For most households, one type of filter covers it: a combined CBRN (chemical, biological, radiological and nuclear) filter on the standard 40mm thread, with a P3 particle stage and a shelf life the maker publishes. This guide explains what the letters and numbers on a filter mean, how to match a filter to your mask, and how to store filters so they're still in date when you need them.
If you've ever looked at a label like A2B2E2K2 P3 R D and wondered whether you were about to spend good money on the wrong thing, you're not alone. Gas mask filters come from industrial safety and military supply, two worlds that assume you already speak the language. Here's the translation, without the drama.
The Short Version
| If you want to be covered for | Look for | Why |
|---|---|---|
| Most things, with one filter | A combined CBRN filter | Gases, vapours and particles in one canister, so you don't need to know what's in the air first |
| Illness and airborne germs | A P3 particle filter | Lighter and easier to breathe through, but it does nothing for gases |
| A chemical leak near home | A, B, E and K together, plus P3 | Chlorine needs B and ammonia needs K. An A filter covers neither |
| Radioactive dust | A P3 stage, and a maker's rating for radioactive iodine | P3 stops you breathing the dust in. No filter protects you from radiation itself |
And one rule that matters more than any rating: a filter only works if the mask seals. More on that below.
Why It's Worth Getting Right
A mask and filters are the kit you're least likely to use, and the one where there's no improvising. You can stretch food and find water, but you can't make clean air, and a wet cloth over your face isn't a plan. That's the whole case for buying them: not because anything is about to happen, but because it's the one thing you can't sort out on the day.
CBRN, NBC and Tear Gas Filters
You'll see three sets of initials used as if they mean the same thing. They don't.
CBRN is chemical, biological, radiological and nuclear, the current term. The "radiological" part covers events that don't involve a nuclear explosion at all, such as a damaged reactor or a transport accident involving radioactive material.
NBC is nuclear, biological and chemical, the older term you'll see on a lot of military surplus.
Tear gas filters, sometimes sold as riot control or RCA filters, are made for tear gas and pepper spray. They're not CBRN filters, and most won't handle industrial chemicals like ammonia.
So if a listing says "NBC" or just "gas mask filter" without a proper rating on it, treat it as unknown until you find the standard printed on the filter.
Reading the Label: EN 14387
Most American guides talk about NIOSH ratings. A filter bought in the UK will usually be certified to EN 14387, the European standard for gas and combined filters, with the particle stage tested to EN 143.
Each letter has its own coloured band, and a combined filter carries several bands, so once you know the colours you can read a filter at a glance.
| Letter | Band colour | Protects against | Examples |
|---|---|---|---|
| A | Brown | Organic gases and vapours that boil above 65°C | Solvents, petrol vapour |
| AX | Brown | Organic compounds that boil below 65°C | Some solvents (single use) |
| B | Grey | Inorganic gases and vapours | Chlorine, hydrogen sulphide, hydrogen cyanide |
| E | Yellow | Acid gases | Sulphur dioxide, hydrogen chloride |
| K | Green | Ammonia and organic ammonia compounds | Refrigeration and farm ammonia |
| Hg | Red and white | Mercury vapour | Industrial mercury |
| NO | Blue and white | Nitrogen oxides | Fumes from combustion and blasting |
| SX | Violet | Specific compounds the maker names | Check the maker's list |
| P | White | Particles: dust, fumes, mists and aerosols | Smoke particles, fallout dust, bacteria and viruses carried in droplets |
A few things worth knowing:
A isn't everything. A filters are common, but an A filter does nothing for chlorine or ammonia.
P is the part you can't skip. The carbon that soaks up gases doesn't stop dust, and radioactive fallout is dust. A gas filter with no P stage leaves a big gap.
"Reactor" is a separate rating. It isn't one of the EN 14387 letters. Some makers give their filters a separate rating for radioactive iodine, including organic forms such as methyl iodide, which ordinary carbon doesn't reliably hold. If a radiological event is part of your planning, look for that rating from the maker.
The Numbers: Capacity, Not Strength
The number after each letter isn't how well the filter protects. It's how much it can absorb before it's used up.
- Class 1: low capacity, for concentrations up to 1,000 ppm (0.1%)
- Class 2: medium capacity, up to 5,000 ppm (0.5%)
- Class 3: high capacity, up to 10,000 ppm (1%), usually big, heavy canisters for industrial use
An A1 and an A2 filter stop the same vapours to the same standard; the A2 simply holds more before it's spent. For a household, we'd choose class 2: it lasts meaningfully longer than class 1 in a canister you can still wear comfortably.
Particle classes are different
The P number is about how much gets through, not capacity:
- P1: stops at least 80% of particles
- P2: stops at least 94%
- P3: stops at least 99.95%
The gap sounds small, but a P2 filter lets through roughly a hundred times as much as a P3. For fallout dust or germs, P3 is the one to have.
