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We all want to make greener choices and help the environment but sometimes what’s best for the planet can be counterintuitive. Like, single-use plastic shopping bags seem to quickly be going extinct.

They’re now banned by stores, cities...even entire countries! And in their place are sturdier, reusable, supposedly greener bags so problem solved, right? Well, when you look at the entire lifecycle of a product, what’s best for the environment can get… complex.

Case in point: By some measures, plastic bags can be the best option. At least, if you leave out one important factor. There are all kinds of bags out there, but here, we’ll focus on five of the most popular: single-use plastic bags, single-use compostable or biodegradable plastic bags, brown paper bags, and two kinds of heavyweight bag: thick, reusable plastic ones, and the classic cotton tote bag.

From that list, you might think you know which bag is best. But sometimes, our intuition does not line up with reality. And that becomes clear when you look at Life Cycle Assessment for this.

A Life Cycle Assessment is a study that looks at the full environmental impact of a product. In it, researchers study and add up each step in how a product is made, used, and disposed of. Several studies have been done like this, and the overall conclusions tend to be the same.

But one major report was prepared in 2018 by the Danish Environmental Protection Agency. This report looked at two types of impact: climate change, and total environmental impact. The climate change bit was pretty straightforward.

They added up all the greenhouse gases emitted throughout the lifetime of these bags. Now, not all greenhouse gases are equal; each has a unique potential to warm the planet. But for easy comparison, this team converted all the gases to the equivalent amounts of carbon dioxide.

Meanwhile, the total environmental impact was a lot more complicated. Here, the researchers looked at 15 effects — everything from ozone depletion, to toxicity, to water and resource use... and a lot more. By putting a number on these things and adding them up, they could compare a broad range of impacts at the same time.

The downside is that this is a big oversimplification, and they couldn’t fit in some important variables. We’ll get to those later. But for now, the big question is, what did this study find?

Well, the first thing to consider is what it took to make the bags because producing them is the stage with the biggest impact. Single-use plastic bags are made of petroleum — also known as oil. Specifically, this study looked at a type called low-density polyethylene.

And the majority of the impact there came from turning the oil into the plastic material itself. For biodegradable plastic bags, they looked at a material called a starch-complexed biopolymer basically, a plastic that incorporates plant starches. Overall, manufacturing these bags releases a similar amount of greenhouse gases as making plastic bags that aren’t biodegradable.

But there are also some additional effects of the agriculture involved in making the plant starches, like more water, fertilizer, and pesticide use. So just from a production standpoint, biodegradable plastic is actually worse than single-use stuff. Similarly, to make a paper bag, you need to start with a tree.

How a specific forest is managed is hard to capture in this sort of comparison, but either way, the process of turning wood pulp into paper can emit a lot of greenhouse gases! This depends on what kind of fuel is used by the paper mill, so different studies can come to different conclusions about paper bags. According to the Danish researchers’ calculations, they have a similar climate impact to single-use plastic.

Now, if you’ve been holding out for the reusable bags to come in and blow everyone away… well, they do. But not how you might think. See, thick, reusable plastic bags are also made from oil, and you need more of it to make a thicker bag — so there’s a bigger impact.

Heavier bags also need more fuel to transport them to the grocery store. And cotton tote bags? These might seem like a green option, but growing cotton requires a huge amount of land, water, fertilizer, and pesticides.

On top of that, processing cotton is an energy-intensive process. So, when it comes to making the bag, single-use plastics win by almost every measure. In this study, paper edged out single-use plastics slightly when it comes to greenhouse gas emissions but others have disagreed with that and calculated that paper bags can be worse.

And either way, when it comes to production, the worst material by far is cotton! But that said, just how something is produced doesn’t really reflect real life. It’s not like we’re just making bags and watching them sit around in a warehouse.

And that brings us to how the bags are used. Using a shopping bag doesn’t cause pollution, but it does affect how we compare these materials. Because like, ideally, you don’t use your nice cotton bag once and then throw it away.

Here, an easy way to compare different materials is by looking at how many times you would need to reuse them to offset their impact, compared to using a new plastic bag every time. If we just consider the climate change impact, paper bags and biodegradable plastic bags are roughly the same as a single-use plastic bag. Meanwhile, heavier reusable plastic bags need to be reused at least six times to make up for their climate change impact compared to single-use plastics.

And cotton bags need to be reused at least 149 times. And this isn’t just something the Danish report found: These numbers are similar to the findings of a 2011 study from the Environment Agency in the UK. To offset climate change impact relative to a single-use plastic bag, they found you’d need to use a paper bag three times, a reusable plastic bag 11 times, and a cotton bag 131 times.

