Most hydrocarbon emissions are residues of the fuel, although they may change their form whilst they are in the combustion chamber. Engines running ‘rich’ produce more hydrocarbon emissions than engines with a ‘lean’ mixture, but even with a lean engine they can be problematic.
In terms of perception a sub-group known as aldehydes is quite significant. Aldehydes are used as preservatives in diesel and propane and have a very unpleasant smell, even at low concentrations. From a health point of view, many hydrocarbons irritate the bronchial passages, which others are known to be carcinogenic. Once released into the atmosphere, some react with oxides of nitrogen in the presence of sunlight to create ozone and photochemical smog. This can be a particular problem in hot countries or in cities with geographical features which prevent dispersion of polluted air.