Methane tank
From Halopedia, the Halo wiki
Methane Tank
The Methane Tank is a device that the Grunts use because they can't breathe oxygen like the other members of the Covenant.
The Heretic Grunts can be seen with a more visabile Methane Tank.
Without Methane Tank
A Grunt is not seen in the games without the tanks. However there is a picture without one.
File:Grunt.jpg
A Grunt without Methane Tank
However if a Grunt was to not wear a tank they would suffocate in an oxygen atmesphere.