Two more letters you'll see after the P: R means the particle stage is tested for use over more than one shift (NR means single shift only), and D means it passed a dust-clogging test, so it's less likely to clog in dusty air. Neither means a filter can be reused after a real incident: MIRA's advice is that once a filter has been exposed to a CBRN threat, it's used up.
The Thread: 40mm Is the One to Buy
The most common expensive mistake is buying filters that don't fit the mask.
The 40mm thread (properly Rd40x1/7", set out in EN 148-1 and the NATO standard STANAG 4155) is the screw fitting used by most military and civilian CBRN masks. Every MIRA Safety mask we sell uses it, including the children's mask.
Bayonet fittings are the twist-and-lock mounts used by industrial brands such as 3M. They're excellent workplace respirators, but their filters fit their own masks and nothing else.
For your household, buy 40mm:
- It swaps. Your filters fit other 40mm masks, and other people's fit yours.
- It's everywhere. There are far more 40mm filters available than for any one maker's bayonet system.
- It lasts. The standard has been stable for decades, while proprietary fittings can change when a maker updates its range.
One practical point: several of our masks come without a filter. The MIRA Safety CM-6M and CM-8M, for example, are sold without one, so check the listing and budget for filters when you buy a mask. Our gas masks and gas mask filters all use the 40mm thread.
Which Filter for Which Worry
One filter for the unknown
For most people buying one type of filter and being done with it, a combined CBRN filter is the answer. It costs more than a single-purpose filter, but you own one thing instead of four and never have to work out what's in the air before choosing.
The one we stock is the MIRA Safety NBC-77 SOF. MIRA rates it A2B2E2K2HgSXP3 D R under EN 14387: the four main gas types at class 2, mercury, an SX rating that covers phosgene, cyanogen chloride and chloropicrin, and a P3 particle stage. It also carries a Reactor rating for radioactive iodine, tested to a separate German standard. It uses the 40mm thread, weighs 335g, and has a shelf life of 20 years in its factory-sealed packaging (all MIRA's figures).
A chemical leak near home
Chlorine is used at water treatment works and swimming pools; ammonia in refrigeration and farming. Chlorine needs B, ammonia needs K, and an A filter covers neither. If you live near an industrial site, a port or a major freight route, we'd treat A, B, E and K with P3 as your starting point, not an upgrade.
Illness and airborne germs
For germs alone you don't need the gas stage at all. A P3 particle filter is lighter, cheaper and easier to breathe through. The MIRA Safety ParticleMax P3 is made for exactly this. MIRA gives it a 20-year shelf life, and at 100g it's under a third of the weight of the NBC-77. MIRA is also clear that it does not protect against gases or chemical threats. Keep one alongside your combined filters, not instead of them.
Radioactive dust
Fallout is dust, so a P3 stage stops you breathing it in. Be clear about the limit: a filter protects your lungs from contaminated particles and, with the right rating, radioactive iodine. It does not protect you from radiation itself.
Shelf Life: Why the Date Matters So Much
Filters are consumables with a shelf life set by the maker, and that's where the real cost hides. MIRA says a lot of CBRN filters have a shelf life of five to seven years. A five-year filter needs replacing four times over twenty years: four times to remember, check and rebuy. A filter with a twenty-year sealed life just sits in the cupboard, still in date.
Shelf life only counts while the seal is intact. Once you open the packaging, the carbon starts soaking up whatever is in the air around it, such as damp, cooking smells and cleaning products, whether or not you've worn the mask. From then on it's a consumable on a much shorter clock. MIRA's guideline is to treat an opened filter, even an unused one, as good for about the same 12 hours as one in use, so keep filters sealed until the moment you need them.
Never guess a shelf life. If there's no maker's production or expiry date, treat the filter as unknown. You can't tell by looking whether the carbon is still good.
Store it somewhere cool, dry and steady. For the NBC-77, MIRA's guidance is ideally between -5°C and 30°C with humidity below 80%, in its sealed bag. A loft that swings from freezing to 40°C is the worst place in the house; a cupboard indoors is fine. And never try to clean or refresh a filter with compressed air or water, which damages it.
Write the dates down. Put the expiry on the box in marker pen and wherever you keep your kit list. In fifteen years you won't remember.
Vacuum-sealing your own
Putting a sealed filter in a second vacuum bag, with a desiccant sachet, is a reasonable extra layer: it protects the original packaging from knocks, damp and punctures. It won't extend the maker's shelf life, though, because that's set by the chemistry inside. And never vacuum-seal a filter you've already opened: once air has reached the carbon, taking the air away afterwards doesn't undo it.
The Seal Matters More Than the Rating
The best filter made does nothing if air can get round the edge of the mask instead of through the filter.
