The air up above is toxic, and without a gasmask, death is assured within minutes, so players must don masks and keep a steady supply of air filters to survive.
The film angered its backers, and veterans groups were successful in having the film censored for, among other controversies, a scene which features horses in gasmasks.
Stick with what you know and what the fans of the game enjoyed... a rocking, kinda scary, claustrapohobic game with hints of laboured breathing through a cracked gasmask.