For most UK households, the sensible choice is a full-face mask that takes standard 40mm filters, bought with a sealed CBRN filter, in a size that seals on the face of the person who will wear it. The mask is the part people spend time choosing, but it is the least important of the three. A mask that doesn't seal, or has no filter, protects nobody.
This guide takes the question people actually arrive with ("which gas mask?") and turns it into the three decisions that matter: the filter, the fit, and then the mask. It also says plainly what a gas mask does not do, because nobody should buy one thinking it does more than it does.
A note on tone before we start. We sell this kit to ordinary households who want to know they could protect the people at home if they ever had to. That is a considered, responsible thing to do, and it does not need a frightening preamble, so there isn't one.
Start with the filter, not the mask
A gas mask is a frame for a filter. The filter is what cleans the air; the mask just holds it against your face with a seal. So the first question is which filter, and for a household the answer is short.
A combined CBRN filter on the 40mm thread. CBRN stands for chemical, biological, radiological and nuclear. A combined filter has layers of activated carbon for gases and vapours and a P3 particle stage for dust, smoke particles and airborne germs, so you don't need to know what is in the air before it is useful. The one we sell is the MIRA NBC-77 SOF, rated A2B2E2K2HgSXP3 D R under EN 14387 and, by the manufacturer's figures, good for 20 years sealed. If you want the letters and numbers explained, our filter guide does that in full.
Why 40mm. Rd40 (STANAG 4155, EN 148-1) is the military and industrial standard thread. Every MIRA mask we sell uses it, including the children's MD-1, so one stock of filters fits everyone in the house and replacements will still be made in 20 years. Avoid masks with proprietary or bayonet fittings: the mask may be cheap, but you are tied to one supplier's filters for life.
How many. MIRA's guidance is about 12 hours of active use per filter, single use after any chemical exposure, and six filters per person to cover a 72-hour event. Most of our customers start with two per mask and build from there. Keep them sealed; an opened filter counts as used.
Filters are sold separately. Every mask on our site says so near the top. Budget for the filter when you budget for the mask.
Then fit, because a seal is the whole point
A full-face mask only works if its edge seals all the way round. Three things decide that:
Size. The CM-6M and CM-8M come in one universal adult size, which MIRA says fits average heads and is not recommended for people with smaller or larger than average heads. The CM-7M comes in three sizes, which is the one to look at if you know you are at either end. If you are unsure, email us your head measurement before you buy and we'll tell you honestly whether the universal size is likely to seal.
Facial hair. A beard breaks the seal. This is not a matter of opinion; it is how the rubber meets the skin. If you will not shave in an emergency, a negative-pressure mask is the wrong kit, and the honest alternative is a powered hood (a PAPR), which we can talk you through.
Glasses. Arms of spectacles break the seal too. MIRA masks have an optical insert option for prescription lenses; contact lenses are the simpler answer for most people.
The check you do every time. Put the mask on, cover the filter inlet with your palm, breathe in. The mask should pull onto your face and stay there. If air leaks in at the edge, adjust the straps and try again. Practise this once when the mask arrives, with the filter still sealed in its packet, so the first time isn't the time it matters.
Children need their own mask
About a quarter of the CBRN orders we send include a child's size, so this is not an unusual question, and it has a straightforward answer: an adult mask will not seal on a child, and no amount of strap tightening fixes that.
MIRA's MD-1 Kids mask is a reusable full-face mask in two sizes. The manufacturer's sizing is Medium for ages 2 to 6 and Large for ages 6 to 12, and we don't go beyond their guidance on fit. It uses the same 40mm filters as the adult masks, carried in a pouch on a hose so the weight isn't on the child's face.
Two things the manufacturer says that parents should know before buying:
- The balloon test. A mask like this works by the wearer breathing air in through the filter against resistance. MIRA's rule of thumb is that a child should only use it if they can blow up a balloon on their own.
