That's probably true, but unburned propane released into the atmosphere also can contribute to pollution. I once saw a television show which hypothesized that much of the smog in Mexico City was caused by propane leaking from poorly maintained tanks. Apparently, according to this show, many people cook using propane there, and there are millions of rusty old propane tanks in use. The leakage of these tanks combined with the sunshine causes smog, in the same way that unburned gasoline and other vapors do. That's not as much of a consideration in places without a dry, sunny climate.