HEPA vs MERV-13: what's the difference?
In one line: both capture the fine PM2.5 particles that matter for health. True HEPA grabs more of the very smallest particles per pass, but restricts airflow; MERV-13 lets a fan move much more air for the same noise. Because room air recirculates many times an hour, the gap in how fast a room actually clears is far smaller than the headline percentages suggest.
What the two ratings mean
HEPA (High-Efficiency Particulate Air) is a certified standard, not a marketing word. Under the European EN 1822 scale, H13 means at least 99.95% capture and H14 at least 99.995%, both measured at the filter's hardest particle size. The American convention usually quoted is 99.97% at 0.3 µm. The two are close but arrived at differently, and they are not interchangeable.
MERV-13 comes from the MERV scale (Minimum Efficiency Reporting Value, 1–16) defined in ASHRAE 52.2 for HVAC filters. It is specified as a set of minimum averages across three particle-size bands rather than a single number:
| Particle size band | MERV-13 minimum | What lives there |
|---|---|---|
| 0.3 – 1.0 µm | ≥ 50% | fine smoke, combustion particles, fine aerosols |
| 1.0 – 3.0 µm | ≥ 85% | most of PM2.5, fine dust, mould spores |
| 3.0 – 10 µm | ≥ 90% | pollen, pet dander, coarse dust |
Those are floors, not typical performance — a good MERV-13 media usually exceeds them. But notice that the headline "50%" applies only to the finest band, while the particles most people actually buy a purifier for sit in the bands above it.
The bit almost no buying guide explains
Filter efficiency is not a straight line that falls as particles get smaller. It is a U-shape.
Large particles are easy: they cannot follow the air as it bends around a fibre, so they hit it and stick. That is interception and impaction. Very small particles are also easy, for the opposite reason — they are so light that they jiggle about on air molecules (Brownian motion) and wander into a fibre almost by accident. That is diffusion.
In between, around 0.1–0.3 µm, neither mechanism works well. Particles there are too light to be thrown into fibres and too heavy to drift about much. This is the most penetrating particle size, and it is the worst a filter ever performs.
Both standards quote efficiency at or near that worst point. So HEPA's 99.95% and MERV-13's 50% are not comfortable averages — they are each filter's low-water mark, and both perform better than their own headline figure at almost every other size. Buying guides that treat 50% as "half the particles get through" have misread the specification.
Why a lower-efficiency filter still clears a room
The other thing that single-pass efficiency hides is that room air goes round again. A purifier does not get one attempt; it gets however many passes the air-change rate gives it. Particle concentration therefore falls exponentially, governed by clean-air delivery divided by room volume.
Take a 25 m² living room with a 2.4 m ceiling — 60 m³. Run two machines in it for one hour, ignoring fresh pollution coming in:
| MERV-13 unit | HEPA tower | |
|---|---|---|
| Clean-air delivery | 369 m³/h | 350 m³/h |
| Air changes per hour | 6.2 | 5.8 |
| Particles remaining after 1 hour | ~0.2% | ~0.3% |
Both clear more than 99% of what was in the room. The filter grades differ enormously on paper; the outcome in the room does not. What does differ is how many watts and how many decibels each machine spent getting there — and that is the comparison worth making.
Where HEPA genuinely wins
Three cases, and they are real. In an operating theatre or cleanroom, where the requirement is near-absolute removal on a single pass and energy cost is irrelevant, HEPA is the correct answer. In a sealed system — ducted, gasketed, pressure-tested — HEPA's performance can be relied on, which matters when the consequence of a leak is serious. And for very fine ultrafine particles in the tens of nanometres, HEPA media holds a genuine edge.
One caveat cuts the other way, though. A filter only performs as well as its seal. Air takes the easiest route, so a dense HEPA element in a loosely built housing invites air to slip around the edges rather than through the media — and bypass is invisible from the outside. A well-sealed MERV-13 unit can outperform a poorly sealed "HEPA" one in practice, which is one reason independent CADR measurement of the whole machine beats reading the grade off the filter inside it.
