CADR Ratings: What the Clean Air Figure Counts and Assumes

A CADR figure counts a volume, never a percentage. ANSI/AHAM AC-1 defines it as the rate of contaminant reduction in the test chamber with the unit on, minus the rate of natural decay with it off, multiplied by the chamber volume in cubic feet. The result is in cubic feet per minute.

Subtracting the natural decay is why the test runs twice. Particles settle out of still air and stick to the walls, so a machine left off would appear to clean a room; AHAM takes that baseline away so the unit is credited only with what it removed.

Two separate properties get folded together. How much air the machine draws in decides one half of the answer. How much 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 says CADR values "are always the measurement of a unit's performance as a complete system". It adds that they "have no linear relationship to air movement or to the characteristics of any particular filter media". The same wording carries into the international documents.

AC-1 runs the same machine against three test particles and reports a separate figure for each: engineered tobacco smoke for fine particles, an engineered fine dust for medium ones, and paper mulberry pollen for coarse ones, spanning 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, candle soot and smoke arriving from hundreds of miles away, with particles 100 to 1,000 times smaller than a human hair. The medium grade is engineered dust, also known as Arizona road dust.

Sizes differ enough that the three results separate. AHAM publishes the 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, 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 existing tests and adds no fourth run.

The chamber is a fixed room, and its size is part of the definition. AHAM gives the volume as 1,008 cubic feet, or 28.4 cubic metres. Most chambers are built at ten and a half by twelve by eight feet, or 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, establishing the natural decay, then the run repeats at the same starting concentration with the machine on. The gap between 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 the floor, and its own airflow pattern decides how much of the chamber reaches it. A recirculation fan against a wall keeps the contaminant mixed, positioned out of the machine's air path so it adds nothing to the score.

One speed setting is tested, the top one. AHAM runs AC-1 at the highest speed because low and medium mean different things across machines, and states plainly that efficacy and room coverage fall when a unit runs slower. A figure quoted off a box belongs to the fastest setting.

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 seal is that arithmetic.

Three assumptions ride inside the 1.55: the room size is set at an 80 percent reduction in smoke particles at steady state, one air change per hour arrives from outside and adjoining rooms, and the ceiling is eight feet. AHAM states the last as a caveat: a higher ceiling 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 predicts: at the top smoke figure of 450, the largest room 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 as the smoke CADR multiplied by 60, divided by the room volume in cubic feet. Floor area alone overstates coverage in any room built taller than the assumption.

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, and that no implication may be made that the seal covers any other feature. Gases, odours and microbiological components sit outside 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. 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, and where a model is sold with alternatives, AHAM requires the literature to say the rating rests on a named filter and may be affected by using another. Power draw enters only for energy-verified models.

The protective move here is short. Where a listing prints a CADR figure beside an odour or germ claim, the two are unconnected: AHAM's rules require non-particulate claims to be kept distinct from certified ones. Ask which pollutant the figure belongs to, and read the rest of the packaging as a separate document.