Weekend Reads | How Much Plastic Do We Ingest?

by Kevin Schofield

Starting about five years ago, the press began reporting about the hazards of microplastics in our environment. Microplastics are tiny pieces of plastic, less than half a centimeter across. For the most part, plastic doesn't decompose like organic matter, though large pieces can break up into smaller and smaller pieces. Since humans started fabricating various forms of plastics about 70 years ago, we've generated enough of the stuff — and enough microplastic — to infiltrate pretty much every corner of the planet: soil, oceans and lakes, and even the air. From there, it moves up through the food chain into the plants and animals we eat and the water we drink (we also inhale plastic into our lungs). This presents two important questions: How much of the stuff are we taking into our bodies, and what are the health effects of doing so?

In 2019, the World Wide Fund for Nature (WWF, formerly the World Wildlife Fund) sponsored a research study by a team at the University of Newcastle in Australia to try to quantify the amount of microplastics humans ingest. WWF then hired a consultant, Dahlberg, to turn that research study into a colorful report sounding the alarm on the dangers of microplastics in the environment and calling for action to start cleaning up our mess. The headline finding in the report, one that news agencies around the world have repeatedly echoed since then, is that humans, on average, ingest about 5 grams of microplastic every week, which is about the size of a credit card.

As a PR headline, it's brilliant. But is it true? (Spoiler: no, not even close.) The original University of Newcastle study has been debunked since its publication, and other research teams have published better studies that arrive at much lower figures. But it's worth looking back at that original research, what the WWF's consultant did with it, and how the scientific community responded, because there are several important lessons for us in how to do this kind of science, how to read these kinds of reports — and when to be skeptical of flashy headlines.

The original research makes several interesting and insightful points worth considering. At a high level, there are many different vectors for us to ingest microplastics through our diet. Those include fish and shellfish, salt, honey, sugar, beer, and drinking water (both tap and bottled). Early studies on microplastics in the environment have focused on our planet's oceans and have documented the tremendous amount of plastic (and other garbage) we have dumped into them. The microplastics there work their way up into fish and shellfish, though in different ways: so-called "filter feeder" shellfish might be more likely to accumulate microplastics than other kinds of shellfish or fish. Also, the fish and shellfish we eat whole, including their guts, are more likely to pass microplastics on to us than fish we "clean" and only eat the meat from. The salt we use, often captured from evaporated seawater, can also contain microplastics. But by far the greatest source of microplastics in our diet comes from drinking water, and much of that from bottled water (often bottled in — you guessed it — plastic). In fact, the research paper cites data showing that water in reusable and recycled plastic water bottles has more microplastic particles than water in single-use plastic bottles, which is not surprising but also not a great endorsement for recycled plastic.

Nevertheless, if we want to understand the total amount of microplastic we are ingesting, we can just focus on drinking water, since most of it is there. So how do we calculate the amount of microplastics in drinking water? It turns out that's complicated. Ideally, one would take a water sample, filter out all of the microplastics, and then weigh them. But the particles range in size from a few millimeters down to a micrometer — a millionth of a meter, 1,000 times smaller than a millimeter — and most of them are at the smallest end of that range. Any filter that would catch them will catch a lot of other stuff floating around in the water too, and it's nearly impossible to separate out the plastic particles from everything else. In practice, researchers filter the water, take a sample of what the filter caught, look at it under a microscope, and conduct a "census" of sorts on the microplastic particles they see. They count the number of particles, estimate their size, and look for telltale signs of what kind of plastic they are, because if they know the type and size of a particle, they can calculate its weight. Then, they multiply the average number of particles by the average weight of a particle, and multiply again by how much water an average human adult consumes, and they have an estimate of how much microplastic we consume through our drinking water.

