The mounting misgivings over microplastics
In this edition of The Naked Scientists, what threat do microplastics pose to our health, and the health of the planet?
In this episode
What are microplastics?
Richard Thompson, University of Plymouth
Plastic production has led to an explosive rise in tiny, pervasive particles - dubbed microplastics and nanoplastics - which have infiltrated biological and even geological systems across the planet. The worry is that these may lead to serious health harms, and last week two papers looking at this subject piqued our interest; one considered baby birds eating diets polluted with plastic, the other looked at the health and productivity of plants upon which we all depend to feed us. So we thought we should explore the topic in more detail, investigating microplastics and asking what impact they might be having, and how.
But what exactly are they? Richard Thompson is Professor of Marine Biology at the University of Plymouth. He coined the term “microplastic” over 20 years ago…
Richard - They're small pieces, if you like, of the everyday plastics that we're familiar with in bottles, in crisp packets, parts of cars and aeroplanes that will break down in the environment over time into smaller and smaller pieces and then there are also sources that come from the wear of products while we're using them. So the pieces enter the environment small and they arise, for example, from the abrasion of tyres as you drive a car along the road or of textiles and clothing or even the walking around in them actually. We've shown that millions of particles can be generated in a domestic wash but over an annual cycle you probably generate as many of these microplastic fibres while you're walking around. It is part of a continuum and we've got evidence of harm stretching right away along that continuum where plastic pollution can cause harm. The larger items, the rope and netting, can cause entanglement. Smaller pieces, a few centimetres in length, are readily ingested by seabirds and turtles and can block the digestive tract. Some of the smaller microplastics have the potential, if they're eaten, to pass from the digestive tract into the circulatory system.
Chris - Where are they then? Is it just an ocean problem or if we look at land, sea, air, are they everywhere?
Richard - I can't of course say that they're everywhere. The science can't tell us at the minute because we haven't looked everywhere. What I can tell you is that pretty much everywhere we have looked we found them. They're in the air we breathe, they're in the water we drink and they're in the food we eat. We found them from shorelines right down to the deep sea. We found them from the poles literally to the equator and when we first did some of that work I was astonished to find concentrations of microplastic in arctic sea ice and in deep sea sediments that were greater than some of the concentrations I was finding in local waters near to me in Plymouth which is, compared to the population of the arctic or the deep sea, considerably bigger and considering these particles come from humans I was really surprised how all pervasive they were. I think there's an important point there to recognise pollution from particles as opposed to substances that can dissolve in seawater. I remember my marine pollution lecturer saying the solution to pollution is dilution. If you're dealing with a particle those particles have the potential to disperse and become re-concentrated at distance and if they're harmful that means that they could have potentially harmful effects at a distance because they can become re-concentrated.
Chris - And how might they cause harm and if they're everywhere presumably nothing is immune from potential impact from them because we know that with say asbestos decades ago we thought this was an amazing material and indeed it is structurally fantastic but when it breaks down into tiny particles it gains a much more insidious behaviour. So is this the sort of situation we're now confronted by with plastic? We're going to start finding health harms that we never thought perhaps were out there when we first began using what we regard as an amazing material.
Richard - Well plastics of course are amazing materials and used responsibly they have the potential to reduce our environmental footprint and to bring immense societal and economic value and I think it's about learning to use them more responsibly than we have so far. You're right they are everywhere now. There are a wide range of laboratory studies demonstrating potential for harmful effects on a range of different organisms for example reducing the capacity of small marine worms to gain weight in the normal way to put on weight and you might think well that sounds like some sort of slimming pill but actually over a lifetime that's going to reduce their fecundity, their reproductive output if you like, it's going to reduce their potential for survival and if you're a predator that feeds on one of those worms it could reduce the food intake that's available to you. But that's just one example, there's a wide range of examples of evidence of harm. We're seeing uptake by plants as well. I mean we humans are really just another species and the scientific consensus here is that while we've got reports of plastic in humans and that doesn't surprise me because they're in the air we breathe the water we drink and the food we eat at the moment understanding how those microplastics cause harm in humans is an emerging field. Is this another asbestos? I mean people often ask me that question. It's impossible to say and I certainly don't want to demonise plastics. They've got the potential to bring a wide range of environmental benefits and economic benefits and you know have the potential to help us fix some of the problems on the planet. I think it's about using them more responsibly and making sure they don't escape to the environment in any form because we've now got considerable evidence of the potential for harm. And that's harm to wildlife, it's economic damage and its potential harm to human health and well-being.
