Composition[edit]
The largest part of most combustion gas is nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except for without carbon); these are not toxic or noxious (although carbon dioxide is a greenhouse gas that contributes to global warming). A relatively small part of combustion gas is undesirable noxious or toxic substances as CxHy, but typically shown simply as "HC" on emissions-test slips) from unburnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, ozone (O3), and particulate matter (mostly soot).