We've already mentioned that microplastic particles are not all the same size; they also aren't all the same shape. Just measuring the longest side doesn't give you the volume, since a cube has much more volume (and weight) than a sphere of the same width. While you can have an average length of a particle, you can't have an "average shape," so the researchers had to make some simplifying assumptions in order to calculate particle weight. In fact, they calculated it two ways: assuming a typical particle is a cube, and assuming it's a sphere. But then they did something weird: They also considered the typical size, shape, and weight of a microplastic particle in the ocean, which has been well researched and documented. On one level, it's an interesting point of comparison, but on another level, it's confusing because the size and weight of aquatic microplastic particles are much greater than those in drinking water. According to the report, a typical aquatic particle weighs about 3 milligrams, whereas a typical drinking water particle (if spherical) is about four-tenths of a microgram — about 10,000 times smaller. They dutifully reported out all three calculations side-by-side: the total weight of microplastics consumed by a person, in a week, if the typical particle is a sphere, a cube, or an aquatic particle. If it's a sphere, they conclude, then we consume about one-tenth of a gram per week; for a cube, about three-tenths. And if it's an ocean particle, 5.5 grams.

You can guess which figure WWF's consultant picked up to use in its report: the big, scary number that has no basis in reality. If you read the fine print in the report, there is some hedging: They emphasize that a person could consume 5 grams per week, and that the whole domain is under-studied and good data is scarce. Nevertheless, the claim that we consume a credit card of microplastic per week is simply not supported — not even by the research paper the WWF report cites.

And it gets worse, because in 2022, another researcher published a paper pointing out several other deep flaws in the methodology used by the Newcastle research team. The most serious one, which casts doubt even on its estimates for "sphere" and "cube" microplastic particles, points to the fact that the researchers not only relied on other studies to estimate the number and size of microplastic particles, but those other studies were incompatible with each other. One study counted particles by filtering water and inspecting the filtered particles; a separate study estimated average size by filtering water and inspecting the filtered particles as well. But the second study used a filter that didn't trap the smallest particles — the vast majority of microplastics — so its calculation for average weight was biased toward larger particles. The Newcastle team's final estimate of total weight used a count of all particles, but an average weight that was too high, so its final total was also too high.

The previous year, yet another research team took on the same questions with a better methodology; it concluded that, on average, a person ingests about 4 micrograms of microplastic per week — about 25,000 times smaller than the original paper's lowest estimate, and more than a million times smaller than the Newcastle team's estimate of 5.5 grams.

Should we be worried about consuming 4 micrograms of microplastic per week? Probably. The research on how much microplastic is in our environment is scarce, but there is even less published research on the health effects of ingesting microplastic (though there is a fair amount of it underway right now, and we should know more in the next few years). One big question still not well understood is how much of the microplastic we ingest passes through us and how much simply accumulates — or is broken down into other chemicals, some of which might be harmful to our bodies. That 2021 paper with the better methodology suggests adults ingest about 900 particles per day, and separate studies have found that we defecate about 200 particles per day; the other 700 aren't currently accounted for. Some we breathe in and back out; a few (not many) leave in our urine; researchers still need to track down what happens to the rest.

But there is also an important lesson here for us with regard to how advocacy organizations — across partisan lines and causes — report science. And in this case, it was scientific work that an organization funded itself. Advocacy organizations try to tell the most compelling story possible to convince people to take action. But people tend not to respond to complex, nuanced results that can be interpreted in multiple ways, so those details get left out in favor of a simple, focused message. Telling people they are eating and drinking a credit card of plastic a week grabs people's attention and shocks them into (hopefully) learning more about the issue of microplastics and what they can do about it. But it's a trade-off: inspiring people to take action now, at the cost of their trust later when the truth comes out and people learn they have been deceived.

This is also a lesson in the power of a meme and an image. For almost three years, it's been known people don't ingest a credit card of plastic per week, yet the idea persists. WWF still has the report and the claim posted on its website. Microplastics are a serious and growing issue that threatens ecosystems around the world, our food supply, and possibly our own health. But whose job is it to hold advocacy groups accountable when they exaggerate the truth and erode our trust?

No Plastic in Nature: Assessing Plastic Ingestion From Nature to People

Estimation of the mass of microplastics ingested — A pivotal first step towards human health risk assessment

Ingested microplastics: Do humans eat one credit card per week?

Lifetime Accumulation of Microplastic in Children and Adults

Featured image via solarseven/Shutterstock.com.