Reference
Every term the box uses, defined the same way this site treats figures: what it measures, what it does not, and where the published number lives. 24 terms, 32 linked guides.
24 terms across 4 groups · every one states what it measures and what it does not
The published table assumes a standard ceiling. Taller rooms change the volume, not the row.
The EPA’s published minimums assume an 8-foot ceiling. A 200 sq ft room with a 10-foot ceiling holds 2,000 cubic feet against the standard 1,600, the correction is arithmetic you apply yourself, and the site says plainly that it is judgement, not a published row.
Read the full page:High ceilings and CADR
Air changes per hour, how many times a machine can turn over the room’s air volume.
Air changes per hour is background arithmetic, not a box rating: room volume times a target number of changes gives the airflow you would need. It is useful for understanding why the published minimums look the way they do, but it is not a certified figure for any machine.
Read the full page:What is an air change per hour?High ceilings change the volume
Adsorbs some gases and odours. There is no published CADR for gases, including this one.
Activated carbon holds gas molecules on its surface, which is why it appears in machines aimed at odours. What it does not have is a published clean air delivery rate: the EPA table rates particles only. Any carbon claim about odour removal is therefore an instruction to read the fine print, not a number to compare.
Read the full page:What is activated carbon?The honest pet-smell page
Measures the air; cleans nothing. Useful for checking whether the plan is working.
A monitor reads particle levels (and sometimes gases) in the room. It is the checking half of the equation, a way to see whether the machine you chose for the room is keeping up. It does not substitute for sizing the machine from the published table in the first place.
Read the full page:Are monitors worth it?
Clean air delivery rate, the volume of filtered air a machine delivers, in cfm.
Clean air delivery rate is the one number that lets you compare two machines honestly. It says how much filtered air comes out, measured in cubic feet per minute. The EPA publishes a minimum CADR for each room area, and every sizing page on this site is built on those published rows.
Read the full page:What is CADR?CADR vs the coverage claim
Cubic feet per minute, the unit the published CADR minimums are written in.
A machine rated at 130 cfm delivers 130 cubic feet of filtered air every minute. That is the whole unit, no hidden conversion, no room-size assumption folded in. When a box quotes a different unit, the comparison stops being honest until you convert.
Read the full page:What CADR do I need?
The room size a maker claims a machine suits, derived from CADR, not independently rated.
Coverage claims are marketing arithmetic performed on the CADR figure, and different makers perform the arithmetic differently. That is why this site sizes rooms from the published CADR minimums and treats coverage numbers as a claim rather than a rating.
Read the full page:Why coverage claims mean very littleThe five sizing mistakes
A certification about electricity use. It is not a performance rating.
ENERGY STAR labels machines that meet an efficiency standard for how much electricity they draw. That is genuinely useful for a device that may run for hours every day, but it says nothing about how much filtered air the machine delivers. Clean air delivery remains the published table’s job.
Read the full page:What ENERGY STAR means here
A filter that removes at least 99.97% of particles at the hardest size, 0.3 micron.
HEPA stands for high-efficiency particulate air. The definition is precise: at least 99.97% of particles at 0.3 micron, the size filters find hardest to catch. A machine can only deliver its rated CADR while the filter is genuinely meeting that standard, which is why the filter and the number belong to the same conversation.
Read the full page:What is a HEPA filter?
Charges particles so they stick to surfaces or plates. Most carry no published CADR.
An ioniser charges airborne particles so they cling to walls, fabrics or a collector plate instead of staying in the air. The approach can lower particle counts, but most ionising machines have no published CADR, and ionisation can produce ozone as a by-product, a trade the box rarely headlines.
Read the full page:What is an ioniser?
A rating for furnace and HVAC filters, from 1 to 16. It is not a room-size rating.
MERV grades how well a filter installed in a central heating or cooling system captures particles. It says nothing about how much air a portable machine moves, so it cannot be read as a room-size rating. If your question is about one room, the published CADR table answers it; MERV answers the central-system question.
Read the full page:What is a MERV rating?Furnace filter or air purifier?
The level the EPA cites as capturing at least 50% of the smallest particle range.
The EPA’s guidance points to MERV 13 as the level that captures at least 50% of particles in the 0.3–1 micron range, while still letting a typical residential system breathe. Higher MERV numbers capture more but load the fan more, the system manual, not the filter aisle, decides the ceiling.
