Knights of the Brotherhood of Steel are sometimes dressed in Environmental Armor. This protects them against radiation, chemical and biowarfare agents. Of course, it also helps with the Brahmin smell.
The game takes place in 2197 in the American Midwest. The Eastern Brotherhood of Steel is trying to claim territory surrounding Chicago. The Warrior, the player character, is a new recruit to the Brotherhood, tasked to lead a squad of soldiers made up of available initiates.