Facial hair is the biggest cause of a poor seal. A mask seals against skin, and even a couple of days' stubble where the mask sits can let air in. If you have a beard, plan to shave when you'd need the mask, and keep a razor with it.
Fit matters. A mask that doesn't fit won't seal at the chin and temples. Some come in one universal size (MIRA's CM-6M and CM-8M) and others in several (the CM-7M comes in three), and MIRA doesn't recommend the CM-8M for heads noticeably smaller or larger than average. Children need a mask made for them, like the MIRA Safety MD-1 Kids, which MIRA sizes as medium for ages 2 to 6 and large for 6 to 12. If you're not sure, ask us before you buy.
Check the seal every time. Cover the filter inlet with your palm, breathe in gently and hold. The mask should pull slightly against your face and stay there. If you feel air getting in, adjust the straps and try again.
Practise wearing it. The first ten minutes in a mask feel strange and a bit claustrophobic, and breathing takes more effort. Find that out at home on a quiet afternoon, not on the day.
A mask only protects your breathing. Chemicals that act through the skin need more than a mask: protective suits, gloves and overboots.
What We'd Never Buy
Some purchases are worse than nothing, because they give you confidence you haven't earned.
Old military surplus filters. Most are decades past any believable shelf life, whatever the packaging looks like. Worse, MIRA warns that older filters, including expired Soviet-style (GOST) filters from the 1980s and 1990s, can contain chromium or asbestos. Surplus canisters are for display, not for breathing through.
Anything without a proper marking. A genuine EN 14387 filter carries the standard number, the letter and class code, the colour bands, the maker and an expiry date. If any of that is missing, or the listing is vague about it, walk away.
Expired filters at a discount. There's no such thing as a bargain expired filter.
Filters with no production date. Without the maker's date, any expiry claim means nothing.
Bayonet filters bought on spec. Check your mask's fitting before you buy filters in any quantity.
Building a Sensible Filter Stock
You don't need a warehouse, just a few decisions:
One mask per person, correctly sized. Including children, who need their own masks.
At least two filters per mask. One ready to fit and one sealed spare. MIRA's rule of thumb is about 12 hours of use per filter, and to cover a three-day event it suggests six per person.
Mix your types if budget allows. A combined CBRN filter for the unknown, plus a P3 particle filter for dust or illness, so you're not using up expensive combined filters on a job a particle filter could do.
Keep the mask and filters together, where you can reach them. Not in the loft behind the Christmas decorations, but somewhere you could find in the dark in under a minute.
Write down the expiry dates. Most of your kit can survive a bit of disorganisation. This can't.
Our CBRN range brings masks, filters, suits, gloves and overboots together in one place.
Questions We Get Asked
Will I be able to breathe in a gas mask? Yes. A properly fitted mask with an in-date filter lets you breathe normally, though with noticeably more effort, and it feels claustrophobic until you're used to it. Practise before you need it. If you have a heart or breathing condition, ask your GP whether a respirator is suitable for you.
What's the difference between P2 and P3? How much gets through. P2 stops at least 94% of particles, P3 at least 99.95%, so a P2 lets through roughly a hundred times as much. For germs or fallout dust, choose P3.
How long does a filter last once it's opened? Far less than its sealed shelf life. How long depends on what's in the air, how hard you're breathing, the humidity and the temperature, so nobody honest can give you one number. MIRA's rule of thumb is about 12 hours of use, and it treats an opened but unused filter the same way. After a real incident, replace it: filters can't be cleaned or reused.
Do filters supply oxygen? No, and this matters. Filters clean the air around you; they don't make it. MIRA says its masks are only for air containing at least 19.5% oxygen. Where there's less, such as a confined space, a cellar filling with a heavy gas, or a room where a fire has used up the oxygen, a filter won't help. That needs breathing apparatus with its own air supply.
Can I use a 40mm filter on a 3M mask? Almost certainly not. Most 3M full-face masks use their own bayonet fitting. Check your mask before buying filters.
Is P3 the same as HEPA? Similar performance, different standards. P3 is tested to EN 143 at 99.95%, and HEPA is graded under a separate standard. In practice they're comparable.
Should I buy surplus filters? Treat any surplus filter the same way: check the maker's production date and rated shelf life on the filter itself, and if you can't confirm both, don't rely on it. Age is age, wherever it was made.
The Bottom Line
It comes down to four questions. What are you realistically trying to cover? Does the filter carry an EN 14387 rating that covers it? Is it a 40mm thread? And does the maker publish a shelf life you can check?
Answer those and you'll end up with the right filter at a sensible price, still in date years from now. Then get the mask sized properly, practise the seal check, and keep a razor with it.
If your question isn't answered here, ask us. We reply within 24 hours, every time. We've shipped over 10,000 orders since 2020, we're rated 4.9 on Trustpilot, and we'd rather spend ten minutes helping you buy the right thing than sell you the wrong one.
