Michele - Kyle answered your question well, but I will add a couple of things to consider.
If the hoods were used with long-lived radioactive isotopes, it would be prudent to take swipes inside the ductwork.
If the hoods were used with perchloric acid and they were not perchloric acid hoods (with a wash-down), you have an entirely different (and much more serious) problem on your hands.
Good luck!