Buy a hybrid, and atone for the sins of your neighbor, who drives a gas-guzzling SUV. Or buy a hybrid, and atone for the sins of your gardener, whose raggedy truck, lawnmower, weed whacker, and leaf blower spew pollutants. The well-off are not the only ones messing up the environment.

Hybrid vehicles are partly about doing something for the environment. They're also about celebrating your conscience. If you drive a Prius, your colleagues know you're a good person. But even for the apolitical motorist, the mostly overlapping categories of hybrid vehicles, high-mpg vehicles, and alternative-fuel vehicles are starting to make sense.

It is not cast in stone (nor tacked to an old-growth redwood endangered by air pollution) that a smaller vehicle with a downsized gasoline engine, a rack of batteries under the back seat, and an electric motor is the only true low-emissions solution. But it is the most typical solution. And the likes of the Toyota Prius and Honda Civic Hybrid are appealing, especially if most of your driving is urban, you don't need a big vehicle, and you go lightly on the throttle pedal. Even compact SUVs can be hybrids: The Ford Escape / Mercury Mariner twins weigh in at just under 2 tons each, yet each has city fuel economy of around 30 mpg. The hybrid price premium is several thousand dollars, but that's improving: The new Civic Hybrid is only about $1,000 more than its gas-engine sibling.

The Japanese have been ahead of the world with hybrids. American automakers are slowly moving to hybrids at the same time that they're pushing alternative-fuel vehicles. The Germans are warming to gas/electric hybrids: At the fall Frankfurt Auto Show, Audi, BMW, Mercedes, Porsche, and VW all promised hybrids, which is an-about face. Previously, the German strategy was efficient diesel-engine cars today, and hydrogen fuel sometime in the future. The U.S. automakers, along with the German makers, have been reintroducing the variable displacement engine -- an 8- or 6-cylinder engine that idles 2, 3, or 4 cylinders when they're not needed.

Passenger-vehicle diesels haven't played well in the U.S., except for big pickup trucks and a loyal Mercedes-Benz passenger car contingent. Diesel deserves another look. Inside the car, there is no -- no, repeat no -- diesel-engine clatter or harshness. Outside, the sound and diesel exhaust smell are muted. BMW has a twin-turbo diesel capable of 0-to-60-mph runs in under 6 seconds. (Some diesel cars sold during the early-1980s fuel crunch needed 15 to 20 seconds.) In 2006, the U.S. mandate for the low-sulfur diesel fuel long available in Europe means diesel-engine pollution will be vastly reduced, although it's still touch-and-go on how closely we'll meet the so-called Tier 2 emissions standards for the 2007 model year (2009 for light trucks) for reduced soot (particulate) and nitrogen oxides.

Diesel vehicles excel on highway fuel economy, the reverse of gas/electric hybrids. Make a hybrid out of the diesel car, and you'd likely cut emissions further. That's the case with DaimlerChrysler hybrid transit buses. Here's why: Diesels are cleanest and most efficient within a narrower rpm range than gasoline engines, and if that's too much power for deceleration or low-speed driving, the excess can be dumped to the batteries. What a turbocharger does now for a diesel, an electric could later, driven either by an array of batteries or eventually an array of capacitors that would never wear out.

Alternative fuels, such as ethanol and biodiesel, are also options. They look better when government subsidies help pay for them. Some research indicates the conversion of corn to ethanol fuel is energy-negative, meaning it takes more energy to create a gallon of ethanol that you get back burning it. Still, corn's a renewable resource, and you don't have to go to the Middle East or Kazakhstan for the harvest.

If you're concerned about pollution and energy use, think big picture, not just about your own car. SUVs are cooler than minivans and don't (yet) carry the soccer-mom connotation, but minivans haul just as many people with a third better gas mileage. Big houses and inefficient houses are also part of the picture. (When did you last change that furnace filter?) For that matter, so is an airplane vacation to Hawaii, when you could have taken a bus-tour vacation of an adjacent state's lake district. So are gas-powered garden tools.

And so are other peoples' vehicles. The dirtiest tenth of the nation's cars account for about half the vehicle pollution. And those vehicles are disproportionately in the hands of people who are low on the economic ladder. Through the end of 2005, the U.S. government offered a $2,000 rebate for hybrid cars (this policy continues, in lesser form, in 2006). If you wanted the greatest social benefit, would the money have been better spent motivating people who drive junkers to buy cleaner, more efficient vehicles?

Americans aren't the only ones at fault. Although we lead the world in energy usage per person, others are catching up, and some developing countries have horrible air- and water-pollution problems. When a butterfly flaps its wings in Asia, some of that soot may land on your shiny new hybrid.