Search "best air purifier for allergies" and you will find dozens of machines promising to trap 99.9% of everything airborne in your home. Some of that is genuine engineering. A lot of it is marketing dressed up as a specification. As an independent consumer-health resource with a long history of separating evidence from sales pitch, we think anyone spending £100-£500 on a home air purifier – particularly an older or disabled person managing asthma, allergies or a damp property – deserves to know which numbers are real standards and which are just words on a box.
This guide explains what HEPA, CADR and the British Standard EN 1822 actually measure, what UK health guidance says about whether purifiers genuinely help, and the questions worth asking before you buy an air purifier for asthma or allergy relief.
What "HEPA" Actually Means – And What It Doesn’t
HEPA stands for High-Efficiency Particulate Air, and in the UK and Europe the filter itself should be tested and certified to the British/European Standard EN 1822:2019. That standard defines several efficiency classes, and only two of them are genuinely "HEPA" grade in the strict sense used by hospitals and cleanrooms:
- H13 – captures at least 99.95% of particles at their most penetrating size (roughly 0.1-0.3 microns), which covers most pollen, dust mite debris, pet dander and mould spores.
- H14 – captures at least 99.995% of the same particles, the grade used in operating theatres and cleanrooms, and generally unnecessary for a domestic living room.
The catch is that "HEPA-type", "HEPA-style" or "HEPA-like" are not certified terms at all. They are marketing language for lower-grade filters that typically capture only 85-90% of particles – noticeably worse than a genuine H13 filter, and not tested to EN 1822. If the listing doesn’t say "H13" (or higher) certified to EN 1822, treat the word "HEPA" on the box as a claim, not a guarantee.
CADR: The Number That Tells You Whether It Will Actually Work In Your Room
A filter can be excellent and still leave you with poor air quality if the unit can’t move enough air through it fast enough for your room size. That’s what Clean Air Delivery Rate (CADR) measures – the volume of filtered air the unit delivers, expressed in cubic metres per hour (m³/h) in the UK. CADR is tested against three separate particle sizes, because a machine can perform very differently against each:
| Particle type | Size range | What it represents |
|---|---|---|
| Smoke | 0.09-1.0 microns | The smallest and hardest particles to trap |
| Dust | 0.5-3.0 microns | Household dust, some mould spores and bacteria |
| Pollen | 5.0-11.0 microns | Hay fever and seasonal allergy triggers |
A higher CADR figure generally means faster cleaning, but the right number depends entirely on your room. As a rule of thumb, a purifier should be able to cycle the air in the room at least two to three times an hour – so a unit rated for a room half the size of yours will underperform no matter how good its filter is. CADR also only reflects performance with a clean, fresh filter; it says nothing about how the unit performs once the filter starts loading up with particles, which is why filter-replacement cost and frequency matter just as much as the headline rating.
Do Air Purifiers Actually Help With Asthma, Allergies or Damp?
This is where a watchdog approach matters most, because the honest answer is more measured than most retailers let on. Reducing exposure to indoor allergens is a recognised part of managing asthma, and a genuine HEPA purifier can meaningfully reduce airborne dust mite debris, pollen, pet dander and mould spores in a room. But UK asthma and lung health guidance is consistent on one point: an air purifier is a complementary tool alongside prescribed medication and proper home maintenance, never a replacement for either.
Damp and mould are a separate and arguably bigger problem in many UK homes, and they are described by UK lung-health guidance as genuinely harmful – linked to rhinitis, respiratory infections, coughing, wheezing and breathlessness, and capable of worsening existing conditions like asthma. A HEPA purifier can capture mould spores already circulating in the air, but it does nothing to fix the underlying damp or condensation producing them. Guidance is clear that existing mould should be removed and condensation tackled at source – painting over it or relying on an air purifier to mask the problem will not protect your lungs. Keeping indoor humidity in the 30-50% range is also commonly advised, since dust mites and mould both thrive above that level.
Five Questions to Ask Before You Buy
- Is the filter certified H13 (or H14) to EN 1822:2019? If the listing only says "HEPA-type" or doesn’t cite the standard, assume it’s the lower-grade version.
- Is the CADR rating matched to your room size, not just the largest room the manufacturer advertises it for?
- What does a replacement filter cost, and how often does it need changing? A cheap unit with expensive, frequent filter changes can cost more over two years than a pricier one with a longer-life filter.
- Does it include activated carbon alongside HEPA? Carbon layers help with odours and volatile organic compounds that a particulate filter alone won’t touch.
- Is it being sold as a treatment, or as a supporting tool? Any product implying it can replace asthma medication or fix a damp problem outright is overselling what the evidence supports.
None of this means air purifiers are a waste of money. A genuinely certified H13 unit, correctly sized to the room, can be a sensible and fairly priced addition to managing allergies or asthma symptoms at home – especially for anyone who spends most of their day indoors. The watchdog verdict is simply this: pay for the standard, not the marketing word. Check for "H13 certified to EN 1822" and a CADR figure that matches your room before you check the price, and treat the purifier as one part of a wider plan that includes tackling damp at the source and staying on top of prescribed treatment.