makes no sense as anything intentional.
Well, it could.
The operation being conducted on Sarek is on his heart and involves major blood transfer from Spock.
There is a process for sealing of blood vessels called cauterization
The practice was once widespread for treatment of wounds. Its utility before the advent of antibiotics was said to be effective at more than one level:
- To stop severe blood-loss and to prevent exsanguination
Cauterization was used to stop heavy bleeding, especially during amputation. The procedure was simple: a piece of metal was heated over fire and applied to the wound. This caused tissues and blood to heat rapidly to extreme temperatures, causing coagulation of the blood and thus controlling the bleeding, at the cost of extensive tissue damage.
So it's possible that this is what the smoke is intended to represent.
It seems highly unlikely that this is related to Kelley smoking on set and the smoke being onscreen but it's not impossible. This seems to be a constant comment without a verified source...other than "it was the 60's and they all smoked on set".
Smoking on the set is a far cry from smoking in the middle of a scene.
It's possible that the smoke is coming from the light source inside the "medical equipment" that is supposed to represent a "sterile field" but again, you would have thought that they would have taken steps to rectify that once seen.
It's much more likely that this is an intentional practical effect meant to be part of the procedure.