On March 7, 2024, researchers from the University of Campania in Italy published a research paper titled "Microplastics and Nanoplastics in Atheromas and Cardiovascular Events" in the New England Journal of Medicine, proving for the first time a direct link between microplastics and human health. The article conducted a three-year follow-up study of nearly 300 cases with carotid plaque, of which 58.4% of patients were detected with polyethylene (PE) and 12.1% of patients were detected with polyvinyl chloride (PVC) micro-nanoplastics. Containing microplastics in the arteries is 4.53 times more likely to cause heart disease, stroke or death in about 34 months after surgery than in cases without microplastics, but this does not rule out other factors that can also cause diabetes or cardiovascular disease. At the same time, the study focused on people who needed surgery to reduce their risk of stroke, and there was a lack of research on healthy people who were also likely to have microplastics, which correspondingly have a wide range of effects. So to what extent does microplastics affect human health, and how should we deal with and prevent it?
1. What are microplastics?
Due to their low cost and excellent performance, plastics and their products are widely used in industry, agriculture and daily life. It is worth noting that global plastics production has risen sharply over the past 70 years, from 2 million tonnes in 1950 to 454 million tonnes in 20181, which can be divided into two categories: thermosets and thermoplastics based on their physical and chemical properties. Thermosets are characterized by high thermal resistance and electrical properties, as well as good mechanical strength and chemical stability. Due to these properties, they are widely used in fields such as electronics, automotive, construction, and aerospace. Common thermosets include phenolic resins, amino resins, and unsaturated polyesters. Thermoplastics can be melted by heating, so they can be recycled continuously, and the heat resistance and chemical stability are relatively poor, which also makes these plastics more likely to be broken down in the environment than thermosets to form microplastics. There are 5 types of thermoplastics commonly used: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and ABS. Among them, PE is mainly used for packaging film of disposable plastics, and HDPE is mainly used for bottle making and packaging, including food, medicine, cosmetics, pallets, film manufacturing, plastic bags and toys, and sheets. For low-density polyethylene LDPE and linear low-density polyethylene LLDPE, it is mainly used for films, including plastic bags and packaging. PP is mainly used in the production of clothing, blankets and other fiber products, medical equipment, automobiles, bicycles, parts, transportation pipelines, chemical containers, etc., and is also used in food and pharmaceutical packaging. PVC is widely used in cables and pipes, flexible packaging. PS is widely used in light industrial market, daily decoration, lighting indication and packaging, etc. ABS plastic is a terpolymer of acrylonitrile (A), butadiene (B) and styrene (S), which has been widely used in machinery, electrical, textile, automobile, aircraft, ships and other manufacturing industries and chemical industries.
The widespread use of plastics in various industries has led to a large number of them entering the natural environment, and new problems are constantly emerging. In 2004, British scientist Thompson et al. first proposed the concept of microplastics in an article on plastic pollution in the UK published in Science, which attracted the attention of the scientific community, governments, non-governmental organizations and others. As for the definition of microplastics, there is no unified standard in the world, and now it usually refers to microplastics with a diameter of less than 5 mm, and then defines plastics with a diameter of less than 1 μm as nanoplastics, collectively referred to as MNP (microplastics and nanoplastics), and such microplastics are collectively referred to as microplastics in this article.
2. Where do microplastics come from?
Microplastics can be broadly divided into two categories from the perspective of source: primary microplastics and secondary microplastics. Virgin microplastics are plastics that are manufactured directly into tiny sizes during the production process and are commonly used in scrubs, facial cleansers, toothpastes, and certain cleaning products in personal care products, and are designed to remove dead skin, clean, or as fillers in products. Secondary microplastics are plastic particles formed by the physical, chemical and biological processes of various plastics such as plastic bottles, plastic bags, fishing nets, and synthetic fiber fabrics.
Figure 1 Sources of microplastics2
In a 2017 report published by the International Union for Conservation of Nature (IUCN), "Primary Microplastics in the Ocean", the top seven sources of marine microplastics are plastic particles, synthetic textiles, tires, road markings, marine coatings, personal care products and municipal dust3, as shown in the figure below.
