Davy Lamp

Carrying a Davy lamp in your hotbar (or placing it) will determine what kind of gas is around it.

If the flame goes out, it is suffocating (so put on a respirator). If the flame goes out, you will need to relight the Davy lamp by placing it in a crafting grid (the 2x2 one in your inventory is fine) with a torch or a flint and steel (but don't relight it in the same place, or it'll go out again).

If the flame increases in size and turns blue, it is explosive (so don't place torches or lava or start any fires).