This week I was directed to a factoid I had somehow missed that is currently making the rounds. That “humans consume the equivalent of a credit card worth of plastic every week”. The factoid was being used by the CEO of Friends of the Earth Canada in a Georgia Strait commentary: “Leave plastic where it belongs—in the tar sands”. Looking around I was struck that I kept finding that same particular value and quote at place like CNN, Reuters, Phys.org etc.. A Google search of the headline got 145 unique hits, almost all leading back to a World Wildlife Fund (WWF) report. This set off my chemist’s antenna and I had to discover whether the reported information was valid. Quelle surprise, it really isn’t. As I will discuss below, it is clear these sources have badly misrepresented the scientific source material and Canadians absolutely do not consume that much plastic.
The “humans consume the equivalent of a credit card worth of plastic every week” factoid is derived from a recent paper: “Estimation of the mass of microplastics ingested – A pivotal first step towards human health risk assessment” by Senathirajah et al.. In the paper the authors do indeed conclude that
we estimated that globally on average, humans may ingest 0.1–5 g of microplastics weekly through various exposure pathways.
But that “may” carries a lot of weight in that sentence. The authors make abundantly clear in their text that the 5 g value is the very top of the suspected range (not the typical as suggested in the news articles) and as I will show, achieving that number requires accepting a number of completely implausible scenarios. Any serious reading of the paper would leave the reader concluding that the correct value was somewhere closer to 0.1 g (which I will argue is likely high) and even that value relies on a sequence of uncommon assumptions.
To begin let’s start with some background on the paper. The paper is a “systematic review and analysis of the published literature” that subsequently attempts to simultaneously estimate the numbers and mass of microplastics ingested. For those not familiar with the language that means this is like a meta-analysis but with less strict inclusion criteria. It is a really interesting piece of foundational research, but like any research of this type it is not terribly robust. The challenge is they are trying to estimate two critical values (with all the associated uncertainties) and then they use those estimates to make further estimates. When you multiply uncertainty that way your accuracy and precision go way down.
Like any analysis of this type, the basic assumptions at the front end will dictate the conclusions at the back end. Thus, it is important to look at the basic assumptions at the front end. In this paper the authors assume typical individuals will drink a lot of bottled water and eat a lot of shellfish. Specifically, they assume that each person drinks 219 L of water a year with 53.2 L of that being bottled water (24%). This is important because bottled water has a LOT more microplastics than tap water. Shellfish is important because many shellfish (especially mussels and oysters) are filter feeders that are eaten whole. Now I don’t know about you, but my family relies entirely on tap water (often run through a Brita which does little to remove microplastics) derived from mountain reservoirs (with virtually no microplastics) and our oyster and mussel consumption is relatively low. Speaking of seafood:
Another key source is shellfish, accounting for as much as 0.5 grams a week. This comes from the fact that shellfish are eaten whole, including their digestive system, after a life in plastic polluted seas.
The problem is most shellfish is not eaten whole. Shellfish includes prawns and shrimp that are cleaned and de-gutted before cooking. Even when you cook a lobster whole, you don’t feast on its digestive system or gills, where the microplastics tend to accumulate. In a typical Canadian diet this shellfish value simply doesn’t make sense.
If you are a family that doesn’t drink a quarter of your water from disposable bottles (not reusable plastic bottles but those fragile PET bottles you get from the store) or eat mounds of mussels and oysters your ingestion numbers will be a small fraction of the total used in the article…but there is more. While the ingestion assumptions are likely a bit high, the biggest consideration in determining the mass of plastic ingested comes from the author’s assumptions about the size and shape of those microplastics. This is what really distinguishes the results of the report.
When scientists discuss the volume of plastic in water they do so by counting particles in the water. They typically use one of two techniques to do the job, either Fourier Transform Infrared (FTIR) and Raman spectroscopies. Both do an excellent job of identifying microplastics, the problem is they are less effective at providing the shape and size of the individual particles (particularly since there are so many particles to size). Also, different types of plastics have different masses (the same volume will weigh a different amount). In the article the authors address this issue by providing different scenarios where they identify typical particle sizes associated with different groups of microplastics.
Microplastics come from a variety of sources and get into our foodstuffs through various means. In the oceans, microplastics tend to be bigger as ocean water doesn’t get treated. So shellfish would be expected to be exposed to these bigger, heavier bits of microplastics. Water treatment facilities aren’t designed to eliminate microplastics, but the treatment process does a reasonable job of eliminating the bigger microplastics through their various filtering systems. As a result, microplastic particles in drinking water tend to be smaller.
A further note, the source of your drinking water really matters when it comes to microplastics. Our north shore reservoirs in Vancouver collect runoff from mountain streams. These streams have very low microplastic loads. Similarly, groundwater supplies from confined and unconfined underground aquifers also have very low numbers of microplastics. So, if your water supplies are coming from the ground or from clean freshwater reservoirs, then this paper really doesn’t apply to you either.
Going back to our discussion. In the 5 g scenario the authors assume the average particle of microplastic is equivalent to the microplastics found in seawater. The authors themselves suggest that this is unlikely, but they are doing scenarios and this is simply Scenario 1. In Scenario 2 and Scenario 3 the authors assume all the ingested particles are consistent with what comes out of water treatment facilities. That would be the scenarios that calculate ingestion between 0.1 g and 0.3 g per week with almost all of that being derived from microplastics in table salt. Ironically, if you want to reduce that number even further you are advised to avoid sea salt and eat rock salts instead.
Finally, the authors do a “medley” where they assume shellfish are exposed to ocean plastics and drinking water to water treatment-sized particles. In that scenario the seafood contribution increases significantly, and ingestion goes up to 0.7 g/wk. This value is significantly less than the 5 g we see in the headlines and may be relevant to communities that consume a lot of mussels and oysters. It absolutely does not apply to most Canadians.
To conclude, it is clear this paper absolutely does not support the headline that “humans consume the equivalent of a credit card worth of plastic every week”. Rather the paper suggests that individuals who rely heavily of bottled water and shellfish (which admittedly could represent a significant community in the developing world) may ingest closer to 0.7 g of microplastics a week. As for your typical Vancouverite who might eat shellfish a couple times a month and drink our beautiful, clean tap water? Your microplastic ingestion rate should be significantly less than 0.1 g/week (or less than a credit card of plastic a year). Admittedly, that result really isn’t going to make international headlines; drive in donations; or convince politicians to ban plastics, so it is understandable how the qualifiers presented in the paper were mostly ignored by the activists discussing this paper in the news.