Everyone knows that many animals, especially those living in the sea, are dying due to the ingestion of plastic. The marine mammals, turtles, big fish and marine birds swallow plastics which remain in some part of their digestive tract, without being able to digest them or eliminate them, and that ends up killing them. The pictures of this reality that we see in the media are very disturbing. We should all be aware of the serious environmental problem represented by the huge amount of plastics that continue to be scattered throughout the planet.
Plastic is a very durable material, which can withstand bad environmental conditions and remain unchanged for a long time. But it can also be broken into smaller fragments by the effect of the sun’s rays and by mechanical abrasion, until it becomes “invisible” particles which can measure micrometres or nanometres1, and which also persist in the environment for a long time. In addition, the additives that are used for the manufacture of plastics are chemicals which are also spread with the plastic2.
Plastics have been found in small fish, crustacean decapods (crabs, lobsters, prawns, shrimps, etc.), bivalve molluscs (clams, mussels, oysters, etc.), barnacles and also in zooplankton (animal plankton: formed by protozoa and larvae). Plastic of very small size and which can be assimilated, among others, by the aforementioned animals, is called microplastic (particles of 1 millimetre maximum) or nanoplastic (less than 100 nanometres)3.
Microplastics end up in the bodies of these small aquatic animals because they are in the sea water and also in rivers, lagoons and other terrestrial water deposits2. Microplastics have been detected in different marine species on which we feed, in all the seas and oceans of the planet1.
Since we are part of the food chain in which animals that ingest plastic are found, it is logical to think that we are also eating plastic. In this case, can the ingestion of plastic affect people’s health? This is a very worrying question, and one about which little is known.
Plastic particles have been detected in water before it is purified for drinking, in the food of aquatic or terrestrial origin that we eat, and also in the air2. But, at the moment, it is not known what effect ingestion or inhalation of plastic particles can have on humans.
Investigations about what happens in small marine animals (oysters, worms, crustaceans, mussels, fish, zooplankton) indicate biological changes in them: inflammatory reactions, increased immune responses, lower food intake, reduced energy reserves, lower reproduction and increased mortality3. Although it may seem that the biology of these animals has nothing to do with ours, this is not the case, because we are animals and the harmfulness of a substance on a living being could also affect the human being.
It could be thought that the microplastics that we take in, not being organic products, could pass through our digestive tract unnoticed, leaving our bodies as they entered them, without passing into the blood or tissues. The foods we eat are organic products and are broken down by the enzymes of the digestive system into molecules that can pass through the wall of the intestine. But since plastic is not organic, it cannot be broken down by intestinal enzymes, and one might think that it has no means of passing through the wall of the intestine. But, unfortunately, this might not be the case. Because there are other mechanisms, other than absorption, such as persorption, pinocytosis and phagocytosis, by which small particles can pass through the intestinal wall without the need to be broken down into absorbable substances2. Through these mechanisms, the microparticles reach the blood and the lymph, and from there they can be distributed through the tissues of the body. So there is a possibility that this happens with the microplastics.
But it has not yet been proved that microplastics are entering, circulating and accumulating in our bodies. If this is happening, what happens when they are in the blood and tissues? Do they produce any type of toxicity? And if so, for how long? What happens if they penetrate inside the cells? At the moment it is not known. It has been observed that nanoplastics can cross the membranes of cells, although it is still not known whether they can affect cellular functions3.
What is a reality is that microplastics are involved in the life of the planet, that of humans and other animal species, in a global way. It seems evident that by eating sea products that contain microplastics, especially those whose flesh is eaten completely, such as oysters, mussels and small fish, we are ingesting plastic. And, for the moment, there is no food regulation that takes into account the presence of microplastics in food from the sea or in other products4.
The physical characteristics of plastics and microplastics also facilitate the concentration of contaminants on their surface3, that is to say, they attract more pollution. To give two examples, bacteria that are pathogenic to humans (such as Escherichia coli) have been found in plastic waste from the coasts of Belgium; and plastics from stagnant waters create an ideal habitat for the larvae of mosquitoes which transmit the Zika virus or the dengue parasite5. And this is related to health, specifically with the risk of acquiring infections.
It is frightening to think that plastic can form part of the tissues of our body. This subject produces a feeling of helplessness, because if we are incorporating plastic into the body, we are not aware of this, and we do not know how we can avoid it. We do not know whether it is harmful to health or not, and if it is, from what amount. There are many unanswered questions that need an urgent response.
At the moment, a theoretical incorporation of plastic into our bodies, in unknown quantities which will be different in each person, has not shown obvious signs of toxicity in the short term, for example, producing disease or killing people quickly, as with certain known toxic substances or with micro-organisms which cause infections in humans. As plastic is an inert material, it could go unnoticed (or not) by the immune system, but this does not mean that it is innocuous; nor do we know how it can alter cellular and molecular functions, nor in what period of time.
It is difficult to evaluate the risk posed by microplastics to humans, because it would be unethical to do experiments on people. But it is possible to investigate, on cellular and animal experimental models, the interactions of microplastics with the cells of the intestine, to know more about their mechanisms and their degree of penetration into the circulatory system, and also about the interactions of the microplastics and nanoplastics in the tissues and inside the cells. The results of these investigations, which we await with impatience, will allow us to know whether contamination by plastic can affect human health, and if so, how.
While plastics are being replaced by other materials of organic origin, which are easier to eliminate from the environment, it is absolutely necessary for everyone to do everything in their power to stop the invasion of the environment by plastics. We will have to be attentive to the solutions that are given, both in local and global environments, and to follow them, in order to stop polluting. Because regardless of what may affect human beings, we must protect nature, which is already very affected, and it is not known whether it can recover from the destruction caused by plastic and other pollutants.
Dr. Ana M. Cerro. PhD and Immunologist.
Bibliografía
1. Hantoro I, Löhr AJ, Van Belleghem FGAJ, Widianarko B, Ragas AMJ. Microplastics in coastal areas and seafood: implications for food safety. Food Additives & Contaminants: Part A. 2019;1–38.
2. Galloway TS. Micro- and Nano-plastics and Human Health. In: Bergmann M, Gutow L, Klages M, editors. Marine Anthropogenic Litter [Internet]. Cham: Springer International Publishing; 2015 [cited 2019 May 8]. p. 343–66. Available from: http://link.springer.com/10.1007/978-3-319-16510-3_13
3. Carbery M, O’Connor W, Palanisami T. Trophic transfer of microplastics and mixed contaminants in the marine food web and implications for human health. Environ Int. 2018;115:400–9.
4. Santillo D, Miller K, Johnston P. Microplastics as contaminants in commercially important seafood species. Integr Environ Assess Manag. 2017;13(3):516–21.
5. Vethaak AD, Leslie HA. Plastic Debris Is a Human Health Issue. Environmental Science & Technology. 2016;50(13):6825–6.