If you ask a gas station attendant to top off your tank, as of today you're asking them to break the law.

A statewide ban on topping off gas tanks goes into effect today.Oregon environmental regulators approved the rule last year that forbids a station attendant from pumping more fuel into a car after the pump's automatic nozzle clicks off. (Though the law does make allowances for so-called "false clicks" that occur before a tank is filled.)

Now the state is in the process of educating gas stations about the new regulation and distributing "Don't Top Off!" posters, which could soon be ubiquitous.

When it comes to enforcing the new rule, the state would first send warnings to gas stations that are repeat offenders. Only in the most extreme cases would they issue a minor penalty, said William Knight, a spokesman for the Department of Environmental Quality.

"We wouldn't be talking thousands of dollars or attendants jailed or anything like that," Knight said.

The department warns that topping off can spill excess gasoline and release benzene and other toxins into the air.

And the agency said modern pumps don't allow extra fuel to feed into the tank anyway, which can lead to customers getting charged for fuel they don't get.

"Topping off the tank helps no one, resulting in a high level of benzene exposure for anyone in the immediate area and increased costs to consumers," said Environmental Quality Commission Chair Bill Blosser. "It is clear that through a simple change in procedure at Oregon gas stations we can better protect public health and the environment."

-- Matthew Preusch, mattpreusch@news.oregonian.com, Twitter: mpreusch