dust first.
• Combined filter cartridges mean you replace the carbon and the HEPA together on the shorter of the two schedules, which is usually the carbon’s.

Our air purifier and humidifier combo guide covers the case where one machine carries two sets of consumables, which is where running costs get least transparent.

The technologies inside the box, and what each is for

Most machines combine several stages, and the marketing usually names all of them without explaining which one is doing the work.

The carbon question is the one that separates similar-looking machines. Odor and VOC removal scales with how much carbon is present and how long air spends in contact with it. A few hundred grams in a proper bed behaves very differently from a carbon-impregnated mesh, and listings almost never give a weight. If odors are your reason for buying, that missing number is the one to chase.

The UV stage is the one most often oversold. Ultraviolet light can inactivate microorganisms given sufficient intensity and exposure time, and air passing a small lamp at speed provides very little of the second. Treat it as a minor addition rather than a reason to choose a machine.

Placement, doors and the habits that decide the result

An appliance’s specification describes a test chamber. Your room is not one, and a few habits close most of the gap.

• Give it clearance. Intakes and outlets blocked against a wall or behind furniture throttle airflow, and airflow is the CADR.
• Close the door. Coverage figures assume a defined space. An open-plan area or an open door turns a bedroom unit into an underpowered whole-floor unit.
• Run it continuously on a moderate speed rather than intermittently on high. Particles are generated continuously by cooking, movement and outdoor air, so a machine that runs all day at a modest rate beats one run in bursts.
• Put it where the pollution is. Near the kitchen for cooking particles, in the bedroom for overnight allergy relief, near the entrance for outdoor smoke ingress.
• Close windows during smoke events, and reverse that advice the rest of the time. Ventilation is the first line for most indoor pollutants and the wrong move when outdoor air is worse than indoor.

The four cases people actually buy for

Allergies. The target is pollen, dust mite debris and pet dander — all particles, all squarely within what HEPA handles. Prioritize a high smoke CADR relative to bedroom size and run the machine overnight with the door closed. Our allergy purifier guide compares on exactly that.

Smoke. Wildfire smoke is fine particulate, which HEPA captures well, plus gases that need carbon. This is the case where the smoke CADR figure and the carbon quantity both matter, and where sizing generously pays off because you will want to run at a speed you can tolerate for days. See our smoke purifier guide.

Pets. Dander is a particle problem and odor is a gas problem, so this is the combination case: HEPA for the first, real carbon for the second, and a washable pre-filter because hair will clog everything otherwise. Our pet purifier guide weighs those together.

Mold. Here the honest answer is uncomfortable: a purifier captures spores in the air and does nothing about the source. Damp is a building problem, and filtration is symptom management until the moisture is fixed. Our mold purifier guide says so explicitly, and it is the category where ozone-generating devices are most aggressively marketed to people who want a shortcut.

How to tell whether it is working

Air quality is invisible, which is why this category tolerates so much unverified marketing. Two ways to get feedback:

A particle monitor. Inexpensive PM2.5 monitors report fine particle concentrations in real time. Watching the number climb while something fries on the hob and fall after the purifier runs is the most convincing demonstration available, and it also tells you whether your machine is sized correctly for the room.

Symptom tracking. Less precise but more relevant for allergy sufferers: note morning congestion over a fortnight with the machine running and a fortnight without. Bedrooms are the best test case because exposure is long and conditions are stable.

Both beat trusting an onboard air quality light, which on most consumer machines is an uncalibrated sensor reporting a color rather than a number.

What this category gets right that food labeling does not

A closing observation, because the contrast is instructive.

Air purifiers are sold with a measured performance figure — CADR — and a filter specification with a defined threshold. Neither is perfect: CADR is measured at maximum fan speed, and coverage claims stretch the arithmetic. But both are numbers, produced by a test method, and comparable between products.

Very little of the food and personal care we cover has an equivalent. There is no CADR for a face cream, no defined efficiency threshold for “natural”. The categories where shoppers feel most confident are often the ones with the least measurement behind them, and the ones that feel most technical frequently have the most.

The practical lesson transfers: when a number with a method behind it exists, use it, and discount everything written around it. That is the whole approach we take to certifications elsewhere on this site.

Reading an air purifier listing

1. Look for the plain word HEPA and treat “HEPA-type” or a bare percentage as a different, weaker claim.
2. Find all three CADR figures. If only one is given, assume it is pollen and look for smoke.
3. Do the room arithmetic yourself using the EPA method rather than trusting the coverage claim on the box.
4. Rule out anything that markets ozone, and ask about ozone output for anything marketing ionization.
5. Price the filters for three years before comparing machines.
6. Check noise at the speed you will actually run it. A purifier that is only quiet on its lowest setting is a purifier running at a fraction of its CADR.

That last point deserves emphasis because it is where specifications and real life diverge most. Published CADR is measured at maximum fan speed. A bedroom machine run on low all night is delivering a fraction of its headline number, which is an argument for buying a larger unit and running it slowly rather than a small one at full power.

Cost over three years, worked through

Because filters dominate the lifetime cost, the comparison that matters is rarely the one on the shelf edge. The arithmetic is simple enough to do in a shop.

