CADR ratings: what the clean air figure counts and assumes
Two air cleaners can print the same clean air figure while one pushes plenty of air through a coarse filter and the other pushes little through a dense one. The rating merges those two facts on purpose. What it merges away is worth knowing before spending.
By Team Snowflake | Updated 9 Aug 2026
What does a CADR figure actually count?
A CADR figure counts a volume, and never a percentage. ANSI/AHAM AC-1 defines it as the rate of contaminant reduction in the test chamber with the unit switched on, minus the rate of natural decay with it switched off, multiplied by the volume of the chamber in cubic feet. The result is stated in cubic feet per minute.
Subtracting the natural decay is why the test runs twice. Particles settle out of still air on their own and stick to the walls, so a machine left switched off would appear to clean a room. AHAM takes the baseline away so the unit is credited only with what it removed itself.
Two separate properties get folded together in the process. How much air the machine draws in decides one half of the answer, and how much of the contaminant that air gives up decides the other. A strong fan behind an open filter and a modest fan behind a dense one can land on the same number.
AHAM says as much in its own terms. Its certification guide states that CADR values "are always the measurement of a unit's performance as a complete system". The same passage adds that they "have no linear relationship to air movement or to the characteristics of any particular filter media". Near-identical wording carries into the international documents built on the same metric.
Why a filter class and a flow figure will not combine into it
Neither input predicts the output on its own, and AHAM is explicit about why. Its testing FAQ says that neither the quantity nor the velocity of the airflow, nor the efficiency of the filter, tells a buyer the amount of cleaned air generated. A filter rating describes the air passing through the element. It stays silent on how much of the room's air ever gets there.
So a dense element in a machine that pulls little air past it scores poorly, and no line on the specification sheet shows why. Reading the filter class and the rated airflow side by side will not reconstruct the figure. The test measures both working together, which is why it is run on assembled machines.
Why one machine carries three different numbers
AC-1 runs the same machine against three test particles and reports a separate figure for each. Engineered tobacco smoke stands in for fine particles, an engineered fine dust for medium ones, and paper mulberry pollen for coarse ones. Between them they span roughly 0.10 to 11 micrometres.
All three are surrogates, chosen for consistency in the laboratory. AHAM describes the engineered smoke as a stand-in for cooking particles, soot from candles and smoke arriving from burning hundreds of miles away. Its particles run 100 to 1,000 times smaller than the width of a human hair. The medium grade is the engineered dust also known as Arizona road dust.
Sizes differ enough that the three results separate. AHAM publishes the tested ranges as 0.09 to 1.0 micrometres for smoke, 0.5 to 3.0 for dust, and 0.5 to 11.0 for pollen. A machine can be respectable at the coarse end and ordinary at the fine end, and the seal will show both.
One derived figure arrived later. AC-1-2019 introduced a PM2.5 CADR, obtained by taking a geometric average of the smoke result across 0.1 to 0.5 micrometres and the dust result across 0.5 to 2.5 micrometres. It recombines tests that already exist, and adds no fourth run.
| What the complaint is | Which figure answers it | The band AHAM will certify |
|---|---|---|
| What the complaint isCooking particles, candle soot, haze | Which figure answers itSmoke CADR, 0.09 to 1.0 micrometres | The band AHAM will certify10 to 450 |
| What the complaint isHousehold dust and fine grit | Which figure answers itDust CADR, 0.5 to 3.0 micrometres | The band AHAM will certify10 to 400 |
| What the complaint isPollen and coarse allergens | Which figure answers itPollen CADR, 0.5 to 11.0 micrometres | The band AHAM will certify25 to 450 |
| What the complaint isOdour, cooking gas, solvent fumes | Which figure answers itNone of the three | The band AHAM will certifyOutside the standard |
Where the scale runs out
Each figure carries a floor and a ceiling written into the certification. The ceiling arrives sooner than most buyers expect. Above the top of the band, the chamber cannot tell one machine from another, because the particles run out before the measurement does.
A machine past the top of the scale gets labelled at the scale. The programme requires the seal to read "450 CADR or greater" for smoke, with the same construction used for the other two. So two units both printing 450 have both saturated the measurement, and nothing on the packaging separates them.
The chamber the number comes out of
The chamber is a fixed room, and its size forms part of the definition. AHAM gives the volume as 1,008 cubic feet, or 28.4 cubic metres, with most chambers built at ten and a half feet by twelve feet by eight feet high. In metric that is 3.2 by 3.7 by 2.4 metres.
Two runs make one result. Contaminant goes into the chamber and its concentration is tracked while the machine sits idle, which establishes the natural decay. The run repeats at the same starting concentration with the machine on. What separates the two rates is the CADR.
Placement is deliberately loose. AHAM notes that particles are not forced through the product; the unit sits on a table or on the floor, and its own airflow pattern decides how much of the chamber reaches it. A recirculation fan runs against a wall throughout to keep the contaminant evenly mixed, positioned out of the machine's air path so it adds nothing to the score.
One speed setting gets tested, and it is the top one. AHAM runs AC-1 at the highest speed because low and medium mean different things across machines, and it states plainly that both efficacy and room coverage fall when a unit is run slower. A figure quoted off a box belongs to the fastest setting that machine has.
What the chamber does not have in it
Nothing keeps producing contaminant during the test, and nothing leaks in. The chamber is sealed, the source is a single injection, and what follows is a decay curve down to whatever the instruments can still read.
