Pyrethrins can harm cats, fish and the environment. Natural pyrethrins can break down and dissipate in about a week. Laboratory versions don't break down as well, so I hope you think about it.
If you want to know what is safe for you animals, check For-the-Love-of-Cats.com for safe alternatives. Read all label directions if you have any concerns.
Never use outdated chemicals. The changes that occur after expiration date can be fatal. Not just to your pet, but yourself as well. A lot of the chemicals we use for fleas are also used in fumigating for termites.
**edited for website name an link Oct 21 2009