Gas mask filter colour codes: what A, B, E, K, P and ABEK mean

Capacity classes: what 1, 2 and 3 tell you

The digit after the letter says how much contaminant the filter can hold: 1 is low capacity, 2 medium, 3 high. A common mistake is to read class 3 as tighter protection. A class 3 filter does not let less gas through than a class 1; it holds more and takes longer to fill under the same conditions. Germany's statutory accident insurance rule on respiratory protection, DGUV Regel 112-190, puts it plainly: the gas filter class has no effect on the protection level.

The class also sets a ceiling. Under the same rule, with unpowered masks a class 1 filter is not used above 0.1% by volume (1,000 ppm), class 2 above 0.5% (5,000 ppm) and class 3 above 1% (10,000 ppm), even if the mask's protection factor would allow more. EN 14387 adds that a class 2 or 3 filter covers everything the lower class covers.

The table below is the laboratory requirement in EN 14387:2004+A1:2008. Each filter faces a test gas at a fixed concentration, with 30 litres of air per minute at 70% relative humidity and 20 °C, and the clock runs until the outlet reaches a set breakthrough level. These figures are not service lives: on site the concentration is usually lower, and during heavy work the breathing rate is higher.

What the table does show is that one class number does not mean one duration for every gas. At class 1, a K filter must hold out against ammonia for at least 50 minutes, while a B filter needs only 20 minutes against chlorine.

Capacity adds weight. EN 14387 limits filters fitted directly to a half mask to 300 g for the set, and filters fitted directly to a full face mask to 500 g. Under DGUV, a heavier filter may only be connected by a breathing hose, with its weight taken by a separate harness, so that it cannot pull the mask off the face and open a leak.

Reading ABEK, ABEK1P3 and A2B2E2K2-P3 labels

The letters ABEK simply list four filter types: A for organic vapours, B for inorganic gases, E for acid gases and K for ammonia. An ABEK filter has met the requirement for each of those types separately. A digit standing alone at the end applies to every letter, so ABEK1 means A1B1E1K1 and ABEK2 means A2B2E2K2. Where the classes differ, each letter carries its own digit, as in A1B2E2K1.

After the gas part comes P with its class, describing the particle stage, followed by R or NR. R means the particle stage can be reused beyond one shift; NR limits it to a single shift. Two examples taken from the standard: A2P3 R is marked brown-white, and ABEK2P3 R brown-grey-yellow-green-white. Suppliers often write the same codes with a hyphen, as ABEK1-P3 or A2B2E2K2-P3. The hyphen changes nothing.

A multi-type filter is not a universal one. An ABEK label claims no protection against low boilers such as acetone, against mercury or against nitrogen oxides, and carbon monoxide lies outside the scope of EN 14387 altogether. Mixtures call for care as well. DGUV notes that breakthrough comes sooner in a mixture than with a single substance. In solvent mixtures, the component held more weakly on the carbon breaks through earlier than it would on its own, and at a higher concentration. HSE tells employers using multi-gas filters against mixtures to get clear instructions from the manufacturer on safe use and replacement intervals.

One more rule: a filter used against one substance group is not refitted later for another. DGUV's example is an AB filter used against acid gases, which may not then be used against organic vapours. The main items EN 14387 requires on every filter are:

• Type, class and colour code, for example A2, ABEK1 or A1B2E2K1
• On combined filters, the P class and R or NR
• The number of the standard, EN 14387
• The year and month when shelf life ends
• The manufacturer, supplier or importer, and the model designation
• On AX and NO-P3, “for single use only”; on Hg-P3, “maximum use time 50 hours”, in words or as a pictogram

P1, P2 and P3 particle filters under EN 143

Gas filters do not stop dust, and particle filters do not stop gas. A P filter is a fibrous medium that captures solid and liquid particles. EN 143 grades it by how much of a sodium chloride and a paraffin oil test aerosol it holds back: at least 80% for P1, 94% for P2 and 99.95% for P3. EN 143:2021 replaced the 2000 edition, which was withdrawn from the EU list of harmonised standards on 9 June 2024. The main changes concern test methods and marking. The P1, P2 and P3 classes remain.

