Plastic Bags

by Craig Good
March 31, 2015

Remember when "paper or plastic" was one of the biggest decisions you had to make at the store? It turns out there's a clear answer to that conundrum, but not to just about any other shopping bag question. It's undeniable that plastic shopping bags reduced costs and increased convenience. They cost something like a quarter of what paper bags do, and take up a lot less space. But then we started hearing scare stories about choking wildlife, and immense patches of floating bags in the ocean. Gradually plastic bags became environmental villains, and shrill advocacy groups started getting them banned all over the place. The entire country of China, not exactly a model of environmentalism in many respects, has banned them. Even paper bags were caught in the crossfire in places, and now single-use bags are discouraged through taxes or mandatory charges in many cities, counties, and states of America. But are plastic bags the environmental moustache-twirlers they're portrayed to be? Let's take a look.

Let me start by stating my bias at the outset. I think that's important because you should never trust a source that claims not to be biased. I went into this thinking the panic was overblown by extremist environmentalist groups that thrive more on fear-driven fund raising than solid science. I remembered Brian's episode about the Pacific Garbage Patch which found it to be rather overstated and nothing like what most people picture. But I really got some surprises doing this investigation. Come along with me and let's see if we can tease some facts out of a highly charged topic.

Researching this episode was the most difficult time I've had yet doing Skeptoid. Not surprisingly, it's hard to find any information on this topic that isn't advocacy one way or another. There are plausible, reasonable claims that plastic bags aren't that bad - thoughtfully provided by the plastic bag industry (PDF). There are horrific tales of disaster - dished up by environmental advocacy groups with their hands out. But actual science? That seems to be pretty rare, and it's hard to dig much up.

Sites you'd think wouldn't, or shouldn't, do host blatant advocacy. Even Scientific American has pages that look sciency, and may even be correct, but which are really blog posts by environmental groups urging the reader to pressure lawmakers into a certain course of action. Here's what I was able to tease out of all the noise.

Do plastic bags contribute to global warming? Yes, though probably not a lot. About 3 or 4% of the world's petroleum ends up as plastics, with about that much again consumed as energy to produce them. But that's for all plastics. Clearly plastic bags are a tiny fraction of that number.

Do plastic bags kill wildlife? It appears so, though bags aren't the only problem. Bags account for a small fraction of the plastic accumulating in the world's oceans, but I can't find how big that fraction is. Plastic does break down in seawater with the help of UV light, and there's evidence that it also gets nibbled by sea life. Again, how much of the break down is munching vs. sunlight is hard to track down. There are at least anecdotes aplenty indicating that turtles and whales are ingesting bags, which might be mistaken for jellyfish in the water. It's reasonable to assume that the plastic accumulating in their gut is bad for them, but we have no idea how prevalent this is or how many might be surviving it fairly well, because nobody posts photos when there's nothing unusual in a dead animal's gut. But it does seem clear that plastics, one way or another, are entering the food chain. Given that there's no nutritional value to plastics this is probably a bad thing. But how much of the problem is plastic bags versus all the other garbage humans dump into the sea? Good question.

Do plastic bags contribute to land fill? Yes but, again, not a lot. And landfill just isn't the problem most people think it is. A single landfill 120 feet deep and 44 square miles (which is, granted a fairly big one but only a speck on the map) could handle all of America's projected landfill needs - for the next thousand years. And state of the art landfills are a far cry from even twenty years ago anyway.

Are there risks to giving up single-use shopping bags? Absolutely. Again, though, watch out for legends. I heard about how San Francisco's hospitals saw a spike in bacterial infections shortly after its plastic bag ban. It turns out that E. Coli and other nasties can easily grow in reusable shopping bags, especially when people don't know that they need regular cleaning, or use them sometimes for gym clothes. Yes, really. But the study that made this claim (PDF) was written by a couple of law professors, published in a non-scientific journal with no peer review, and was, to put it mildly, severely flawed (PDF). Which is not to say that people haven't actually gotten sick from reusable shopping bags. An entire girl's soccer team was sickened by one which had been left in the bathroom where one team member was experiencing diarrhea. (Skeptoid pro tip: Don't store your shopping bags in the bathroom.)

So how much of the plastic in the oceans comes from plastic bags? Some, but probably not a lot. We don't know. How often do the bags fool sea life into eating them and end up lining their stomachs? Clearly more than we'd like, but we really don't know. Good predictors of how much plastic in general finds its way from land to the sea are the population of the country and how modern its waste disposal infrastructure is. Not a surprising result.

What little science I found about plastics in the ocean has wildly varying estimates of how much there is. One study estimates approximately 5.25 trillion plastic particles, weighing a total of just north of 250,000 tons. Most of it is tiny particles, though, and apparently some unknown mechanism is slowly removing the smallest of them. Is it being eaten, broken down, or precipitating out? They don't know. Another estimates somewhere between about 5 and 12 million metric tons of plastic enter the oceans from the land each year. But note that these are total plastics, not just bags. Besides, most of the plastics in the ocean come from ships. It's old fishing nets, lines, and other marine waste. Still, it seems like switching to a reusable bag would be a good idea. Right?

Exactly how environmentally friendly are those reusable shopping bags? That turns out to be a tricky question. The biggest impact to global warming and pollution from shopping bags is in the sourcing and manufacturing. When the UK Environment Agency did a life cycle assessment of supermarket carrier bags (PDF) they found that non-woven polypropylene bags needed to be re-used at least 11 times to have lower global warming potential than single-use HDPE, or High-Density Poly-Ethylene, bags. Cotton shopping bags need to be used at least 131 times. Paper bags were the big losers. They aren't likely to survive the 4 uses needed to reach the same global warming potential, but are much more toxic to produce than plastic. Go figure. They also noted that HDPE bags tended to be re-used as garbage can liners about 40% of the time, which further reduces their global warming impact. I'll bet some of you are thinking, "Hey, what about recycling? Those bags are recyclable now." Here's what the UK study found:

The reuse of conventional HDPE and other lightweight carrier bags for shopping and/or as bin-liners is pivotal to their environmental performance and reuse as bin liners produces greater benefits than recycling bags.

So what can we say with confidence? We can say that plastic bags contribute to global warming, though only a tiny bit. They are a small part of the detritus in the oceans that may be entering the global food chain, but we don't really know by how much. We don't even know exactly what happens to all the plastic that does make it into the sea.

So there's my unexpected plastic bag adventure. Using bags for garbage can be better than recycling them, some non-zero but vague number of animals are killed when they make it into the oceans, and cotton shopping bags aren't such a great idea. I really wanted to be able to present simple, solid facts, and only found a problem that's marginally understood and much more complex than you'd think. But I do at least have some reasonable suggestions for those who want to minimize their shopping trip's impact on the environment.

Surprisingly, to me at least, if the choice is paper or plastic, take plastic. Then re-use the bag for something like lining garbage cans, and make sure it eventually ends up in a landfill, not floating around loose. Especially near the beach. If you're hunting for reusable shopping bags, get the synthetic ones, not cotton. Re-use them as much as you can, but launder them regularly. Yes, I know that takes both water and energy, and puts more detergents in the water. I told you this was complicated.

What's obvious to me is that more real science is needed to understand the scope and severity of the impact plastic bags have on the environment. Policy is outside the scope of Skeptoid, but it's hard to talk productively about policy if the science isn't well understood. For now, this is a reminder that the best and most honest answer to some questions is, "I don't know, let's find out."

By Craig Good