S90 gas mask
From Halopedia, the Halo wiki
The S90 Gas Mask is a mask that provides protection from hazardous gases. This piece of UNSC equipment is not seen in gameplay but is labelled on a cargo container. The gas masks might also be used as part as a survival kit like the ones that Master Chief mentioned at Sigma Octanus, in the Halo Novel, Halo: The Fall of Reach.
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