Propane is a constituent of crude oil, and I understand that North Sea oil is particularly rich in the "higher" hydrocarbons. However, as they are difficult to store, a proportion of these volatile gases is simply flared off at the refinery - so we get the nitrogen dioxide and the carbon dioxide and hydrocarbons anyway. At least if they go through vehicles first, it is instead of rather than in addition to other pollution.
Batley & Spen Green Party
Batley, West Yorkshire