A flat does neither of those things. Cooking, the corridor, window gaps and the front door keep feeding it while the machine works, so the concentration settles at a balance point instead of falling away. That balance is the thing a resident actually experiences.
AHAM is direct about the boundary. Its FAQ says the test chamber is not intended to represent a certain size room in a home, and describes it instead as a standard chamber allowing accurate, uniform and repeatable measurement. The EPA puts the consequence plainly: a portable air cleaner may not achieve its rated CADR under all circumstances, though the value does allow comparisons among portable air cleaners.
So the figure describes a machine, and the room stays a separate question. A test that let the room vary would produce numbers nobody could line up against each other, which is the reason the chamber was fixed in the first place.
Turning a rate into a room
AHAM publishes one conversion, and it uses only the smoke figure. Room size in square feet equals the smoke CADR multiplied by 1.55, and the square metre version multiplies that result by 0.093. The suggested room size printed on a certification seal is that arithmetic and nothing more.
Three assumptions ride inside the 1.55. The room size is set at an 80 percent reduction in smoke particles held at steady state, it assumes one air change per hour arriving from outside and from adjoining rooms, and it assumes an eight foot ceiling. AHAM states the last one as a caveat: a ceiling higher than eight feet means the square footage covered comes out below the figure shown.
The relationship was not invented for marketing. AHAM records that it was verified by scientists at the National Institute of Standards and Technology, and recognised as reasonable by the US Federal Trade Commission. The same arithmetic caps what the method will predict: at the top certified smoke figure of 450, the largest room it covers is 698 square feet, or 64.8 square metres.
For a ceiling that is not eight feet, the honest calculation runs on volume. AHAM gives the clean air changes a machine delivers in a given room as the smoke CADR multiplied by 60, divided by the room volume in cubic feet. Floor area on its own will overstate coverage in any room built taller than the assumption.
| Clean air changes per hour | Particles held out, at steady state | What that point represents |
|---|---|---|
| Clean air changes per hour1 | Particles held out, at steady state45 percent | What that point representsThe infiltration the room size assumes |
| Clean air changes per hour2 | Particles held out, at steady state62 percent | What that point representsWell below the certified basis |
| Clean air changes per hour4.8 | Particles held out, at steady state80 percent | What that point representsWhere the suggested room size is set |
| Clean air changes per hour6 | Particles held out, at steady state83 percent | What that point representsThree points for a quarter more machine |
| Clean air changes per hour10 | Particles held out, at steady state89 percent | What that point representsTwice the rate, nine points more |
Where the five air changes came from
The familiar advice to aim for about five air changes an hour is the same rule written a second way. Divide 60 minutes by an eight foot ceiling multiplied by 1.55 and the answer is 4.84 changes per hour, which AHAM rounds down to 4.8 for its seal. Room size and air changes are one statement, not two pieces of evidence.
AHAM's own table shows what buying past that point returns. It puts steady state removal at 45 percent for one clean air change per hour, 80 percent at 4.8, and 89 percent at ten. The curve flattens hard above the certified point, so a machine sized far beyond the room is spending on the shallow part of it.
Ventilation is a different measure with a similar shape, and the two get conflated. AHAM notes that air changes per hour describes outdoor air entering a building, tied to refreshing the air instead of cleaning it, and uses the term equivalent air changes for what filtration contributes. Filtration recirculates, and brings nothing in.
What the seal deliberately leaves out
The certification covers three particle results and stops there. AHAM's programme guide states that only the CADR for tobacco smoke, dust and pollen is certified. It adds that no implication shall ever be made that the seal covers any other feature. Gases, odours and microbiological components sit outside the scope of AC-1, and so do sound and ozone emissions.
Each of those went off to a separate document. Sound got its own standard in AHAM AC-2, filter loading was drafted as AC-3, and microbial reduction became a separate international document reporting its result as m-CADR to keep the two apart. A high particle figure predicts none of them.
Filter life and running cost are absent for related reasons. The certified result is tied to one identified filter. Where a model is sold with alternative filters, AHAM requires the literature to say the rating rests on a named filter and may be affected by using another. Power draw enters the certification only for models that are also energy verified.
The protective move here is short. Where a listing prints a CADR figure beside an odour or germ claim, the two are unconnected, and AHAM's own rules require non-particulate claims to be kept distinct from the certified ones. Ask which pollutant the figure belongs to, and read the rest of the packaging as a separate document.
Which machines the number was built for
AC-1 was written for portable room air cleaners, meaning appliances placed in a room during use. AHAM excludes whole house cleaners mounted in ductwork, on the grounds that their particle dynamics and airflow patterns differ, and states that CADR claims for those units are not comparable to AC-1 claims. A ducted system carrying a filtration figure was measured some other way.
Split systems sit in a third position. The international method, IEC 63086-1, covers combination products including air conditioners with an air cleaning function, and says its test methods "are only aimed at their air cleaning function when tested". A cooling machine can therefore be given a clean air figure, and that figure will describe the filtration alone.
Singapore names the metric without setting a number against it. The NEA page on air cleaning devices tells haze buyers what the rate means: the amount of clean air a device can produce for a room. Knowing that alongside the room size, the page says, supports an informed purchase. No minimum is published, and no local scheme certifies the figure. Whatever appears on a box here was measured somewhere else.
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