SS13's robust atmospheric system has been simplified, but you will still have to be wary of the air around you. To quickly check whether or not you can take off that gas mask, you can use an atmos analyzer or the built-in version on your PDA.
Gas comes in four storage containers: emergency tanks (can fit on the belt or in a pocket), ordinary tanks (can fit on the back slot), canisters (can only be dragged or picked up by the engineering cyborg module) and reserve tanks (fixed).
Note: Due to the
utterly alien will of eldritch Atmos gods semi-convoluted but slightly sensible atmos system, you can't just repressurise a room simply by sticking a gas canister in there, or else all the gas will collect and stay in that one tile. Instead, move the gas canister around the room from time to time and hope that the gas evenly distributes itself then.
Note: Don't be tempted to top off your jetpack with regular air. It only works with pure oxygen.
In the future, pure oxygen is safe to breathe! Canisters are readily available in toxin research, engineering, EVA, Medbay's cryo treatment cells, and on the mining outpost. Tank dispensers have loads of these, and you might find some in those blue emergency oxygen closets. The oxygen vendor can fill up your tanks with this--for a price. By default, jetpacks also contain 100% O2.
Note: Keep in mind that oxygen fuels fire.
The true purpose of SS13's research. It is a purple gas that is highly toxic and highly flammable, hence breathing plasma is a bad idea. It is the main ingredient in making bombs, powering the station and hellish station-consuming fires. Logically enough, plasma canisters are stored in toxin research and engineering.
Note: It's extremely flammable. Creating sparks or carrying any sort of lit object (welder, cigarette, cakehat) into a cloud of plasma will ignite it, setting off a fire that will quickly spread.
Invisible but deadly. Inhaling CO2 will make you cough, eventually fall unconscious and suffocate if you're not moved to an area with proper atmosphere. CO2 canisters are available in toxins research and engineering.
Part of the standard air mixture. It is a safe gas to use in the engine, but does not produce much power. A couple of canisters start in toxin research and engineering, which spawn with a default temperature of 80 K or -193° C.
Knock-out or laughing gas, exposure to which will quickly render you unconscious. Recognizable by the white particles it leaves in the air. A canister can be found in the armory and the tanks in medbay's anesthetic locker also contain N2O.