I mean even campfires and smoking in public have negative externalities which cause cancer. The marginal cost of each is tiny and probably hard to price, but it has a price. Adding this price would slightly offset the cost but more importantly act as a disincentive for buying a polluting vehicle.

Smoking in public is widely banned. Campfires in populated areas are usually limited to very specific recreational areas. Home wood fires in populated areas are often banned and should be as long as there are reasonable alternatives.