But! All those numbers change if you look at the total environmental impact if you add in the other 15 categories with toxicity, ozone effects, runoff, and everything else. In that case, in order to be greener than a single-use plastic bag, biodegradable plastic bags, paper bags, and reusable plastic bags need to be used about 40 to 50 times each.

And cotton bags need to be reused 7100 times! That means even if you grocery shop three times per week, you need to use that same cotton bag for the next 45 years to have the same impact as using over 7000 single-use bags! And this estimate was even higher for organic cotton because organic crop yields tend to be lower.

You’d need to reuse that bag 20,000 times! So there’s clearly an issue with some of these materials. Like, the numbers for reusable plastic bags are well within the expected lifespan.

I have reusable plastic bags that I have been using since 2004. But for biodegradable plastic, paper, or cotton bags, the number of times you’d need to reuse them is well beyond how long you would expect an individual bag to last. Now, only the Danish report calculated the total impact like this, so we don’t yet have multiple studies to support those numbers.

Still, just based on that, it might seem like plastic comes out ahead. And the plastics industry is happy to use this logic and these comparison studies to lobby against the regulation of single-use plastics. But there is one more key variable: disposal.

Here is plastic’s Achilles’ heel: there’s no good way to dispose of it. So, if you add in litter and impacts on marine ecosystems, single-use plastics look a lot less green. It’s hard to get good data on what fraction of plastic bags are recycled, but we know it’s low, possibly around one to three percent.

Many recycling programs don’t accept them for the simple reason that they get caught in sorting machines. And bags that aren't recycled sit in landfills, clog sewers, and pollute waterways. Plastic bags are especially bad since they’re easily picked up by the wind and strewn across a large area.

They also take a long time to break down and pose a direct threat to wildlife. They can become wrapped around creatures, mistaken for food and eaten, and even spread invasive species that hitch a ride. And this is one area that life cycle assessment studies fall short: they don’t have a way of comparing the effects of litter on ecosystems.

Regardless, new materials like biodegradable or compostable bags are supposed to solve this problem. But while a 2019 study found that these are better than traditional plastics, after three years, none of the bags tested broke down in all environments. Paper bags are biodegradable, so the impact of litter is low.

And they’re recyclable! But left to break down in a landfill, they release methane, a potent greenhouse gas. For the heavier reusable plastic and cotton bags, disposal should be a smaller part of the overall footprint since hopefully they’ve been reused many times.

Both can be recycled, but that doesn’t mean that they always are. For example, only around 15% of textiles a broad category that includes cotton bags — end up recycled. And now, after three intense rounds, and a surprisingly long episode of SciShow, the greenest bag of them all is… it’s complicated.

The best material depends on many factors, including your individual habits, like how many times you reuse each type of bag and how you dispose of your bags. Overall, making single-use plastic bags has a relatively low environmental impact, but waste is a massive problem with no good solution. The best option for the end of a plastic bag’s life is to reuse it as a trash bag.

Presumably, you’d be using something to contain the trash anyway, and if nothing else, the bag is a lot less likely to float away if it’s weighed down by all the stuff inside it. Meanwhile, manufacturing paper or biodegradable plastic has higher impacts, but these materials reduce the problem of litter. And heavier reusable plastic bags are a great option if you reuse them enough.

Cotton tote bags have by far the biggest environmental impact. They look very green when you’ve got them on your shoulder… turns out, that’s kind of a lie. They need to be used hundreds of times to counteract their climate footprint and possibly thousands if you consider multiple environmental impacts.

So, we’re not saying plastic bags are good. They’re not. But it is important to remember that all the alternatives have an impact, too.

It’s not worth going out to buy a snazzy new reusable product if you already have one that works. So, no matter what your bags are made of, the best way to minimize their impact is to reduce how many you use and reuse them as many times as possible. And when you can’t use a bag anymore, do whatever you can to make sure it doesn’t become litter.

It’s also worth keeping in mind that despite all the attention grocery bags get, they’re only a small part of our impact on this planet. But looking at the entire life cycle of a product can be a useful way to analyze nearly any aspect of our lives. From the clothes we wear, to the food we eat, to how we get around by thinking through the full effects of daily decisions, individuals and corporations can get a better idea of the best way to reduce our environmental footprint.

Here on SciShow, we love exploring questions like these because often, the things that seem the most simple are actually really nuanced and fascinating. And we wouldn’t be able to do things like this without the support of our Patreon community. So to all our patrons: Thank you for helping us make videos like this.

If you’re not a patron but want to learn more about supporting free science education on the internet, you can head over to patreon.com/scishow. [♩OUTRO].