How could microplastics affect human health?
Tamara Galloway, University of Exeter
What might be the health impacts of microplastics on us humans? How heavily are we being exposed, and when we are, what happens? Studies have shown plastics entering human lungs, and intestines, and getting into the heart, our brains, the walls of blood vessels, and even being present in semen. This suggests there may be potential consequences such as increased stroke risk, dementia and fertility costs. But is it too early to tell? Tamara Galloway is Professor of Ecotoxicology at the University of Exeter…
Tamara - There are all sorts of different ideas about how microplastics might impact human health. Some of those relate to the presence of the small pieces of plastic themselves, also physical damage, and others relate to the presence of different chemicals, particularly plastics additives that might be present in those plastic particles and then might leak out into the body. And most of the suggestions relate to things like oxidative stress and inflammation. The presence of those tiny little bits of plastics cause inflammatory changes to different tissues in the body.
Chris - The other thing that people have worried about is that plastic is made from oil and it will go around collecting up a cargo of other oil-based molecules. And when you put these into your body, it might offload that cargo into your tissue. So although there is a physical plastic effect, there may also be chemical damage going on as well.
Tamara - That's right, the so-called Trojan horse effect. The suggestion is that that can happen based on lots of hydrophobic chemicals or contaminants in the environment, and also metals that might be floating around, particularly in things like combustion from cars or air traffic.
Chris - How do we think, though, that these particles replete with their cargo get into the body in the first place?
Tamara - The most obvious route into the body is that most microplastics are ingested with food or drink. They pass from the gut. There is evidence to suggest that the smaller particles can pass across the gut and to the circulatory system and circulate through the blood.
It's also becoming obvious that there are plastics floating around in the air, so particularly fibrous microplastics. These can be inhaled and enter the body through the lungs and then enter from the lungs into the bloodstream.
Chris - And what sort of level of exposure do we all have? Are we talking about a handful of particles here and there, or are we talking about significant burdens of this stuff that we are breathing in and also eating?
Tamara - The suggestion is that the vast majority of the microplastics that we ingest are in things that we eat, and they're passed into food substances from packaging. So packaging is most likely a much larger source than, say, ingesting seafood that might already have microplastics in it. As to exactly how much we're exposed to, well, that's one of the problems because we're not entirely sure.
It's very difficult, it's very technically challenging to do those kinds of measurements and know exactly how much there is in any particular human body at any particular time. Just about all of us have microplastics in our bodies or are ingesting them and excreting them at any particular time.
Chris - Is it a one-way street then? Once it's in, does it stay in, or does it come and go?
Tamara - The suggestion is that it comes and goes. Particles that are taken up across the gut pass into the lymphatic system and the blood system, and then they can circulate through the body. And ultimately, they'll be cleared through the liver and the kidneys, and they'll be excreted in faeces and urine. So although they might circulate in the body, the vast majority will be excreted after a certain period of time. What we don't really quite understand is what happens to the particles that might be retained within the body, particularly the smaller-sized particles that are much harder for us to measure.
Chris - And what organs are we particularly concerned about?
Tamara - Well, a lot of the organs that have been studied have been highly vascularized organs, so that means organs that have a high blood flow. So things like the liver, the heart, even the testes and the ovaries, these have all been studied and shown to have tiny pieces of plastic present in them. What we don't know is whether that has any significance yet for human health.
Chris - Indeed, it must be a hard one to prove, because I suppose we've got different people alive in different places for different lengths of time with different levels of exposure, getting different diseases. So trying to tie this together and show a kind of causal relationship must be really, really tricky.
Tamara - Absolutely, and you've got to the heart of the problem there, because we're not talking about one individual thing that people are exposed to. We're talking about lots of different kinds of plastics of different shapes and sizes. We're talking about people in the general environment who are exposed to all sorts of different things at any particular time. They'll be eating different food, they'll be living different lifestyles. And a lot of the ill effects that we are expecting or we are predicting might occur due to plastics and microplastics are very diffuse things. They're chronic disorders.
