The 2016 EgyptAir plane crash of flight MS804 which had been, at the time, attributed by Egyptian authorities to terrorism, was eventually found to have been caused by the pilot smoking a cigarette in the cockpit, French investigators have concluded.

Crashing isn't the only risk:

    In-flight air quality measurements in approximately 250 aircraft, generalised by models, indicate that when smoking was permitted aloft, 95% of the harmful respirable suspended particle (RSP) air pollution in the smoking sections and 85% of that in the non-smoking sections of aircraft cabins was caused by SHS (Second Hand Smoke).

    Typical levels of SHS-RSP on aircraft violated current (PM(2.5)) federal air quality standards approximately threefold for flight attendants, and exceeded SHS irritation thresholds by 10 to 100 times.

    From cotinine dosimetry, SHS exposure of typical flight attendants in aircraft cabins is estimated to have been >6-fold that of the average US worker and approximately 14-fold that of the average person.

https://pubmed.ncbi.nlm.nih.gov/14985612/