An externality if Many individuals are insured by entities that spread the health costs of smokers among all of the insured; also, the health costs of the uninsured are passed on to others.
Employers do not adjust wages according to individual productivity, so that they must lower wages for all workers to offset productivity loss.
The fires damage nonsmokers property, raise the cost of the local fre department, or raise fre insurance premiums for all.
Nonsmokers save money because smokers die too earty to collect full Social Security benefits and because their deaths reduce the high health costs near the end of life (a positive externality).
The effects are serious and smokers do not account for their families' utility when deciding to smoke.