Glossary
Quick answer
CADR is tested airflow, ACH is how many times an hour a room turns over, MERV rates ducted filters and HEPA is a definition rather than a brand. The two thirds rule is a convention, not a standard, and a coverage area claim in square feet means nothing without the air change rate behind it.
| Term | What it means |
|---|---|
| ACH, air changes per hour | How many times the entire volume of air in a room passes through the machine in an hour. Two is a general comfort target, four to five is the figure usually given for allergy households, and six is where smoke guidance sits. It is calculated as CADR times sixty, divided by the room volume. |
| AHAM AC-1 | The ANSI standard test method behind a CADR number. It is run in a sealed 1,008 cubic foot chamber against three particle sizes, which is why a purifier carries three CADR numbers rather than one. It is a standard, not a rule of thumb. |
| CADR, clean air delivery rate | Cubic feet per minute of genuinely filtered air a machine delivers, measured under AHAM AC-1. It is not the same as the fan rating: a machine can move plenty of air through a poor filter and post a high airflow with a much lower CADR. |
| Two thirds rule | A shopper convention published by AHAM: look for a smoke CADR of at least two thirds of the room floor area. It is the standard sizing arithmetic with an eight foot ceiling and five air changes an hour already filled in, since eight times five divided by sixty is 0.667. |
| HEPA | A filter that captures 99.97 percent of particles at 0.3 micron. That size is the hardest for a filter to catch rather than the smallest it catches, because smaller particles are removed efficiently by diffusion. H13 and H14 are the equivalent classes under EN 1822. |
| HEPA type | Not HEPA. It is marketing language for media that did not meet the definition, and the accompanying percentage is usually quoted at a particle size where filtration is easier. Read what size the percentage refers to before comparing two numbers. |
| MERV | Minimum Efficiency Reporting Value, defined by ASHRAE 52.2, running from 1 to 16. It rates filters for ducted systems on their worst performance across three particle size bands, so a filter cannot advertise its easiest result. |
| PM2.5 | Airborne particles under 2.5 microns across, measured in micrograms per cubic metre. This is the fraction that wildfire smoke, cooking and combustion produce most of, and the one indoor monitors report. |
| Activated carbon | Media that adsorbs gases and odours, which a particle filter cannot do. Capacity depends on mass: a thin carbon coated mesh in a combination filter holds very little, and a carbon stage with real weight behind it is a different product. |
| Pre-filter | A coarse screen in front of the main filter that catches hair and large dust. Where it is washable, cleaning it is the cheapest way to keep the expensive stage at the long end of its replacement interval. |
| Ozone | A reactive gas that irritates the airways. Ozone generators produce it deliberately and are sold as air purifiers; some ionizing and plasma type devices produce it as a by-product. California CARB regulates ozone emissions from indoor air cleaning devices and certifies devices sold there. |
| Ionizer | A mode that charges particles so they clump and settle onto surfaces rather than being captured. It is optional on machines that have a real filter, and it is the mode to leave switched off where the technology can produce ozone. |
| Pint rating | A dehumidifier capacity figure, in pints of water removed per day. Current labels are measured under the 2019 DOE test conditions, which run cooler than the old test, so the same machine is labelled roughly thirty percent lower than it once was. |
| Relative humidity | How much water the air holds as a percentage of what it could hold at that temperature. Indoors, 30 to 50 percent is the band to aim for. Above 60 supports mould and dust mites, and in cold weather high indoor humidity condenses on windows. |
| Static pressure | The resistance a filter adds to a ducted system. It is why a denser furnace filter is not automatically better: past what the blower can pull through, a system moves less air and delivers less clean air per hour than a lesser filter would. |
| Coverage area claim | The square footage on a purifier box. It is meaningless without the air change rate it assumes, because the same machine is honestly described as covering a large room at one change an hour and a much smaller one at five. |