Nature is basically cyclical, but in the "manufacturing process" of engine combustion, the cycle becomes linear. Fuel that has been burned, rather than being returned to the cycle, is emitted into the environment as a pollutant.
本來，大自然是循環的，但在汽車燃燒這種「工業製程」中，物質的循環卻是「線性」的，一旦被使用過，排放出的物質已回不到循環，被拋離大自然之外，成了「汙染物」。