1. Purchase price of the machine.
2. Filter price for a full replacement set, including carbon if it is separate.
3. Replacements per year, using the manufacturer’s interval rather than an optimistic one. Most quote 60 to 90 days for combined filters, six to twelve months for separated HEPA elements.
4. Multiply and add: purchase price plus three years of filters.

Run that on two machines and the ranking frequently reverses. A unit that is cheaper by fifty dollars and takes proprietary cartridges twice a year at sixty dollars costs substantially more across three years than a dearer machine with a washable pre-filter and an annual HEPA element.

Two traps in the filter market are worth naming. Proprietary cartridges with no third-party alternatives give the manufacturer pricing power for the life of the machine. And “filter life” counters on many units are timers rather than sensors: they count days of running, not particles captured, so they will tell you to replace a filter in a clean room on the same schedule as a filthy one.

Noise, and why it decides whether the machine gets used

Every specification on the box is measured at maximum fan speed. Every real installation runs slower than that, because maximum is usually too loud for a room people occupy.

The consequence is direct: a machine’s usable CADR is its CADR at the speed you can live with. That is why buying a larger unit and running it at a low setting generally beats buying a smaller one and running it flat out. The larger machine moves the same air more quietly, and the filters last longer because they work less hard for the same result.

Three practical notes:

• Check the decibel figure at the setting you plan to use, not the headline “as quiet as 24 dB” figure that applies to the lowest speed only.
• Sleep modes usually dim the lights and drop the fan, which is good for sleep and bad for clean air delivery. Size for that reality.
• Low-frequency hum travels through floors more than the specification suggests. If it is going in a bedroom, the tone matters as much as the number.

What we look for in a listing, and what is usually missing

Applying the standard we use across this site, here is what a genuinely informative air purifier listing contains. Very few do.

That distribution is the same one we find in food and textiles: the numbers that would let you compare products are exactly the ones omitted, and the adjectives that cannot be checked are abundant. The difference here is that the two most important numbers, CADR and the HEPA threshold, do exist and are standardized — so the shopper who insists on them is not asking for something unreasonable.

Where organic certification does and does not apply

It does not, and it is worth saying so on a site called Organic Connection.

There is no organic certification for appliances. What exists instead are performance measurements — CADR, HEPA efficiency — and the absence of ozone. Those are the equivalent evidence in this category, and they are considerably more checkable than most food certifications, because they are numbers rather than seals.

Where our usual reading applies is the surrounding marketing: “natural”, “chemical-free air”, “removes 99.9% of viruses” and similar phrases are unregulated in the same way as elsewhere. The numbers above are the part with definitions behind them. Our non-toxic home guide places air quality alongside the other household decisions, and our allergy purifier guide is the one where the HEPA specification does the most work.

Common questions about air purifier specifications

What does HEPA actually mean?

A HEPA filter removes at least 99.97% of particles at 0.3 microns, the hardest size to capture. The EPA notes particles larger or smaller are trapped with even higher efficiency. “HEPA-type” and similar phrases are not the same claim.

What is CADR?

Clean Air Delivery Rate, a measured rate of clean air a machine delivers. It is normally published as three figures for smoke, dust and pollen. The EPA’s guidance is that a higher CADR removes more particles and serves a larger area.

Which CADR number matters most?

Smoke, for most buyers. It covers the smallest particles tested and is the most relevant to wildfire smoke, cooking particles and fine particulate generally. It is also usually the lowest of the three, which is why single-figure listings rarely quote it.

How do I size a purifier for my room?

Multiply the room’s length by its width, then compare that area against the machine’s specifications using the EPA’s minimum CADR guidance. Doing the arithmetic yourself avoids coverage claims that assume very few air changes per hour.

Are ozone air purifiers safe?

The EPA advises avoiding air cleaners that intentionally produce ozone, notes that ozone is a lung irritant, and states that no federal government agency has approved these devices for use in occupied spaces.

Do air purifiers remove smells and gases?

Only with activated carbon, and only in proportion to how much carbon they carry. HEPA filtration captures particles rather than gases, so odor and VOC performance depends on a different component that many machines include only in token quantities.

Does one purifier clean a whole house?

No. The EPA describes portable air cleaners as designed to filter a single room or area. Whole-home filtration is an HVAC question rather than a portable appliance one.

How often do filters need changing?

The EPA notes all filters need regular replacement and that an overloaded filter does not work well, with manufacturers typically recommending changes every 60 to 90 days. Washable pre-filters extend the life of the HEPA element behind them.

Is a more expensive purifier better?

Not reliably. Compare smoke CADR against room size, filter cost over three years, and noise at the speed you will actually use. Those three decide satisfaction more than price does.

Is there an organic or natural certification for air purifiers?

No. This category is judged on measured performance rather than certification, which in practice makes it easier to evaluate: CADR and HEPA efficiency are numbers, not seals.

Sources

• EPA: Guide to Air Cleaners in the Home
• EPA: What is a HEPA filter?
• EPA: Air Cleaners and Air Filters in the Home

Related reading

• Clean Air and Water hub
• Air purifier vs humidifier, explained
• Best Air Purifiers for Allergies
• The non-toxic home guide
• Organic Product Database — compare brands across every guide

Organic Connection is reader supported. We read every product listing in full, check the claims in it, and say plainly when a claim cannot be verified. How we research and rate.