Air purifiers are marketed as health devices, but their real-world performance hinges on two technical figures that most buyers never learn to read: the filter class and the clean air delivery rate. Understanding both is the difference between a unit that genuinely clears a bedroom and an expensive fan that circulates dust. TechNews DevTop breaks down what the official guidance from the US EPA, the WHO and Spain’s Ministry of Health actually says — and where the marketing outruns the evidence.
- HEPA has a precise definition: Under US EPA rules, a true HEPA filter captures at least 99.97% of particles at 0.3 microns, the hardest size to trap. In Europe, only classes H13 and H14 qualify under standard UNE-EN 1822.
- CADR is the sizing metric: Clean Air Delivery Rate measures how much filtered air a unit delivers. A simple formula — room area × ceiling height × 5 — gives the minimum CADR in m³/h.
- Purifiers have hard limits: HEPA filters trap particles, not gases. They do not reduce CO₂, eliminate volatile organic compounds, or fix the damp that causes mould.
What ‘HEPA’ Actually Means
A HEPA filter is a pleated mechanical filter designed to retain nearly all airborne particles, and the term carries official definitions with concrete numbers. According to the EPA, a HEPA unit must capture at least 99.97% of particles measuring 0.3 microns — the most penetrating particle size, or MPPS. Larger and smaller particles are captured even more effectively.
In Europe, the reference is standard UNE-EN 1822, which classifies filters into groups. Spain’s Ministry of Health, in a November 2020 technical document on aerosols, reproduces the 2020 classification table: the EPA group spans classes E10, E11 and E12 with minimum efficiencies of 85%, 95% and 99.5% respectively; the HEPA group includes H13 at 99.95% and H14 at 99.995%; and the ULPA group runs from U15 to U17 at 99.9995% or higher.
The practical takeaway: only H13 and H14 are HEPA under the European standard. An E11 or E12 is not, and packaging that says ‘HEPA-type’ without naming the class tells you nothing verifiable. The Ministry translates this into a concrete example: with an H13 filter, 50 particles out of every 100,000 at the hardest-to-catch size escape.
CADR: The Number That Decides Fitness for Your Room
CADR (Clean Air Delivery Rate) measures how much filtered air the device delivers, and it is the figure used to size a purifier. An H14 filter moving little air cleans a large room worse than an H13 with a generous fan.
The US manufacturer association AHAM, which certifies these measurements, reports three separate CADR values: tobacco smoke, pollen and dust. The EPA explains these represent small, large and medium particles, and that anyone concerned about fine particles — PM2.5, the ones of greatest health interest — should look at the smoke CADR. Note that CADR is typically measured at maximum speed, and units filter less at the low speeds used overnight to keep noise down; the EPA cites 50 dB as roughly equivalent to a modern refrigerator.
In Europe, CADR is usually given in m³/h; in the US, in cubic feet per minute (cfm), where one cfm is about 1.7 m³/h.
How to Calculate the Right Size
The most widespread rule is AHAM’s: the smoke CADR in cfm should be at least two-thirds of the room’s floor area in square feet, assuming 8-foot ceilings (about 2.44 metres). The EPA publishes the same table: for 200 square feet (about 18.6 m²), a minimum of 130 cfm.
Converted to metric units, that rule equates to renewing the room’s air roughly five times per hour. Hence a simple formula:
Minimum CADR (m³/h) = floor area (m²) × ceiling height (m) × 5
- A 12 m² bedroom with a 2.5 m ceiling: 12 × 2.5 × 5 = 150 m³/h.
- A 25 m² living room with a 2.5 m ceiling: 25 × 2.5 × 5 = about 310 m³/h.
With high ceilings or open-plan spaces, count the entire volume. For wildfire smoke, AHAM raises the bar by 50%, requiring a smoke CADR equal to the floor area in square feet. And if the required figure exceeds what a single unit can deliver, the EPA suggests using several.
What a Purifier Cannot Do
A HEPA filter traps particles, not gases: it does not eliminate CO₂ or volatile organic compounds, nor does it resolve the damp that causes mould. Gases require an activated carbon filter, and the EPA warns that carbon only works well when a large amount of material is present — the thicker the better — and that no widely used rating system exists to measure its effectiveness: CADR applies only to particles.
On CO₂, the Ministry is blunt: these devices do not reduce CO₂ levels. CO₂ is the indicator used to judge whether a room is well ventilated, and the same document sets 800–1,000 ppm as the guideline threshold that should not be exceeded. A purifier can leave the air dust-free while the CO₂ meter reads red.
Mould is similar: the EPA explains that the filter retains some spores, but the cause is moisture, and no device fixes that.
As for extras, the EPA advises avoiding units that intentionally generate ozone, a lung irritant, and notes that ionisers, electrostatic precipitators, some ultraviolet lamps and plasma devices may emit it.
Does It Improve Health? What the Official Bodies Say
The official answer is a qualified ‘possibly’, and all three bodies agree on the hierarchy: remove the pollution source and ventilate first, with filtration as a complement.
- EPA: several studies of portable HEPA purifiers have shown small improvements in cardiovascular and respiratory health, usually not perceptible to the person but measurable. In allergies and asthma, improvements have been found in one or more symptoms, but not always significant and not across all symptoms. And filtration does not replace ventilation.
- WHO: in its ventilation Q&A, it states that air filters do not ventilate and do not replace other ventilation methods, though they can reduce airborne virus concentrations; as a complement it suggests a standalone unit with a MERV14 or ISO ePM1 70–80% filter.
- Ministry of Health: in 2020 it recommended them only when sufficient ventilation was not possible, warning that their area of influence is limited and that users find it hard to verify they work as they should.
The conclusion is that the purifier is sold as a health device when, according to these bodies, it is a support tool with a modest effect. It makes sense in specific cases: pollen allergy, a heavy-traffic street, smoke episodes, or a home where frequent ventilation is not viable. What makes no sense is buying one to stop opening the windows. If you are considering a unit, run the CADR calculation for your room, demand H13 or H14 class in writing, rule out any ozone-generating feature, and budget for replacement filters.