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.

Air

Nitrogen and oxygen. What you are generally breathing. Spare canisters of air are available in Toxins, and air is created every time a floor tile is placed in space, resulting in space wind. Putting air in a jetpack will make it stop working.

Oxygen

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. Oxygen fuels fires and is necessary for the blob to evoluate.

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. Breathing plasma will give a biohazard indicator next to your regular health marker. 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. Inhaling CO2 will cause a player to eventually fall unconscious and suffocate if they are not moved to an area with proper atmosphere. Carbon dioxide is also produced in tiny amounts by humans natural breathing.

NO2

Knock-out gas that will render players unconscious. No longer available in canister form, tanks of NO2 can be found in Medbay's anesthetic locker.