Aired out is relatively easy because it's something you can take some blame for yourself if you feel minded to mention the issue. You can either open your windows, or make some comment about getting a bit sweaty so the hosts might want to crack the windows for a bit (or substitute something else appropriate to the conditions).
If the house is filthy and desperately needs to be cleaned it depends on your relationship with the hosts. Hopefully if you know someone well enough to stay with them you know how to present something a bit delicate, so you don't come across as saying "this accommodation you gave me free of charge wasn't really good enough".