We're not talking about acute toxicity here, where you could definitely say somebody's collapsed from plastic poisoning, say, in the same way that you might look at lead or arsenic or highly toxic substances. We're looking at very subtle effects and they're very hard to identify in population studies.
Chris - Do you think it's the new asbestos?
Tamara - I think there are all sorts of things that we don't yet know about microplastics. And one of the most interesting things that we can try to understand is how plastics relate to other particles that we might have in our body. So what is our exposome? How are we exposed to different particles? How are we exposed to different chemicals? How many of those might we be able to avoid? Is it an issue that we're breathing in all sorts of different things altogether? And what can we do to reduce our risk? I think those are the most important things to focus on.
13:21 - Microplastics linked with cognitive decline in seabirds
Microplastics linked with cognitive decline in seabirds
Alix de Jersey, University of Tasmania
Meaningfully tying plastic exposure to health harms is extremely hard to do. But there are ways to begin to build a body of evidence. A study looking at baby birds eating plastic just came out in Science Advances, and it does address some of the outstanding questions. The team in Hobart have collected samples of blood and stomach contents from sable shearwater seabird chicks. Those with higher exposures to plastics had proteins circulating in their blood indicative of damage to the stomach lining, and they also showed signs of neurological impacts too. Alix de Jersey, from the University of Tasmania, authored the study…
Alix - Seabirds have been well documented to consume plastics. We've learned about this since the 60’s, but what we know about plastic and ingestion in seabirds really comes down to the lethal impacts. We're seeing birds that are entangled or we're seeing them dying of starvation or really emaciated and sickly looking individuals.
But we wanted to look really early on in the condition and understand what are the health impacts of just having a few pieces of plastic in your stomach, because there's seemingly many individuals out there living what looks to be a normal life while carrying this plastic burden in their stomach.
Chris - How did you pursue it?
Alix - We head down to Lord Howe Island every April and May to contribute to a long-term monitoring program of the sable shearwaters and to document their extremely high rates of plastic ingestion. We'll go and grab some birds and we will take a blood sample. And blood samples are awesome. If you've got something wrong, a human will go down to the GP and the first thing our GP will do is probably request a blood test and it can be a really great tool to start to begin to investigate what's going wrong.
At the time of blood sampling we also encourage the birds to vomit. This process is called lavaging. And so we'll also be able to remove their stomach contents and see just how much plastic is sitting in the stomach of that bird.
Chris - Right, okay. And so you can then marry up what the blood is showing you with what the plastic burden is. You can say, well, one might be associated with the other.
Alix - Yes, exactly right. And we've got two groups. We've got birds that don't have any plastic and then birds that do have plastic. So we can really start to disentangle what the health consequences are of that plastic burden.
Chris - And presumably the origin of that plastic is what the parents are feeding them.
Alix - That's correct, yeah.
Chris - And what do you find then? When you marry up those blood samples with the plastic burden, is there an impact?
Alix - Yes, there's definitely an impact. And to do this, we looked at proteins. And proteins are a great tool. We used a technique called proteomics, which allows us to look at the most abundant proteins. Whether you're digesting food or responding to an infection or, getting ready for a migration, we can start to see proteins change within the body to help your body prepare for those things. There are certain hallmark proteins that will help us to look at the health of your overall body or how your organs might be functioning.
Chris - So what sorts of things are wrong with the seabirds?
Alix - The less surprising thing was that we saw a change in the stomach proteins. But what's really interesting about this is that the stomach is quite unique in nature, as these proteins should stay within the stomach because they're highly specialised to that acidic environment. They're working in an environment that's got a pH of 1-2. These proteins have no biological function outside of the stomach. But fascinatingly, we were seeing a higher concentration of stomach derived proteins within the blood. This is showing that, in those high plastic impacted birds, we're getting a breakdown in that stomach permeability. This isn't massively surprising. You can imagine, having big pieces of plastic in your stomach rubbing up against your stomach wall, we’re likely to get perforations and this plastic causing leakage within the stomach.
Next up would probably be the filtering organs. So with the kidneys and liver, they produce copious amounts of proteins, one in particular that the liver produces is the albumin. This is one of the most abundant proteins within the blood. Within the high plastic impacted birds, we saw quite a large decrease in the production of albumin. So it shows that these liver and kidneys are not functioning as they should be.
Chris - Right. And what else did you see?
