I hesitate to publish this post.  Because it deals with smoking in apartments.  With the exception of service animals, smoking is probably the next most emotional issue — with tempers running high on both sides.  One of my properties received a reasonable accommodation request (with a note from a physician) requesting that a resident be moved to a non-smoking section of a community because of a medical condition.  Management does not have a non-smoking section at this property. Whether we should or not is academic — right now we do not.  And we have two valued residents we want to keep.
On the one hand, smoking is legal for adults in our country.  On the other hand, some individuals are allergic to smoke or otherwise need to be away from it.  I understand both sides. 
As such, management is in the unenviable position of trying to keep harmony between two neighbors and find a way to work out a solution in which good residents are maintained.
I located a 2007 letter from HUD in which the Department makes clear that there is no HUD policy, statute, or regulation that restricts management from adopting a prohibition of smoking in common areas or individual apartment units.  However, that same letter provides that management should think carefully when crafting a policy in order to avoid complaints filed by current residents based on theories of constructive eviction or breach of contract by residents who have current leases and who smoke (or have guests who smoke) in their own homes.
As alluded to above, one option for the long term is to discuss whether to designate a wing, section, floor, or building as "no smoking" — and find residents and applicants who would affirmatively want to live in such a place.
However, management needs to respond to the pending reasonable accommodation request.  Some options include:  (a) transferring the requesting resident to a different unit; (b) asking the smoker to move to a different unit (to be sure, management will likely need to sweeten the pot to entice him or her to move); (c) provide extra interior fans and/or air cleaners; (d) increase weather stripping and/or insulation around doors and windows; or (e) permit either resident to break their lease and move.
This list is not exclusive.  But it provides some real world guidance concerning dealing with an issue that comes up again and again.
Just A Thought.