Thrifty Car Rental and Dollar Rent a Car recently joined Avis, Budget and other national car agencies in banning smoking in their rental vehicles.

The latest policy change leaves Hertz - the world's largest car rental agency - as the only major car rental company to officially allow smoking in rental cars. (Hertz says you can request a "smoking designated" car, but if none are available, you won't be allowed to smoke in the car you get.)

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The policies for Enterprise, Alamo and National remain a bit hazy. An Enterprise Holdings spokeswoman said all three rental firms offer smoke-free cars, but some locations also have smoking cars. You need to call your local agency to reserve the car you want.

Meanwhile, if you light up inside a car from Thrifty or Dollar, be ready to pay a hefty fine. The two agencies are now inspecting returned cars to sniff out smoke residue. Violators of the policy face a fine of up to $250.