According to StarTrek.com it's a deliberate effect intended as
"cryogenic fog" from the cryogenic-surgery (mispronounced as 'serogenic' in the episode)
Question: In “Journey to Babel,” Sarek (Mark Lenard) becomes
critically ill while being transported on the Enterprise and,
according to Mr. Spock, needs a “serogenic” surgical procedure on his
heart to repair it. What’s a serogenic surgical procedure?
Answer: That’s a good question. Checking the script, it appears that
the word “cryogenic” – which relates to very low temperatures – was
mispronounced as serogenic by Nimoy. Here’s the dialogue from the
I’d say a cryogenic open-heart procedure is the logical approach.
As further proof that the word cryogenic was intended, the surgery
that McCoy (DeForest Kelley) later performs on Sarek makes use of a
“cryosurgical frame” that generates fog from condensed atmospheric
water caused by the very cold temperature. (The effect was likely
created by the effects team using smoke or dry ice).
hat-tip to https://www.trekbbs.com/threads/journey-to-babel-deforest-kelley-smoking.300756/ for the link