When did it become inappropriate for a professor to smoke during lectures? Sidney Coleman smoked during lectures he gave. Was this universally acceptable behavior at the time? When did smoking during lectures become inappropriate?
Some older universities have lecture theatres of mainly timber construction.  At those places, I'd suggest it was always inappropriate.  Otherwise, I'd say it became inappropriate as soon as there was clear evidence in the peer-reviewed literature that passive smoking has serious deleterious health effects on otherwise healthy adults.  A quick Google Scholar date-range search suggests that this happened between about 1979 and 1981.
The &quot;universally accepted&quot; half of the question, of course, may be a very different matter.