ihm
It's probably either because your sims' house is very dirty (smelly green dishes everywhere, dirty counters, etc.) or your house has a bug.
Is it every lot you go into or only one family?
I remember when I played The Sims Online there was a bug that made dirty plates and garbage go invisable, so there would be flys everywhere and you couldn't fix it. I haven't seen a bug like that in The Sims2.
If you go to this link
The Sims 2 you might be able to find posts about it if other people are having the same problem. If it's going on in every house, you might have to uninstall everything and reinstall the game and all expansions. If it's just one house you probably just need to clean it. Another option is moving your sims into a brand new house and destroying the old one, if the bugs won't go away with general house keeping.
Good luck.