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

Where does it come from? Where does it go?

Making a plastic bag

What does it take to make a plastic bag? First take some oil, coal, natural gas or even plant material, all of which are made up of carbon and hydrogen atoms. Apply pressure to “crack” or break these hydrocarbons into single molecules called monomers. Ethylene, one type of monomer, serves as the basic building block of a plastic bag. Next, chemically treat the monomers so that they combine to make polymers. A chain of ethylene monomers is called polyethylene (PE). (If you’ve seen “HDPE” or “LDPE” on plastics, that’s what the PE means. The “D” means density and the “H” stands for high, the “L” is low.)

Now, you have a bunch of pellets of PE. Heat the pellets in an extruder, which is basically a long, horizontal tube around a big screw. You dump your pellets into one end and move them through the tube by turning the screw. At the other end, the melted plastic gets squished through a hole onto a die that shapes it into a thin film. A little cooling, and you have your plastic film, which can be shaped into a bag, complete with handles, logos and recycling information.

Plastic bags, sacks and wraps accounted for over 4 million tons of plastic waste in 2007, the third-highest category of plastics.

Plastic bags, sacks and wraps accounted for more than 4 million tons of plastic waste in 2007, the third highest category of plastics. (Source: EPA 2007 “Municipal Solid Waste in the United States.”)

Using a plastic bag

The bag then travels to the store via ship and/or truck, is stuffed with groceries, and then handed to me, the customer. I handle the bag for 30 seconds as I walk to the car. I then carry the bag into my apartment, which may take as long as one minute, and unload my groceries, taking perhaps another two minutes. Then the bag goes under the sink, along with scores of others. I will probably reuse most of them one more time, to transport lunch, before tossing them. That means each bag has a useful lifetime of less than five minutes before settling into a near-eternal landfill afterlife. Or, I could recycle it.

What happens to recycled plastic film:

What happens to recycled plastic film may change in coming years due to many plastic bag makers agreeing to make plastic bags with 40 percent recycled content by 2015. (Source: Plastics Division of the American Chemistry Council’s “2007 National Post-Consumer Recycled Plastic Bag & Film Report”)

Recycling a plastic bag

Many grocery stores now feature bag-recycling bins—ask yours why if it doesn’t. After drop-off, the bags are either sent to a domestic recycling plant or exported. The recycling plant sorts and bundles the plastic, and sells it directly to manufacturers. Some uses of recycled plastic require the starting product to be clean and free of contaminants, such as food or other types of plastics. To turn an old bag into a new one, for example, you have to start with very clean recycled content. Then it just takes a little chopping, melting and re-extruding to produce a new, moldable film.

To make composite lumber, the rough plastic is mixed with sawdust and extruded into a plank of desired dimensions. The result is waterproof, splinter-free decking that often come with a limited lifetime warranty, and that can in theory be recycled itself. Recycled PE film also ends up in signs, railroad ties, plastic pallets and other items. (One could argue that making these items out of plastic simply delays its arrival at a landfill, but replacing wood with old plastic also helps to keep trees standing. Which is better in the end is probably fodder for a new question.)

Plastic bag bans

The plastic bag has become a symbol of environmental wrongdoing due to its prevalence, disposability, and precious starting materials. Light weight makes them easy for shippers and users to transport, but makes them ideal for wind transport, too. They end up in our streets and oceans. And wherever they end up, plastic bags stay for a long, long time because they don’t biodegrade. All of these factors have led to some cities and even entire countries banning recyclable plastic bags (versus compostable plastic bags).

Bans and taxes

In 2003, Ireland hit plastic bags with a substantial tax (cleverly called the “PlasTax”) and saw a 90 percent reduction in their use. San Francisco became the first city in North America to ban them in 2007. Supermarkets there with more than $2 million per year in revenues can now offer only paper or compostable plastic bags to their customers. Oakland is considering a similar ban. States including New York and New Jersey require recycling programs at stores using plastic bags. Countries on every continent—including China—already have plastic-bag bans in place or are curbing plastic bag use.

But are bans the right way to go? If the objective is to prevent turtles and birds from literally choking on plastic bags—a too-common occurrence at sea—then bans fit the bill. But the obvious, convenient alternatives are not obviously better for the environment. Compostable plastic bags and paper bags require more energy to make than do regular plastic bags. Paper bags are substantially bulkier, and thus require some seven times the energy to transport than the plastic equivalent. While more paper bags are recycled than plastic bags (21 percent vs. 5 percent), the destination of the rest is landfills, where conditions are far from ideal for breaking down paper. Paper, simply put, isn’t perfect either.

The best option

Enter the reusable bag. While perhaps not as thoughtlessly convenient as the plastic or paper bags at the grocery store, they are by far the best environmental option. The difficulty is motivating people to buy them and then bring them to the store. One option might be to take Ireland’s solution one step further—instead of just taxing plastic single use bags, tax them all. This would create an incentive for reusable-bag toting and prevent the waste created from plastic and paper bags alike. It might also relieve some of the overhead costs of stores, who are directly paying for the bags. Who really ends up paying those overhead costs? That’d be the customers. And you thought bags were free.

Some stores do already provide shoppers with incentives to bring reusable bags. At Whole Foods, I receive 5 cents for each reusable bag I use. At Safeway, I receive 3 cents per bag. My reusable bag also seems bigger and stronger than either a paper or plastic bag, so I can carry more in one bag. Better yet, it was a gift: With no bag overhead myself, I’m making money every time I shop. Only pennies, you say? You’ve clearly never been a grad student.

Andrew Hellman is a PhD candidate in biology.