- MIRA recommends a powered unit for children, even those who pass the test, because a PAPR pushes filtered air into the mask so the child isn't breathing against the filter. For the youngest, MIRA's MD-2 escape hood is a powered hood with the filter included, from about age 2.
If you are buying for the household, the usual order is masks for the adults first, then the children's sizes, with one filter stock for everyone. Email us the ages and we'll say what fits.
Now the mask
With the filter and fit settled, the choice between the adult masks comes down to a few practical differences. All three are full-face, EN 136 Class III, take two 40mm filters, and have a speech diaphragm so you can be understood. None includes a filter.
| CM-6M | CM-7M | CM-8M | |
|---|---|---|---|
| Sizes | One universal | Small, Medium, Large | One universal |
| Drinking system | Yes, with canteen | Yes, with canteen | No |
| Visor | Panoramic, anti-fog inner mask | Panoramic | Widest field of view (MIRA: over 92%); shaped for optics |
| Storage life (MIRA) | 20 years | 20 years | 20 years |
| Best for | Most adults; long wear | Anyone outside the average head size | People who want the widest view and don't need to drink in it |
What our own buyers choose. The CM-6M is the mask we send most, by some way: it fits most adults, it has the drinking system, and the anti-fog inner mask is the thing ex-forces reviewers mention. The CM-8M is next, chosen for the view. The CM-7M sells to people who need a size.
Price and oxygen. The masks cost between about £220 and £240 at the time of writing, with the filter extra. And one line that belongs on every mask page: a filter mask needs at least 19.5% oxygen in the air. It cleans air; it does not supply it.
What a gas mask does not do
This is the section the sales copy on most sites leaves out.
- It does not protect you from radiation. A P3 filter stops you breathing in radioactive dust, which matters a great deal. It does nothing about gamma radiation passing through you. Distance, shielding and time do that.
- It does not work in a fire. A house fire eats the oxygen and produces carbon monoxide, which a CBRN filter does not remove. For smoke, the answer is a smoke hood or getting out, not a gas mask.
- It does not protect the rest of you. Chemical agents can act through the skin. That is what the HAZ-SUIT, gloves and overboots are for, and why people who are serious buy the mask first and the suit second.
- It does not last forever once opened. The mask stores for 20 years in a cool, dark place by MIRA's figures. A filter is good for about 12 hours of use and is single use after chemical exposure.
- It does not fit everyone. See the section on fit. A universal size is universal for most, not all.
None of this is a reason not to own one. It is the reason to own the right one, with filters, that fits.
A sensible order to buy in
- One adult mask per adult, in a size that seals, plus two sealed NBC-77 filters each.
- Children's masks in the manufacturer's size for their age, and a serious look at the powered option for the youngest.
- More filters, towards six per person.
- Suits, gloves and overboots.
- A pouch to keep each mask with its filters, so it is in one place when you want it.
Store it all together, cool and dark, and put a note in your calendar to check the dates once a year. That's it. No bunker required.
Questions we get asked
Is it legal to buy a gas mask in the UK? Yes. There is no restriction on owning or buying a civilian gas mask or filter in the UK.
Will one filter cover everything? A combined CBRN filter like the NBC-77 covers gases, vapours and particles. No filter covers carbon monoxide or a lack of oxygen.
Can I reuse a filter? Unopened, it keeps for its rated life. Once opened, treat it as used after about 12 hours, and as finished after any chemical exposure.
Can I wear it with a beard? Not with a reliable seal. A powered hood is the honest alternative.
How do I know it fits? Put it on, cover the filter inlet, breathe in. It should pull onto your face and hold.
Which size for my child? MIRA's own sizing: MD-1 Medium for ages 2 to 6, Large for 6 to 12, and the balloon test first. Email us if you're unsure; it isn't a silly question.
If you want a second opinion on any of this before you spend £300 or more, email us with who it's for and we'll reply within 24 hours. We'd rather you bought the right kit once.
Related: Gas Mask Filters Explained: Which CBRN Filter Rating Do You Need?