From Halopedia, the Halo wiki
The CBRN module has been observed to be compatible with the following helmets:
The CBRN module has three observed variants, each with its own distinct physical appearance.
The standard variant is single-piece rebreather covering the "mouth" area of the given helmet.
The EOD-style variant of the CBRN module is a pair of filter attachments on either side of the helmet. It is employed on the standard Mark V EOD-class Mjolnir helmet, but it could be removed for other helmet attachments.
AKIS-style variant was developed in parallel to the AKIS helmet, and has only been observed as compatible to the AKIS helmet.
The CBRN module and its variants serve as cosmetics and have no impact on gameplay.
The first two variants of the CBRN module are available as attachments on several helmets in the armor permutations of Halo: Reach. The device is priced at different levels of Credits, depending on the base helmets. In Halo: The Master Chief Collection, the same CBRN-attached helmets are unlockable in Season 1: Noble, while the AKIS-style variant added in Season 5: Anvil.
Halo 5: Guardians
A variant of the CBRN module will be available on the Commando helmet in Halo Infinite.
"CBRN" stands for chemical, biological, radiological, and nuclear, and is pronounced "C-BURN". It is the same abbreviation used in real life, hence its association with the HAZOP armor permutation.
List of appearances