Alix - The most surprising to me was a decline in a protein called BDNF, which is a brain derived protein. There's this fantastic researcher at UTAS called Jessica Collins, and her research has found that low levels of BDNF is a signature of cognitive decline in elderly people. And we are seeing a similar signature in these chicks. This is fascinating because these chicks are 90 days old, and they should be living well into their 30s. So to be seeing evidence of cognitive decline at such a young age is quite alarming.
Chris - Obviously, in this paper, you have taken a measure of plastic, and then you've measured these other physiological markers of health. We don't know at this stage that one causes the other, but it's very, very tempting to speculate that they're connected in some way. So what do you think the mechanisms are behind all this?
Alix - It is incredibly hard to pick that apart. We've talked about a few different hypotheses within our paper. But microplastics could really be at play. We've seen evidence of them embedded in tissues when we've necropsied the same species previously. Unfortunately, we didn't have access to tissue, so it's really hard to start to make those links.
Chris - To what extent does this begin in the egg? And to what extent does it then get reinforced by feeding? Have you looked at eggs to see if parents are making eggs that already contain a plastic burden and therefore might already be setting their chicks up to fail, and then they make the problem worse by what they feed their chicks?
Alix - We would love to do an egg study, but it can be incredibly challenging to A) find those eggs, and then B) have enough of a cohort to create a study. So that's an incredible big-wish study.
There are some birds that are fine, and they're not getting plastic, and they're going off on their merry way, so we can attribute a lot of this to the chicks that are being fed copious amounts of plastic from their parents.
Chris - And what is the food that the parents bring back? Is it a fish-based diet, and are they therefore getting it out of the oceans?
Alix - Yeah. So these birds will typically eat fish or squid, and within the plastic that they pick up, we see that they tend to favour white or blue pieces. So it's thought that they're selecting this plastic by misidentifying prey.
Chris - I suppose then there's two possibilities, aren't there? There's physically giving the young big bits of plastic we can see, but then feeding them animals like squid and fish that may have themselves ingested small particles of plastic with whatever toxic burdens attached to that. So they could be getting a double whammy here?
Alix - Yeah, it can be really incredibly challenging to start to pick that apart. Proteomics is a great tool. Proteins are highly conserved across taxa, and so it is quite likely that we're seeing these impacts across numerous species that consume plastic, which include humans. I would love to be seeing this tool more commonly used within wildlife conservation to start to really piece together the full picture of plastic impacts on our wildlife.
Microplastics block plant photosynthesis
Richard Lampitt, National Oceanography Centre
There’s growing concern that microplastics are affecting plants' ability to carry out efficient photosynthesis. This really matters, because plants are nature’s solar panels that turn solar energy into food for other living things. Microplastics, some scientists are showing, can physically block sunlight from reaching the right parts of the plant, and they can also lead to the formation of reactive molecules that further compound the damage. With this in mind, Chinese scientists have this week attempted to model the extent and the productivity costs of the problem. It could, they argue, increase the number of people at risk of starvation by 400 million in the next two decades. Richard Lampitt, Professor at the National Oceanography Centre, has been taking a look at the paper, just out in PNAS…
Richard - Well, this is a really important study because it focuses on one of the most important issues of environmental pollution and that of producing food crops. What they do is they've produced a model which leads to a calculation of what is the loss of productivity of our farms and our seas as a result of plastic pollution, microplastic pollution. These are really big numbers they come up with, 10 to 12% reduction in the chlorophyll content, that's the pigment which enables plants to grow. What we've got is a reduction in main crop production of 4 to 13% and 0.3 to 7% for the aquatic side of things. So these are really big reductions as a result of plastic pollution. And it's something we need to be very careful about and very concerned about.
Chris - And looking at it another way, how many people does that not feed?
Richard - There's of the order of several hundred million people who are going to be deprived of food as a result of this pollution.
Chris - Now, is that the here and now, is that the status quo or is that taken to its logical conclusion with the current trends being what they are?
Richard - That's in the here and now, but what they did was what is going to happen in the future and, if there is a reduction in the amount of plastic pollution, how many people would be saved from this level of starvation.
Chris - What do they invoke as the mechanism, though? How do they think that the plastic particles are cutting down the photosynthesis in the plants so that it has this dramatic knock on effect for productivity?
