Gas
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 analyser, the built-in analyser in your PDA, or look at any of the wall-mounted atmos alert systems that will blare a lethal alarm the moment there is more than 0.01 kpa of carbon dioxide present.
Gas comes in three storage modes: Emergency tanks (can fit on the belt), Tanks (can fit on the back slot), or Canisters (can only be dragged).
Air
Nitrogen and oxygen. What you are generally breathing. Spare canisters of air are available in Toxins. Putting air in a jetpack will make it stop working.
Oxygen
File:Emergencytank.png
In the future, pure oxygen is safe to breathe! It is readily available in blue canisters in Toxins and Engineering, as well as in oxygen tanks and jetpacks.
Plasma
The true purpose of SS13's research, plasma is a purple gas that is highly toxic and highly flammable. It is the main ingredient in making bombs and hellish station-consuming fires. Plasma canisters are available in Toxins and Engineering. Breathing plasma will give a biohazard indicator next to your regular health marker. Making 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.
Carbon Dioxide
Invisible but deadly, canisters of CO2 can be found in Toxins and Engineering. Inhaling CO2 will make you cough, eventually fall unconscious and suffocate if you're not moved to an area with proper atmosphere. Carbon dioxide is also produced in tiny amounts by humans natural breathing.
N2O
Knock-out gas, also known as laughing gas, recognizable by the white particles it leaves in the air. Exposure will quickly render you unconscious. Canisters can be found in Toxins and the Armory, and tanks can be found in Medbay's anesthetic locker.
N2
Two canisters of N2 start in Engineering. It is a safe gas to use in the engine but does not produce much power.