Warning-33-512 Filters listed in this article are known to contain materials hazardous to health
Exercise caution when handling and storing.
Refrain from using these filters.
Seek medical advice if you have any concerns.

The C2 filter canister was developed in Canada, and saw widespread use in the US military. It is threaded in compliance with STANAG 4155 (Rd40 x 1/7" thread), making it compatible with most NATO masks. The filter provides CBRN protection, but offers minimal protection against ammonia.[1]

SAM 0178

Current issue C2a1 filter.

C2 Filters Edit


The older, obsolete C2 Filter Canister.

C2 filter canisters are considered hazardous waste, as they use ASC Whetlerite carbon, which contains hexavalent chromium. C2A1 filters contain no chromium.[2]

In 1982, black streaks were found on the faces of some US C2 users, indicating exposure to the chromium-containing carbon used in the filters. Testing indicated that the chromium released was within NIOSH limits, and medical follow-up of users was deemed unnecessary.[3]

C2a1 Filters Edit

The modern production C2a1 filters are sealed in a plastic container to keep the filter components fresh and unexpired. The latest run of C2a1 gas mask filters are marked (ASZM-TEDA), C2a1. The (ASZM-TEDA) signifies that these canisters are manufactured with a new type of Impregnated Carbon which offers increased protection against Chemical agents. The filter offers minimal protection against ammonia.

C2a1 filters can be dated using the lot number. The first number pair with letter dates the filter's date of manufacture. MMB05A035-01, for example, indicates that the filter was manufactured in February [B] of 2005 [05]. The number cluster has been underlined for ease of reference. These filters have an official shelf life of 10 years, but unofficially some studies place the effective lifespan at somewhere around 12 years (for the more current C2a1 canisters packaged in the plastic containers, not for the older ones packaged in the metal cans).

Older C2a1 filters were packaged in metal cans. It is important to ensure that it is indeed a C2a1 filter and not a C2 filter, as the C2 filters are almost certainly all expired (indeed, some C2 filters have found to have potentially hazardous amounts of Chromium contaminants). Both the C2 and early C2a1 models are packaged in the same kind of metal can, but are plainly marked with 'C2' and 'C2a1', respectively.

Defective C2A1 filters Edit

A large number of C2a1 filters manufactured during 2006 were defective. The C2a1 filter lots that were suspected to be defective were tested in order to identify which filters within these lots were defective. Those within the suspect lots that tested as being functional filters were marked with a large T to signify that they were functional. A full list of defective lot numbers can be found here

Us canister

Colour differance between the C2 filter (Left) and the C2A1 filter (Right)

C2a1 filters can be found on the surplus market fairly easily, though it may be increasingly difficult to find filters which are more recently manufactured. Those due to expire sometime in the current year are for instance much easier to find that those that expire in 2020. C2 filters may be dangerous to wear, but might make for good collection pieces.

Colouration Edit

It has been noted that not all C2A1 filters are coloured with the vibrant green colour noted in the pictures above. Some share the darker green/black colouration of the C2. It is clearly stated on the casing which version of the filter they are. - If you are unsure, please do not hesitate to ask in our forums

Standards compliance Edit

It should be noted that the C2 and C2a1 filters used by the U.S. Military are not NIOSH approved unlike most CBRN filters manufactured today. They do however pass set standards by the US Military (MIL-C-51560 and EA-C-1704) against various chemical warfare agents.

References Edit

  1. 1.0 1.1 1.2
  2. p3
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