Where they differ in practice
| True HEPA | MERV-13 | |
|---|---|---|
| Captures PM2.5? | Yes (99.95%+ at its worst size) | Yes (≥85% through most of the PM2.5 range) |
| Airflow resistance | High (chokes the fan) | Low (fan moves more air) |
| Clean-air delivery | Lower per watt of fan | Higher per watt of fan |
| Noise for same clean air | Higher | Lower |
| Filter cost | Often proprietary, £50–80/yr | Standard media, cheaper |
| Replaceable anywhere? | Usually one supplier | Standard sizes, many suppliers |
The pressure point is the one that drives the rest. Fan power rises with both the air moved and the pressure it must be pushed against, so dense media costs watts twice over: once directly, and again because you need a bigger fan — which is louder — to get the air through at all.
So which should you choose?
For a home dealing with dust, pollen, pet dander, smoke and everyday PM2.5, the thing that matters is moving a lot of clean air quietly and cheaply enough that the machine stays switched on. That is where high-airflow MERV-13 does well, and it is the principle behind the Corsi-Rosenthal box: surround a fan with several MERV-13 filters so it delivers a very high CADR at low noise and low cost.
The practical test is not "which filter grade is highest" but "which unit delivers the most clean air, quietly, at a running cost that means I never switch it off". A superb filter in a machine too loud to sleep beside cleans nothing at night.
Real UK figures: clean air per pound
What the "airflow" difference means in practice — reference figures at a UK rate of ~£0.25/kWh, 12 hours a day:
| Unit type | Filter | CADR | Noise (quiet) | Power | Filter cost/yr | Running cost/yr* |
|---|---|---|---|---|---|---|
| Luggable 5-Fan | MERV-13 | 369 m³/h | under 35 dB | 6–16 W | ~£20–30 | ~£7–18 |
| Typical HEPA tower | True HEPA H13 | 300–400 m³/h | 50–60 dB on high | 40–70 W | ~£50–80 | ~£44–77 |
| DIY Corsi-Rosenthal | 4× MERV-13 | 500–800 m³/h | ~45 dB | 30–50 W | ~£40–60 | ~£33–55 |
*Our estimate from published wattage and current UK electricity pricing. The point: a low-restriction MERV-13 unit hits the same or higher CADR as a HEPA tower while drawing far less power and staying quieter, because the fan isn't fighting a dense filter.
Free calculator: how much clean air does your room need?
Whichever filter you pick, it's the clean-air delivery rate that has to match your room. Work out the target:
Live UK air quality
When outdoor PM2.5 spikes, a higher-airflow unit clears it faster. Here's the reading now:
Next step
Compare units on measured clean-air delivery, noise and running cost at Best Air Purifier UK, or read the Corsi-Rosenthal explainer.
Need replacement filters? Standard 3M Filtrete MERV-13 in 16×25 and 20×25 — ordinary sizes, no proprietary cartridge and no lock-in.
FAQ
Is MERV-13 as good as HEPA for viruses and smoke? Both capture the fine aerosol and smoke particles that matter. HEPA edges it per pass at the smallest sizes, but the recirculation maths above shows why that rarely changes how fast a room clears.
Why is 0.3 µm always the number quoted? Because it is roughly where filters are worst. Quoting efficiency there is the honest, conservative convention — it is a floor, not an average.
What about "HEPA-type" filters? "HEPA-type" or "HEPA-like" is not the certified standard and can capture far less. Look for true HEPA (H13/H14) or a genuine MERV-13 rating.
Does MERV-13 handle smells? No. Odours and VOCs are gases, and no particle filter of any grade removes them — that needs activated carbon.
Is a higher MERV always better? Not in a portable unit. Above MERV-13 the media gets denser and the airflow penalty grows faster than the capture benefit, so delivered clean air can fall even as the rating rises.