SAGE (Sound Advice for a Green Earth) is a project by advanced students in Earth Systems and other programs to answer sustainability questions.

Q: I’ve heard that reusable shopping bags can be less sustainable than plastic shopping bags. Is this true? Which shopping bags are most sustainable: reusable, plastic or paper bags?

Asked by Connie McNair, Medford, Ore.

The Essential Answer

A trip to the grocery store entails a multitude of decisions. Many of us are drawn to discount prices; claims of health benefits; bright, sexy packaging — and, recently, environmental considerations. But the decisions don’t end with the products themselves. Those of us striving to be green want to make sure we’re using the most eco-friendly carrier. Thus, the question: paper, plastic or reusable cloth bags?

The plastic bag has a bad reputation. According to the United States Environmental Protection Agency, Americans go through hundreds of billions of these each year. The result has been called a “modern menace,” with stray bags gathering in the streets, the ocean and landfills, where they endanger wildlife and never biodegrade. They can be recycled but usually aren’t.

Paper bags are biodegradable and easy to recycle or compost. But producing them in quantity requires a lot of water, fuel and cut-down trees. And they usually aren’t made from recycled material, because new paper has longer, stronger fibers. Cloth bags are typically made from cotton, a particularly pesticide-intensive and water-guzzling crop. Reusable bags made from nonwoven polypropylene plastic are also common, and they’re actually less carbon-intensive to produce.

Reputation aside, single-use plastic bags have the smallest carbon footprint, at least in terms of single-bag production. But that’s only the beginning. How we use and dispose of bags matters even more.

Two of the most important considerations for the eco footprint of a bag (or any other item) are whether we reuse it and, if so, how many times. An exhaustive Environment Agency (U.K.) report from 2011 found that paper bags must be reused at least three times to negate their higher climate-warming potential (compared with that of plastic bags). A cotton bag would have to be reused 131 times to break even with a plastic bag, in terms of the climate impact of producing each bag. Of course, plastics can be reused as well — they just don’t look as trendy.

But the answer isn’t quite as complicated as these details might make it seem. Which bag is the most “sustainable”? It really depends on how many times you’re going to use it and, especially with plastic, how carefully you dispose of it once its useful life is over. Use the bags that you’ll reuse the most — and check out previous SAGE answers at stanfordmag.org for advice on filling them with food that’s as gentle on the environment as the carriers you’ve chosen.

Photo: velkr0/Visualhunt.com

The Nitty Gritty

Let’s dish the dirt on each type of bag individually — then we can compare.

Paper

According to the previously cited U.K. study, it takes three reuses of a paper bag to neutralize its environmental impact, relative to plastic. A bag’s impact is more than just its associated carbon emissions: Manufacturing a paper bag requires about four times as much water as a plastic bag. Additionally, the fertilizers and other chemicals used in tree farming and paper manufacturing contribute to acid rain and eutrophication of waterways at higher rates.

Paper may not be the first choice for your reusable grocery bag, since it tears easily and doesn’t hold up in the rain. However, paper bags can be repurposed once they’ve been carted home — for bagging lunches; making arts and crafts; or collecting compost, trash or recyclables.

In terms of disposal, paper bags are better than their plastic counterparts. Paper is compostable. If you have access to composting, just tear it up and toss it in. Or if, like me, you’ve grown pots of mold in your kitchen too many times and are now a little compost-shy, recycling is the next best option. As long as they’re not overly contaminated with food, paper shopping bags can go in any municipal recycling bin.

Plastic

The standard grocery store plastic bag is made from high-density polyethylene (HDPE). Studies agree that plastic bags are by far the least costly (i.e., carry the smallest ecological footprint) to produce. Still, there is no way around the fact that plastic is derived from petroleum. Petroleum is a finite resource, and as it becomes increasingly limited, obtaining it becomes increasingly damaging to the environment.

Recycling plastic bags can be difficult. They often fly out of bins or cling to machinery. For these reasons, many cities do not accept them in the municipal recycling stream. Some large grocery stores offer plastic bag recycling options on site (find a recycling center near you). But unlike metal or glass, plastic can be reincarnated a limited number of times before it is too costly to revive.

Many people do reuse plastic bags. And this lowers their carbon footprint — but only to a point. Most plastic shopping bags are fated to become waste-bin liners, or dog pooper-scoopers. They do an excellent job fulfilling these duties — but when done, they’re off to the landfill. Plastic bags simply don’t have the reusable potential of cloth or even paper bags.

Reusable cotton or polypropylene

Reusable bags may be made from many different materials (hemp fiber, for instance, is especially good for people who fancy themselves as hip), but the two most common types are cotton and nonwoven polypropylene (PP), a more durable type of plastic. Even these chic reusable bags have caught flak from some environmentalists. Are they really better than plastic bags? The answer depends on how faithfully you reuse them. As mentioned in our essential answer, above, an average cotton shopping bag would need to be reused 131 times to account for its higher impact on the production side. So if you’re going to use this bag for the next five years, have at it.

Nonwoven PP, on the other hand, is less costly than cotton. These bags need to be reused only 11 times to break even with the conventional plastic (according to the same U.K. study). Remember — not all bags are created equal! If you do opt for a reusable, be sure to consider the material, its origins and how much you will reuse the bag. Of course, the best option is to use a tote you already have (or buy one secondhand).

In the end, your actions will make the greatest difference — not the bag itself. The most sustainable choice is one that’s sustainable for you. What are your preferences? Which considerations, environmental or otherwise, are most important to you? And which lifestyle changes will you make for the long-term?

Take these questions as food for thought the next time you’re on your way to the grocery store.

Claire Thompson, ’16, plans to receive her MA in Earth Systems in 2018. Edited by Dylan Anslow, ’16, MS ’17.