The largest part of most combustion gas is [[نيتروجين]] (N<sub>2</sub>), water vapor (H<sub>2</sub>O) (except with pure-carbon fuels), and [[ثنائي أكسيد الكربون]] (CO<sub>2</sub>) (except for fuels without carbon); these are not toxic or noxious (although carbon dioxide is a [[غازات دفيئة]] that contributes to [[احترار عالمي]]). A relatively small part of combustion gas is undesirable noxious or toxic substances, such as [[أحادي أكسيد الكربون]] (CO) from incomplete combustion, [[هيدروكربون]]s (properly indicated as C<sub>x</sub>H<sub>y</sub>, but typically shown simply as "HC" on emissions-test slips) from unburnt fuel, [[أكسيد النيتروجين]]s (NO<sub>x</sub>) from excessive combustion temperatures, and [[Atmospheric particulate matter|particulate matter]] (mostly [[سناج]]).