Combined filters exist for jobs where gas and particles share the same air. Spray painting is the textbook case, with solvent vapour and paint droplets arriving together. HSE points out that a particle filter on its own is not effective against organic solvent mists or sprays; on a combined filter the A stage deals with the solvent and the P stage with the paint droplets. The same guidance advises against using P1 or P2 filters on fume, welding fume for example, unless the manufacturer says otherwise.

As the particle stage loads up, breathing resistance climbs. Unlike the gas stage, this is a warning the wearer can feel, and it deserves attention: harder breathing pulls air in around the edge of the mask. An NR filter is discarded at the end of the shift. An R filter can be stored as the manufacturer instructs and refitted for a later shift.

Half mask (EN 140) or full face mask (EN 136)?

However high the filter class, protection only reaches as far as the seal between mask and face. A half mask covers the mouth, nose and chin. A full face mask seals from forehead to chin and covers the eyes as well. With eye irritants such as ammonia, chlorine or sulphur dioxide, a half mask is not enough on its own and separate eye protection is needed; DGUV considers a facepiece that also covers the eyes the better choice in such cases.

The gap between the two shows in the protection factors national bodies assign. In the UK, HSE gives a half mask with a gas filter a factor of 10 and a full face mask with a gas filter 20. In Germany, DGUV credits the same pair with protection up to 30 and 400 times the exposure limit. The figures differ between the two countries, but both put the full face mask well ahead.

EN 136 sorts full face masks into three classes: class 1 for light duty, class 2 for general use and class 3 for special duties such as firefighting. All three give the same protection level; they differ in mechanical strength and in resistance to flame and radiant heat. The usual connection on single-filter full face masks is the EN 148-1 standard thread (Rd 40 × 1/7″). To rule out dangerous mix-ups, DGUV does not allow that thread on class 1 masks.

Glasses and beards are separate problems. Spectacle arms break the seal of a full face mask; the fix is a spectacle frame made to fit inside the mask. On a half mask, the arms can disturb the fit as well. Beards and stubble leak on any tight-fitting mask: HSE expects the wearer to be clean-shaven where the mask seals, and points those who cannot be towards loose-fitting hoods or helmets. The same guidance describes a seal check to do every time the mask goes on: cover the filter inlet with your palm and breathe in. The mask should pull towards the face and stay there while you hold your breath for ten seconds. If it loosens, readjust and check again. If it will not seal, do not use it.

Which filter on site? Examples from chemicals, energy and municipal work

The first step in selection is knowing the name of the substance. Section 8 of the safety data sheet gives the recommended protective equipment and section 9 the boiling point, which is often the one line that decides between A and AX. Then come the concentration, the length of the task and whether the eyes are affected.

In chemical plants the usual pairing is solvent vapour and acid vapour. Toluene and xylene call for A; acetone and methanol for AX. Around hydrochloric acid baths, hydrogen chloride needs E, and ammonia filling needs K. When several of these groups appear in the same shift, ABEK is worth considering. Because it does not cover low boilers, acetone still means assessing AX separately.

On energy sites the risks are more scattered. Hydrogen sulphide at geothermal plants (B), ammonia used in flue gas treatment (K) and sulphur dioxide in combustion gases (E) are the main examples. Where mercury-containing instruments or lamp waste are handled, Hg-P3 is the filter, and its 50-hour limit is tracked by writing the first-use date on the label.

Municipal and utility crews meet two typical situations. Chlorine used to disinfect drinking water and swimming pools calls for a B filter, and since chlorine also irritates the eyes, a full face mask is the sensible choice. Wastewater work points to B for hydrogen sulphide too, but if the job is inside a manhole, sewer shaft or closed tank, oxygen is measured first. DGUV allows only breathing apparatus independent of the surrounding air in wastewater facilities once oxygen falls below 19%.

