Which bag should you use? - Luka Seamus Wright and Imogen Ellen Napper

You’ve filled up your cart and made it to the front of the grocery line when you’re confronted with yet another choice: what kind of bag should you use? If you’ve seen the images of plastic bags strewn across the ocean, it might seem obvious that plastic is bad for the environment. Surely a paper bag or a cotton tote would be the better option. But is that really true?

Each of these three materials has a unique environmental impact that’s determined by its carbon footprint, its potential to be reused and recycled, and its degradability. So, to get the full story on these grocery bags, we need to look at how they’re made, how they’re used, and where they ultimately go.

Let’s start with plastic. The typical thin and flimsy plastic bag is made of high-density polyethylene, commonly known as HDPE. Producing this material requires extracting petroleum from the ground and applying extreme heat. The resulting polymer resin is then transported alongside additional ingredients like titanium oxide and chalk to a bag manufacturing plant. Here, coal-powered machines melt the materials down and spin them into sheets of plastic, which are then folded into bags.

By the time a bag reaches its final destination, it’s contributed an estimated 1.6 kg of carbon dioxide to the atmosphere. That’s the same amount of carbon a car produces, driving a little over 6 kilometers. But the alternatives actually possess a much larger carbon footprint. Paper is made from wood pulp, and when you account for the carbon cost of removing trees from their ecosystems, a single paper bag can be responsible for about 5.5 kg of carbon dioxide. Meanwhile, growing cotton is an extremely energy and water-intensive process. The production of a single cotton tote emits an estimated 272 kg of carbon dioxide.

When we compare carbon footprints, plastic bags are the clear winner. But environmental impact is also determined by how the bag is used. Reusing or recycling these bags significantly offsets their environmental toll by reducing demand for new production. To quantify that offset, we can divide the bag’s carbon footprint by the number of times it’s reused. For example, if a typical paper bag is reused three times, it has a lower net impact than a single-use plastic bag. The carbon footprint of a cotton tote can similarly be lowered if it’s reused 131 times.

Of these three options, durable cloth totes are most likely to be reused. Evidence shows paper bags are quickly discarded due to their tendency to tear. This issue plagues HDPE plastic bags as well. But even when they’re made to avoid tearing, their widespread availability makes it easy to treat them as single-use items. Fortunately, researchers estimate that 40% of HDPE bags are reused at least once for throwing out waste. Recycling these bags also offsets their carbon footprint, but it’s not universally possible for each material. Many countries lack the infrastructure to efficiently recycle plastic bags.

Cotton totes are perhaps even more difficult to break down and process, but since they’re often reused for long periods, they’re still least likely to end up in landfills. Whenever these bags aren’t recycled, the third factor in calculating environmental impact comes into play: degradability. Since HDPE bags are heat-resistant and insoluble, they stick around long after we’re done with them. Partially broken down plastic can circulate in ecosystems for centuries. Cotton, on the other hand, degrades substantially in a matter of months, and paper bags break down completely in just 90 days.

So, which bag should you use? It turns out the most environmentally friendly bags have features of several materials we've discussed. They’re durable and reusable, like cotton, but made of plastic, which has a lower carbon footprint than cotton or paper. These sturdy shopping bags consist of polyester, vinyl, and other tough plastics, and are already used worldwide. Most importantly, they should last a lifetime—making them the best option for the planet and your groceries.