Read the full page:How MERV is gradedCentral filter vs portable machine
A micrometre, one millionth of a metre. The scale every particle rating is written in.
A micron (µm) is one millionth of a metre. Human hair is roughly 50–100 microns across; the particles the ratings care about are far smaller. The 0.3 micron mark in the HEPA definition matters because it is the size filters find hardest, not the smallest they catch.
Read the full page:Why 0.3 micron is the benchmark
Decibels measured at one speed under one condition, not comparable between brands.
A decibel figure on a box is a measurement taken at a specific fan speed under conditions the maker chose. Two machines quoting the same number can sound very different in the same bedroom. The honest use of the figure is for one machine across its own speeds, and the sizing fix for noise is headroom: a larger CADR run on a lower speed.
Read the full page:Noise and the sizing fixQuiet machines, honestly framed
Deliberately produces ozone. The EPA advises against using one in an occupied space.
Ozone generators are sold as air cleaners, but ozone is a lung irritant and the EPA advises against using devices that deliberately produce it in occupied spaces. This site gives these machines no rating, no comparison and no purchase link, the honest answer is the refusal itself.
Read the full page:The full ozone page
The first stage, catches hair and larger debris so the main filter lasts its interval.
A pre-filter is the coarse first line: hair, lint, visible dust. It protects the expensive main filter from loading up early. On most machines it is washable, and keeping it clean is the one maintenance task that clearly pays for itself.
Read the full page:What is a pre-filter?Pre-filter maintenance intervals
Fine particles 2.5 microns and smaller, the range with the published ratings.
PM2.5 describes fine inhalable particles 2.5 microns across and smaller. Particles in this range are exactly what the published CADR ratings are about: smoke, fine dust and the like, where a machine’s clean air delivery rate is measured and comparable.
Read the full page:What is PM2.5?Wildfire smoke, rated
Particles 10 microns and smaller, the coarse end, also covered by the published rating.
PM10 covers the coarser particle range up to 10 microns: dust, pollen fragments, carpet fibres. Machines carry published CADR for these too, and because larger particles are easier to catch, coarse-particle delivery rates tend to be the strongest figure on the sheet.
Read the full page:What is PM10?Household dust, rated
How often to change a filter, set by the manufacturer, because no public standard exists.
There is no public standard for how long a filter lasts; the manufacturer’s stated interval is the only figure on the table. The practical guidance on this site is that the signs beat the calendar, visible loading, smells that return, airflow that drops, with the stated interval as the outer bound.
Read the full page:HEPA replacement intervalsWhere the money actually goes
The EPA’s six rows, 100 sq ft needs 65 cfm, up to 600 sq ft needs 390 cfm. Nothing beyond.
The published table runs from 100 sq ft (65 cfm) to 600 sq ft (390 cfm) in six steps, assumes an 8-foot ceiling, and stops there. The site treats those six rows as the whole dataset: sizes between rows round up to the larger published row, and rooms past 600 sq ft get an honest note instead of an invented figure.
Read the full page:The full sizing methodWhole-house vs per-room sizing
A marketing label with no standard definition behind it. Check the stated efficiency instead.
There is no independent standard that separates “True HEPA” from ordinary HEPA, the prefix is marketing shading, not a higher grade. The useful check is the same either way: the stated efficiency at the stated particle size, written as a figure rather than an adjective.
Read the full page:What is a True HEPA filter?
A supplementary light stage. Residential effectiveness is not published as a CADR.
UV-C light can inactivate microorganisms given enough dose and dwell time, conditions a moving airstream in a small box struggles to guarantee. Residential UV stages carry no published CADR, so the site treats them as supplementary at best and sizes nothing by them.
Read the full page:What is a UV air purifier?
Volatile organic compounds, gases, and gases have no published CADR to size by.
Volatile organic compounds are gases released by paints, cleaning products, furnishings and similar sources. Because they are gases rather than particles, they fall outside the published CADR table entirely. Source control and ventilation lead; any machine claim about VOCs is a claim, not a rating.
Read the full page:The formaldehyde pageWhat machines are rated to do
A filter you rinse instead of replace. Cleaning the schedule is the whole job.
Washable filters trade replacement purchases for a cleaning routine. They work, with two honest caveats: cleaning intervals are set by the manufacturer like any other filter, and a washed filter must dry fully before going back or it becomes its own smell source.
Read the full page:Washable filter intervalsWhen a machine starts smelling