Gas is a word transliterated from a foreign language, which seems to be Russian; Gas is a general term for the mixture of gas and solid particles.

Ordinary gas refers to the mixture of methane, carbon monoxide and pulverized coal in a mine, which will explode in case of open fire or electric spark, that is, blow up the mine.

Tear gas is a kind of gas, similar to what Altman used to deal with monsters. It is a chemical spray, which can stimulate people's eyes or respiratory tract and produce tears.