Richard - Well, there is quite good evidence, certainly in terrestrial environments, that plastics do affect productivity. It certainly does occur. What it's doing is in some way affecting the defence mechanisms of plants so that, as a result, they react in a way which is not appropriate and then their productivity decreases.
And certainly in areas, particularly in China where we've got about 12% of the farmland covered with plastic mulch (which does lots of good things in general, retaining moisture and keeping the soil warm) but in spite of that, there is this very large amount of plastic which gets into the soil and does affect the productivity of those plants that are growing there.
Chris - This is very much a model. So they've taken what we think is the case, scaled it globally, and then said, this is the consequence. So is that a reliable way of doing this?
Richard - This is really where I have a bit of a problem with the paper because it's taking an enormous number of assumptions. Every step of the way is plagued by major uncertainties. And so you end up with conclusions, which really, in my opinion, are not very robust.
It’s really important, as I said earlier, to get this sorted out, to understand this, but this sort of implied accuracy is really not supported by the data as far as I could understand. And in fact, a lot of the data which they say they are using is not quoted. So one's not able to find out how good those data are.
So lots of big uncertainties right away along the line, going, for instance, to the effects on the chlorophyll, the pigment concentration. How does that translate into productivity? How does productivity of the plants convert into food production, particularly in the aquatic area, in the seas and rivers, which I'm a bit more experienced in? There's really big uncertainties in those links.
Chris - Nevertheless, if we have got half a billion people potentially going hungrier than they should because of a lower productivity as a consequence of this, it is still an important viable mechanism and presumably deserves more attention.
Richard - Oh, gosh, yes. There's absolutely no doubt about that. Much more attention has got to be given to this because this is obviously a fundamental issue of our ecosystem and the ecosystem services: how much food do they produce?
If those are getting impacted in the way that is indicated in this paper, then we've got to be concerned about it, along with all the other effects of plastic on ourselves and on the environment from an ethical as well as a practical point of view.
What can we do about microplastics?
Richard Thompson, University of Plymouth
At the moment, there’s evidence of a smoking gun linking widescale plastic pollution with health impacts across the biosphere. But what, I asked marine biologist and microplastics specialist Richard Thompson, should be our next move?
Richard - Yeah, it's a really good question and often I feel it can be a distraction that, you know, if we make a mess I think it's almost in human nature to want to clean it up immediately. The challenge here is though at the minute the quantity of plastics going into the environment across the whole size spectrum is so immense that cleanup and focusing on cleanup alone would be a little bit like trying to mop the bathroom floor while the tap was still running and the bath was overflowing. We need a more systemic answer and some of that is going to involve reduction in the quantities we use.
If we think about the waste hierarchy it's reduce, reuse, recycle and it's clear to me there are some plastics that we could manage without but some plastic products, you know, whether that's a single-use carrier bag, a single-use water bottle, are we using plastics only in ways that are essential? What we really need is criteria to ensure that the plastics that are used and the products that are used are safer and more sustainable than they are today and that's safer in terms of the chemical composition. We also need to make sure that the plastics are more sustainable.
You can do that by reusing some plastic items, the single-use carrier bags, the refillable water bottles and beyond that for the plastics that are essential to us we need to try to design them to be more circular. And that's going to need us to simplify the format so that we can scale up recycling efforts. People will say to me, you know, we can't recycle our way out of here and recycling has failed and it has to an extent because we're only recycling globally less than 10% of all the plastics we produce. It's not just a problem associated with recycling technology.
The challenge as I see it is that we're failing to make items that are compatible with circularity. You look at the thousands of different chemical additives, the dozens of chemical permutations, the complex products that we're making for single-use applications that have multiple layers of different types of plastic that make them almost impossible to recycle. So in short, in order to improve that third R, the recyclability, the circularity, so that we're using end-of-life plastics as the carbon source for new plastics and if we can do that we decouple ourselves from the waste that we've talked about in the oceans and on the land but we also would decouple ourselves from the use of fossil oil and gas as a carbon source.
When I talk to product designers they say they were never asked to think about end-of-life. You know, even for a single-use bottle that seems incredible to me that you're designing something that brings convenience to humans for an instant yet can persist on the planet for hundreds if not thousands of years. That's the bit that's got to change.
We've got to think about plastics right from the design stage to make sure they're safer, more sustainable, don't escape to the environment and don't therefore release microplastics.
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