Air purifiers & water filtration, decoded
What HEPA grades, CADR and filter types actually mean — and what they can and can't do for your home. No brands, no hype: just the criteria that matter so you can choose well.
Two honest starting points. UK mains water is among the safest in the world — the Drinking Water Inspectorate (DWI) reports compliance at around 99.96%, so for most homes filtration is about taste, chlorine and limescale, not safety. And no air purifier fixes the cause of bad indoor air: if you have damp or mould, the fix is removing the moisture, not masking the spores. Treat both products as helpful tools — not cures, and not medical devices.
Air purifier filter grades, side by side
"HEPA" is widely abused on packaging. "HEPA-type" or "99% HEPA" is not the same as a true, individually-tested H13. Here's what the grades actually mean.
| Grade / label | What it really means | Captures | Watch for |
|---|---|---|---|
| "HEPA-type" / "99% HEPA" | Marketing term, no standard behind it | Variable — not certified | Avoid relying on this alone |
| H11 HEPA | Entry HEPA under EN 1822 | ≥95% at 0.3µm | Lets more fine particles through |
| H13 ("True HEPA") | Medical-grade, individually tested | ≥99.95% at 0.3µm | The sensible minimum to aim for |
| H14 HEPA | Cleanroom / hospital grade | ≥99.995% at 0.3µm | Overkill (and pricier) for most homes |
| Activated carbon layer | Separate to HEPA — adsorbs gases | VOCs, smells, some fumes | HEPA alone won't touch gases or odours |
Efficiency figures are at or near the Most Penetrating Particle Size (~0.3 microns), per the EN 1822 / ISO 29463 standards. "True HEPA" is a US term (99.97% at 0.3µm) and is broadly equivalent to H13.
What to look for in an air purifier
Beyond the HEPA badge, these are the criteria that separate a genuinely useful unit from an expensive fan.
A true H13 (or "True HEPA") rating
Aim for H13 / 99.95%+ at 0.3 microns. Be sceptical of "HEPA-type", "99% HEPA" or unlabelled "HEPA filtration" — they meet no fixed standard.
A CADR matched to your room
Clean Air Delivery Rate (an AHAM measure) combines airflow and efficiency. The rule of thumb: CADR should be at least two-thirds of the room's floor area. Under-size it and it never clears the air.
A separate activated-carbon stage
HEPA captures particles — pollen, dust, pet dander, smoke particles — but not gases. For VOCs, cooking fumes or odours you need a carbon layer as well.
Honest, ongoing filter costs
Filters are consumables. Check the replacement price and interval before you buy — a cheap unit with dear, frequent filters can cost more over time.
Sensible noise on the setting you'll actually use
Headline CADR is often quoted at top speed, which can be too loud for a bedroom. Check the noise (and clean-air output) at the quiet setting.
No ozone
Avoid "ioniser" or ozone-generating modes used as the main cleaning method — ozone is a lung irritant. Mechanical HEPA + carbon is the safer route.
Water filter types, side by side
There's no single "best" filter — it depends on what you're actually trying to fix. For most UK homes that's taste and chlorine, sometimes limescale.
| Type | Best for | Removes well | Watch for |
|---|---|---|---|
| Jug / dispenser (carbon) | Taste, chlorine, on a budget | Chlorine taste & odour | Frequent cartridge changes; not for safety |
| Under-sink carbon | Taste & chlorine, whole-tap | Chlorine, some organic compounds | Won't soften water or remove minerals |
| Reverse osmosis (RO) | Maximum purity, dissolved contaminants | Most dissolved solids & minerals | Wastes some water; strips beneficial minerals too |
| Water softener (ion-exchange) | Limescale in hard-water areas | Calcium & magnesium hardness | Adds sodium; softened water isn't for drinking by all |
Indicative only; performance varies by product and is not a safety judgement on your mains supply. Check independent reviews (e.g. Which?) and the manufacturer's certified claims.
Filtered water: the honest trade-offs
- Improves the taste and smell of chlorinated tap water
- Carbon filters reduce chlorine and many taste/odour compounds
- RO removes the widest range of dissolved contaminants
- Softeners protect kettles, showers and boilers from limescale
- UK mains water is already safe — filtering is mainly comfort, not protection
- Carbon filters don't remove limescale minerals or microorganisms
- RO wastes some water and strips beneficial minerals like calcium and magnesium
- Neglected cartridges can become a breeding ground — change them on schedule
Air purifiers and water filters are genuinely useful for the right problem — allergens and odours indoors; taste, chlorine and limescale at the tap. They are comfort and quality-of-life tools, not medical or safety devices. Buy on the spec that matters (a true H13 plus carbon; the right filter type for your actual concern) and ignore the badges and buzzwords.
Allergy and asthma households, smoky or odour-prone rooms, hard-water areas, and anyone who simply prefers the taste of filtered water.
"HEPA-type" labels, ozone-based cleaning, undersized CADR, and treating either product as a substitute for fixing damp, mould or a genuine water-quality concern.
Frequently asked questions
Do I actually need to filter UK tap water?
For safety, generally no — the Drinking Water Inspectorate reports UK mains compliance at around 99.96%. Filtering is mostly about taste, removing chlorine, or tackling limescale in hard-water areas. If you have a specific concern (an old lead pipe, a private supply, or a health condition), speak to your water company or a pharmacist rather than relying on a filter.
Is "HEPA-type" the same as True HEPA?
No. "HEPA-type", "99% HEPA" and similar are marketing terms with no fixed standard behind them. A genuine H13 ("True HEPA") filter is individually tested to capture at least 99.95% of particles at 0.3 microns under EN 1822. If the grade isn't stated, treat the claim with caution.
What does CADR mean and what number do I need?
CADR (Clean Air Delivery Rate, an AHAM measure) tells you how much clean air a purifier actually delivers, combining airflow and efficiency. A common rule of thumb is that the CADR should be at least two-thirds of your room's floor area — too low and the unit can't keep up.
Will an air purifier remove smells, gases or mould?
HEPA captures particles, not gases — so for VOCs, cooking fumes and odours you need a separate activated-carbon stage. And a purifier won't cure damp or mould: it may reduce airborne spores, but the real fix is removing the excess moisture. If mould affects your health, see your GP and address the source.
Does reverse osmosis remove anything good?
Yes — RO is thorough, which means it also strips beneficial minerals such as calcium and magnesium, and wastes some water in the process. For most UK homes a good carbon filter is enough; RO makes more sense where you have a specific dissolved-contaminant concern.
Is this medical advice?
No. This is independent consumer guidance to help you compare categories and criteria. For personal health decisions — allergies, asthma, or a specific water-quality worry — speak to your GP, a pharmacist, or your water company. For standards, see the DWI, NHS and Which?.
Choose with confidence, not on the packaging
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