Next time you slide in behind the steering wheel, think about this: every summer, air pollution drives thousands of Massachusetts residents indoors and into emergency rooms; and most of that pollution comes, not from factories or power plants, but from cars, trucks, and buses.

Motor vehicles on the road account for about 40 percent of the pollutants - hydrocarbons and nitrogen oxides - that react with sunlight on hot days to produce ground-level ozone, the main ingredient in smog. And every summer, smog spells real trouble for nearly three-quarters of a million people with heart or lung ailments, the elderly, and the very young. Even the healthiest of us find it harder to breathe on a smoggy day.

Not Just from the Tailpipe

Some air pollution escapes in your tailpipe exhaust, even if your car is running as clean as it should. But harmful emissions also escape from under the hood and along the fuel system, when gasoline evaporates before it's burned. That happens especially when it's hot and sunny, after the car is shut off but the engine and fuel system are still warm, and, to some agree, when you're filling at the pump.

Most tailpipe pollution is released in a "cold start," the first few minutes it takes your car to warm up. Your car warms up faster when you're driving it, so it's a good idea to not let it sit idling in the driveway too long.

Any time your car idles for more than half a minute, it's burning more gas than it takes to restart the engine. So when you're on the road, think about avoiding long lines at drive-through windows, for example, and take a walk inside instead.

What Else Can You Do?

Some of the best ways to prevent air pollution from cars are just common sense. Drive less by combining errands. Carpool if you can. Take the train or bus to work. You meet the most interesting people. There's more:

  • When you're on the road, travel at steady, moderate speeds.
  • Keeping your tires properly inflated will reduce fuel consumption by as much as 10 percent.
  • After you fill the tank, check to make sure the gas cap fits tight.
  • Perform routine maintenance according to the manufacturer's instructions. Replace oil and filters at recommended times, using an energy-saving grade of motor oil (labeled EC II or Energy Conserving II).
  • Pay attention to a loss in fuel economy. This usually signals an increase in emissions.
  • Watch for signs at the tailpipe. Black smoke means there's too much gas in the air/fuel mixture, so the fuel injection system needs to be checked. Blue smoke, and your engine is burning oil. That means a lot of hydrocarbons just going into the air.

Can You Really Make a Difference?

Absolutely. Consider this: One person using mass transit for a year instead of driving to work keeps about 9 pounds of hydrocarbons, 62 pounds of carbon monoxide and 5 pounds of nitrogen oxides out of the air. And just a 10 percent nationwide increase in mass transit ridership would save 135 million gallons of gasoline every year.

Will It Save You Money?

You bet. By carpooling every day, for example, you can save up to $3,000 a year on gas, insurance, parking fees, and wear and tear on your car. Your insurance premiums can go down as much as 20 percent by designating your automobile for pleasure use only. Idling and stop-and-go traffic cost motorists about 750 million gallons of gasoline a year. That works out to $1,200 per driver in wasted fuel and wasted time.