Q: It's obvious that reusing plastic bags is a good idea, but it's not obvious that recycling them is worthwhile because they have so little mass. How much energy and raw materials are actually required to make plastic shopping bags, and how much do we save by recycling them?

Asked by Jim Salvia, PhD candidate in electrical engineering, Menlo Park

You’re right, reusing plastic bags is a good idea, but recycling them may not seem as important as recycling weightier items like cans and bottles. And most people seem to agree: less than 10 percent of plastic bags are recycled in the United States, according to the EPA. (If my apartment is any indication, the other 90 percent are crammed under kitchen sinks.) But I should, you should, we all should recycle them. The alternative is sending them to the landfill, which is like burying bottles of good oil.

According to the EPA, the equivalent of nine barrels of oil is saved when we recycle one ton of plastic bags. By my calculations, that means one barrel saved for every 10,000 grocery bags recycled. Put another way, one shot glass full of oil is saved every time three grocery bags are recycled. That may not seem like much, but Americans use some 100 billion plastic bags each year. Recycling all of them would annually save 10 million barrels of oil—or more than 100 shot glasses per person.

Besides, are plastic bags really all that light? Sure, they’re relatively flimsy and inexpensive, so recycling one may seem insignificant. But let’s compare them to more commonly recycled items. I weighed a piece of printer paper (4.5 grams), a plastic grocery bag (5.5 grams), and an empty soda can (13.5 grams). A plastic bag weighs more than a piece of paper, and three plastic bags exceed the weight of a can. Bags also have the added advantage of being easy to transport: You can’t stuff an empty six-pack into a single can.

So where do plastic bags go when you drop them off for recycling? First, they are sorted and bundled together with other plastic films like stretch films, which are the plastics commonly used to wrap shipping pallets. In 2007, more than one-third of recycled plastic film in the United States was made into composite lumber, a mix of sawdust and plastic used in construction.

More than half of recycled bags, however, are exported from the United States. Other countries execute the recycling process themselves or incinerate the bags to harvest energy—plastic is just re-formed fossil fuel, after all—though it’s hardly what you’d consider clean-burning. The rest (8 percent) are used to make more plastic film or other products, such as buoys. While these alternatives beat a landfill, there is still a better option.

Even if you recycle, you’re still consuming resources. Plastics generally are made from natural gas, oil or even plant materials, and there are plenty of environmental impacts no matter how they’re produced. Making 1,500 plastic bags from natural gas, the most common feedstock, consumes almost 33 pounds of fossil fuel and 58 gallons of fresh water, and produces some 15 pounds of solid waste and 0.04 tons of CO2, according to a report prepared for the Progressive Bag Alliance. We could avoid using that water and fuel altogether—if only someone would invent a reusable, cloth-like bag . . .

Andrew Hellman is a PhD candidate in biology.