People interested in environmental issues love a good game of this vs. that. Which is better for the environment: Paper towels or hot-air hand dryers? Cash, check or credit? Going to the theater or renting a DVD? While it’s an interesting diversion, such cases are all cocktail party chatter compared with whether, how much and what you choose to drive. One-fifth of U.S. greenhouse gas emissions come from transportation, according to the advocacy group Environmental Defense Fund.

Environmental impact is just one of many considerations when buying a car. Quality and price — both of the car and, these days, of gasoline — come into play. Green cars are currently struggling to be price-competitive, leading GM recently to put a production hold on the highly publicized Volt.

But if what if you wanted to base your choice on environmental merits alone? The biggest decision then is what sort of engine to buy. The traditional and still most common choice, the gasoline engine, is unsupported by an electric battery of any kind. Then there are hybrid vehicles such as the Toyota Prius, Honda Insight and Ford Fusion, which use gasoline to power the car but can capture the gas engine’s surplus energy as electricity to charge a battery. There are also plug-in hybrids, which add the ability to charge the battery from an electrical socket while maintaining a gasoline backup. The Chevrolet Volt has grabbed headlines in this category and was the co-winner of the car of the year at this year’s Geneva Auto Show. Finally, there are such pure plug-in automobiles as the Nissan Leaf. No gas, just a rechargeable electric battery.

Unfortunately, neither manufacturers nor the government has done much to help environmentally conscious car-buyers choose among these options. Stickers on the windows of new cars at dealerships — these signs are known in the industry as “Monroney stickers,” after the Oklahoma senator who decades ago proposed the law that mandated them — only tell you each car’s fuel efficiency during operation. As readers of this space already know, that’s just one factor in a product’s overall environmental impact. Other considerations include the energy required to extract the vehicle’s raw materials, the land taken up by the mines and factories, the energy used to transport the raw materials and the newly assembled car from place to place, the actual manufacturing of the product, the disposal of the car at the end of its useful life and the infrastructure required to support the vehicle (gas stations, charging stations, etc.).

You’re probably expecting an answer here: All things considered, are the new engines environmentally superior? Unfortunately, the answer is “It depends.” But there are some factors to consider when making this decision.

When choosing a new car, environmentalists consider engine among other aspects. (Bigstock)

Jeremy Michalek, a professor of mechanical engineering at Carnegie Mellon University, published a study on the relative impacts of these categories last year. He found that a variety of factors affect a car’s overall environmental impact. Some have to do with location, some with consumer behavior and some with manufacturing decisions beyond your control.

Consider, for example, what kind of fuel powers the car. There’s no question that 100-percent electric vehicles have lower tailpipe emissions, because no fossil fuels are combusted during use. Total greenhouse gas emissions, however, are a different story. The electricity that runs a plug-in car has to come from somewhere. Coal is the most common source of electricity in the United States, and it emits 27 percent more carbon dioxide than oil, per unit of energy produced, by some calculations.

So, according to Michalek, your best environmental choice depends on your location: “Consumers in Seattle obtain much of their electricity from hydroelectric power, so a battery electric vehicle that uses no gasoline will be at its best. Consumers in West Virginia obtain much of their electricity from coal, so a regular gasoline-powered hybrid electric vehicle like the Toyota Prius is better.”

The Washington metropolitan area gets a large portion of its energy from coal — inspiring a lively debate about sustainability in the Post — but even that fact doesn’t end the conversation.

“The time of day that the consumer plugs in matters, because at a certain time of day the power plant that would respond to the increased demand might be a coal plant even if average electricity in the region comes from cleaner sources,” says Michalek.

While there’s no definitively “best” car type, Michalek’s study did make some points of general application. The life cycle environmental impact of a hybrid or pure plug-in car is largely influenced by the size of its batteries. Large batteries store more energy and extend a car’s range, but they come with an environmental cost. They’re heavy, require more extensive manufacturing resources and have to be replaced if they don’t outlive the rest of the car — which they often don’t. Disposal of the batteries is also resource-intensive, because they must be disassembled carefully rather than tossed into a landfill. Based on the average consumer’s energy mix and driving patterns, Michalek found that the pure plug-in cars, with large, long-range batteries, were no better for the environment than conventional gasoline cars. Hybrid engines with smaller batteries, however, are better than conventional vehicles and pure plug-ins.

Does this mean we should abandon plug-in all-electric cars? Not necessarily. Michalek’s study found that there is a conceivable future in which the plug-in car would be the best choice. But a lot of pieces would have to fall into place, such as expanding renewable energy use in the electricity grid and figuring out how to extend the life of the batteries. In other words, if you want a greener car, work to green the grid by pushing for greater use of renewable energy sources.