From Halopedia, the Halo wiki
The CBRN module is employed for operations in hazard zones, wherein chemical, biological, radiological and nuclear threats are present.
There are two variants of CBRN module; the first is a single-piece rebreather covering the "mouth" area of the given helmet. The second takes the form of the pair of filter attachments on either side of the helmet, seen on the EOD helmet as a default attachment.
"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.
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