In the AD&D 2e supplement, In The Cage: A Guide to Sigil, there are rules given for the impacts of a noxious smog, as follows:
Any PC exposed to [the smog] must make a Constitution ability check, with the usual bonuses for poison saving throws for dwarves and halflings.
The paragraph goes on to explain the effects of a failure, which are easy enough to transfer into 5e, so they aren't relevant here.
Based on the question Determining how to apply Bonuses and Penalties to Ability Checks, my understanding is that the DC of this check would be equal to the overall relevant ability score, which is Constitution in this case. (Please correct me if I'm wrong here; I've only ever played 5E.)
Is there a way to set the DC on this to appropriately difficult per 5E rules, or is this a case where the games are so dramatically mechanically different that it basically comes down to DM judgement? I am not asking for input on what DC everyone thinks this effect might have; I am asking if there is a tidy way to port this mechanic.