I recently went over to a friend's home for the first time in a while. The first thing I noticed walking in the door was the unmistakable smell of cat urine. By the end of the evening my eyes and throat were burning. (They have had these cats for years, this is the first time it's been a problem for me.)
They have announced an intention to start regular hosting of social events. How can I tactfully suggest that they air the house out better beforehand? As it is I would not feel comfortable bringing friends over there.
(Note: I created a new account to ask this question, for the obvious reasons.)
[Edited 2 months later to add:] As I mentioned in an answer below, I raised the issue directly and my friends were already aware, and expressed gratefulness for my honesty. A subsequent vet visit led to the longer-term solution: one of the aging cats has arthritis, and once she was given a litter box with lower sides started using it consistently.