Recirculating fans are little more than pricey white-noise machines. To effectively exhaust steam, smoke, odors—and, if you cook with gas, carbon monoxide—you must vent outdoors. You've also got to match the hood to the range, both in terms of dimensions (a 30-inch range needs a 30-inch hood) and of air flow, as expressed by cubic feet per minute (CFM), to firepower, or BTUs. Factor the latter using this calculation: BTU/100=CFM
Note: The distance between the hood and cooktop, where the hood is installed (island or wall), the length of the duct run, and the number of bends can skew this number. Consult with an appliance pro before you buy.