Cats
Are easily hurt by many commercial insecticides, especially some of those marketed for dogs.
Please, read the labels before using any around cats.
All three of my cats from the humane society came with fleas. My body temp. is too low for them to be interested in me, but after my roommate got bit in 1986, we started what was to be a two year trip through hell. Since he was/is a super-environmentally-conscious-organic-no-chemicals-Green, everything which might have worked was out of the question.
The good part was, our apartment was so clean, it could have been used as a surgical theater. The bad part is, unless you can kill all the eggs, it is impossible to get rid of the beasts.
Finally, after 18 months of annoyance to him and the cats (I never even saw one, much less got bit) we got a feline-safe insecticide from the vet. Collars for the cats.
And that was the end of that.