My shop is infested by fleas, what can I do to kill them?

I have a 40x60 enclosed heated shop where I used to work on a race car. Durning the cold,last winter I let my cats stay in there when it was cold. Now I have a flea infestation problem! What is the strongest poison I can use to kill them with? Do I need to hire a exterminator to get ride of them and will that take care of my problem?
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