Methane tank

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 withound the tanks. However there is a picture without one. However if a Grunt was to not wear a tank they would suffocate in an oxygen atmesphere.