Mayor Rocky Anderson's order that Salt Lake City employees not idle their vehicles for more than five minutes is a small effort to help clear the air. But it's an important one because there is no magic bullet to improve air quality in the Salt Lake Valley. It will take a thousand different silver bullets.

We hope that the mayor's policy on idling will be an example that other Utahns will follow.

The reason for the policy is obvious. When a car is idling at a stop, it gets zero miles per gallon. Yet the engine still is churning out carbon dioxide (a major greenhouse gas), carbon monoxide and the small particles of soot that can lodge in people's lungs and create disease.

According to the California Energy Commission's Consumer Energy Center (www.consumerenergycenter.org), the rule of thumb is this: "If you're in a drive-through restaurant or business line or waiting for someone and you'll be parked and sitting for 10 seconds or longer, turn off your car's engine."

Why?

"For every two minutes a car is idling, it uses about the same amount of fuel it takes to go about one mile."

And, 10 seconds of idling can use more fuel than turning off the engine and restarting it.

The Energy Center says it is a myth that shutting off and restarting a car is hard on the engine. "Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money that will likely be recovered several times over in fuel savings from reduced idling."

What's more, excessive idling can damage engine cylinders, spark plugs and exhaust systems because fuel is not fully burned.

A second myth is that idling an engine for five minutes before driving is good practice. In fact, according to the Energy Center, modern engines heat up fast, and idling for 30 seconds is a sufficient winter warm-up.

The mayor's policy is not a blanket one. It makes numerous exceptions for cars stopped at traffic signals, for police vehicles on duty and fire trucks that must idle to operate pumps, for heavy equipment that operates auxiliary devices to lift heavy loads, for heating and air conditioning to protect police dogs.

But the general idea is not to leave an engine running when it is not necessary. The motto? Reduce idling and we'll all breathe easier.