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Grunts (aka Unggoy) use a Methane environmental suit to mimic their homeworld's methane filled atmosphere. The methane is supercooled for storage (allowing their thermal signatures to be picked up easily). The triangular backpack attached to a Grunt's back contains several methane tanks. Attached to every Grunt's wrist is a readout (measured in Covenant units) of the amount of methane left in the Grunt's pack. When the tank gets low, a resupply of methane must be found quickly, or the Grunt will suffocate.
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A grunt in it's methane suit. Note the triangular backpack and the methane mask.
In Halo 3 it is possible to kill Grunts by knocking their breathing apparatus off, or shooting their tanks which will explode, killing the Grunt and others around it.