Maybe they were smokers too?

Just like you can choose to not rent to a smoker, an insurance company can choose to not insure a homeowner/renter with a pit bull.  

She's not a smoker.

Have a set pre-screening message ready to go for interested parties (ex. 3 x rent in income, great credit, no pets, no smokers, etc.), and make sure you keep it the same throughout so you don't expose yourself to any problems with fair housing.   

The upper unit is fine, but the lower unit…former smoker lived there.

Does other side rent to loud messy smokers with 3 pit bulls and a car on blocks in the front yard while you want to attract a clean, quiet, non-smoking non pet owning tenant on your side?

While I don’t smoke if they are smokers it puts them at ease. 

On the flip side most weed smokers are pretty laid back chill people lol.

For a heavy smoker you may need to paint the rooms, including the ceiling, to get rid of the smell.

If the previous smoker was a heavy indoor smoker (probably the case in light of the lingering smell) then the odor can penetrate sheetrock more than most people realize.