“Topping off,” or attempting to fill a vehicle’s gas tank to its maximum capacity, is a bad idea for many reasons. For one thing, much of the additional gasoline which consumers think is going into their tank actually remains in the hose for the next customer.

Topping off also prevents the pump’s vapor recovery system from working properly, as the system requires air space to return vapors from the gas tank to the pump. Over-filling prevents this from happening and can damage this protective feature.

As with all volatile chemicals, if you can smell it, you are being exposed, and the resulting fumes are not healthy. Most people can smell gasoline in the air at just 0.25 parts per million. Gasoline contains benzene, toluene and other chemicals; benzene is a recognized carcinogen, and has been shown to interfere with normal development and reproduction. Toluene is a recognized developmental toxicant, and is linked to kidney and respiratory problems.
 
Finally, gasoline spilled on the ground as hoses are returned to the pump continues to volatilize into the air, contributing to air pollution. Residue remaining on the ground eventually runs off into bodies of water as non-point source pollution, where it impacts water quality, habitat and wildlife.