How long does a gas mask filter last, and when should it be changed?

There is no single number of hours. Filter life depends on the substance and its concentration, the wearer's breathing rate, humidity and temperature. Humidity is hard on type A filters in particular, because water vapour also binds to the activated carbon. The same A2 filter can be spent within one shift in a solvent-heavy booth and last for days on low-level checks in a store.

For employers who want a rule, HSE's HSG53 guidance offers a starting point: change class 1 filters at least every two days, or daily against a carcinogen or a respiratory sensitiser; class 2 filters at least once a week; class 3 filters as the manufacturer instructs. These are upper limits. If measurements or the manufacturer's calculation point to a shorter interval, that interval applies. In the US, OSHA requires a change schedule based on objective data unless the cartridge has an end-of-service-life indicator.

Smell is not a reliable trigger. Hydrogen sulphide at around 100 ppm deadens the sense of smell within 2 to 15 minutes (OSHA), and carbon monoxide has no smell at all. If the wearer smells or tastes the contaminant inside the mask, breakthrough has already begun and the area is left at once. DGUV notes that under unfavourable conditions this can happen within a few minutes.

In Turkey, the regulation on the use of personal protective equipment at work requires the conditions of use, and in particular the duration of use, to be set according to the degree of risk, the frequency of exposure, the characteristics of each workplace and the performance of the equipment (Article 6). On site, that translates into a written change schedule and the fitting date marked on every filter.

Storage is part of filter life. The end of shelf life is printed on the filter and its box as year and month, and the box states the storage temperature and humidity conditions. An out-of-date filter is not used, even if it has never been opened. A filter that will be refitted is kept capped and plugged or in an airtight container; DGUV limits that storage to six months from first use. Change the filter straight away when:

• You smell or taste the contaminant, or your eyes sting: leave the area first
• Breathing resistance on the particle stage rises noticeably
• The filter has been dropped, is cracked, has a damaged thread or is visibly contaminated
• The scheduled time is up, or 50 hours have passed on Hg-P3
• An AX or NO-P3 filter has been used once
• Shelf life has expired, even with the packaging unopened

Where a filter mask gives no protection

A filter cleans air; it does not add oxygen. Normal air holds about 21% oxygen. In Germany, DGUV bars filter devices once oxygen drops below 17%; in the US, OSHA defines an atmosphere below 19.5% as oxygen deficient. HSE gives no figure; its guidance simply rules out respirators wherever there is too little oxygen.

The places where oxygen can fall are familiar: tanks, silos, wells, trenches, manholes and sewer shafts. A filter mask does not go on in these spaces until the air has been measured. The same applies where the make-up of the atmosphere is unknown or may get worse during the work, and DGUV points to breathing apparatus independent of the surrounding air for such cases.

Some gases have no filter at all. A type B filter does not take carbon monoxide, and EN 14387 leaves CO filters outside its scope. Nor are there filters for nitrogen or carbon dioxide; with these gases the danger usually comes from displacing oxygen.

Welding and open flames add one more risk. EN 14387 requires the manufacturer's information to warn that carbon-filled filters can ignite near open flames or molten metal droplets, especially filters that are not fitted directly to the facepiece.

What to include when asking for a mask and filter quote

Which mask and filter to use is decided by the workplace risk assessment; the supplier's job is to provide the technical data that decision needs. The Turkish regulation on PPE use at work also requires products covered by the PPE product regulation to carry the CE marking and come with user instructions in Turkish (Article 6).

When you ask Formasan for a quote on respiratory protection, sending the details below means the quote can be prepared with the filter type, class and connection already settled. If you have the safety data sheet, attach it; it answers most of these questions on its own.

• The substance name, or its safety data sheet
• Measured or estimated concentration and daily exposure time
• The mask type and connection in use: EN 148-1 thread, bayonet or a manufacturer-specific fitting
• Whether dust, mist or fume is present as well
• Headcount, shift pattern and the planned change interval