Clean fuels produce less pollution when burned compared to conventional fuels. Propane, being a gaseous fuel (part of LPG - Liquefied Petroleum Gas), generally burns more cleanly than solid fuels like coke or liquid fuels like petrol and wax, which produce significant amounts of smoke, particulate matter, and other pollutants like sulfur oxides and nitrogen oxides. While no fuel is perfectly "clean," propane is significantly cleaner than the other options listed in terms of common air pollutants.