Thanks to our love affair with the automobile, sucking in a lungful of air is akin to puffing on a Marlboro. If you want to save the world (and save your lungs), make your next car a low-emissions vehicle. But you don't have to lay out the bucks for a pricey hybrid--lots of mainstream cars made in the last few years emit a modest amount of ozone-causing pollutants and greenhouse gases. But how do you determine which are the greenest and cleanest? This guide will show you how to wade through the acronyms and technical minutiae to make the right choice.

Getting green and clean
What comes out of a tailpipe--carbon monoxide, carbon dioxide, nitrogen oxides, and hundreds of hydrocarbons--is pretty nasty stuff for you and the planet. But thanks to the Federal Clean Air Act and, notably, California's tougher air quality standards, new cars are far cleaner than their predecessors.

How much cleaner? A new car built in 1965 spewed a ton of hydrocarbons into the air in its first 100,000 miles. By 2010, cars adhering to California's Low Emission Vehicle II standards (LEV II) will emit a mere 10 pounds. The health payoff could be enormous. In one famous study, Atlanta's efforts during the 1996 Summer Olympic Games to cut ozone pollution by 28 percent resulted in a 42 percent drop in acute asthma cases.

Down on greenhouse gases? There's no federal auto standard, but a California law that takes effect in 2009 mandates 30 percent lower greenhouse gas emissions from new cars. So far, 11 states (including New York, Massachusetts, Connecticut, Oregon, Washington, Maine, and Vermont), have adopted the law, which automatically means they must also live up to the LEV II standards. (Five other states are considering the law.) "Car makers are suing to overturn the law," says Patricia Monahan, deputy director of the Clean Vehicles Program for the Union of Concerned Scientists. "But we hope they'll embrace the 'innovate don't litigate' model soon."

Buying green
The EPA has its own scheme for rating auto emissions, but as Monahan points out, the average "cleanest" car by federal standards is the "dirtiest" car according to California standards. To buy a truly clean and green car, use California's LEV yardstick. We explain what each emissions rating means and offer a few examples of cars that fall under it. To complicate an already complicated matter, the first two ratings, LEV and ULEV, only apply to cars sold before 2004. Cars from 2004 on must meet the stricter LEV II standards.