Figure 2. Percentage of sources of microplastics
3. Health risks of microplastics
After clarifying the definition and source of microplastics, we have come to the most concerned question in this article - what are the exposure routes of microplastics to the human body, what are the harms that can be produced, and whether we can continue to use plastic products?
The main routes of human exposure to microplastics include dietary intake, respiratory inhalation, and skin contact. Dietary intake is the main way for microplastics to enter the human body, whether it is the food we eat, the drinking water and beverages we drink, or their packaging, which may become the source of our ingestion of microplastics. Microplastics produced by road traffic and textiles enter the air and enter the lungs with breathing. The skin barrier inhibits the absorption of microplastic particles larger than 100 nm, while microplastics smaller than this size may cross the dermal barrier and enter the human circulatory system. The main exposure routes of microplastics are as follows:
Figure 3. Exposure routes to microplastics
In order to explore the physiological changes of human exposure to microplastics, this paper summarizes the results of animal experiments, human organoid experiments and epidemiological investigations in 37 toxicological literatures in the past five years, and discusses the risks of microplastics to human health.
Table 1: Summary of literature on toxicity of microplastics
Thirty-one articles on animal experiments have shown that in studies using mice and rats as animal models, the main toxic effects mainly occurred in the digestive, neurological, respiratory, reproductive, and cardiovascular systems. Among them, the toxicity of PS microplastics has been widely confirmed, such as causing intestinal barrier damage, neurotoxicity, causing lung inflammation, reducing the number of mature germ cells, and inducing cardiac fibrosis.
Organoids are three-dimensional structures made up of a variety of cell types produced by stem cells. As a new model that more closely resembles natural human organs, organoids are well suited for microplastic exposure and toxicity studies. On the one hand, among the five articles that took human organoid experiments as the way of exploration, two articles with polyester fiber, PP, PA, TPU microplastics as the research objects showed that microplastic fibers would not inhibit the growth of airway organoids, nor would they cause related inflammation or oxidative stress, nor would they cause cytotoxicity of intestinal organoids, nor would they change their barrier integrity. On the other hand, the remaining three studies on PS microplastics reported that PS microplastics would have toxic effects on intestinal organoids, liver organoids and forebrain organoids, impairing their physiological functions.
An article published in the New England Journal of Medicine in March this year on the relationship between microplastics and heart disease, stroke and other diseases proved for the first time through epidemiological investigation methods to prove the direct relationship between microplastics and human health. The new study, led by researchers from the University of Campania in Italy, looked at 257 people with atheroma in their carotid arteries. These plaques can restrict blood flow to the brain, increasing the risk of stroke. The average age of the participants was 72 years, and the researchers followed them for an average of 34 months. The results of the study showed that traces of PE were detectable in carotid plaques in 58% of study subjects and traces of PVC in carotid plaques in 12% of study subjects.
As can be seen from Table 1, PS appears most frequently in microplastic toxicology articles, and its toxic effects have been found in different experiments on different organs. PE, which occurs with the second highest frequency, has also been confirmed by animal experiments and epidemiological investigations to have adverse effects on the hematopoietic system, digestive system, and cardiovascular and cerebrovascular systems. In addition, there are a few articles that verify the health risks of PP, PVC, PMMA, PA, TPU, polyester fibers and rubber to different organs and tissues. Among them, PP had an adverse effect on the intestinal barrier function of mice, but no cytotoxic pro-inflammatory response was found when PP, PA, TPU were exposed to intestinal organoid models closer to human tissues, and polyester fibers had no significant effect on the growth and function of airway organoids. PVC can induce liver injury and intestinal dysbiosis in mice, PMMA can inhibit the self-renewal ability of hematopoietic stem cells, and inhalation of rubber microplastic particles produced by tire wear can induce pulmonary fibrosis injury.
As mentioned above, diet is one of the main ways to contact microplastics, and the food contact plastics that we are exposed to in our daily diet usually include PP for plastic crisper boxes, plastic bottles/water bottles, plastic lunch boxes and PET for beverage bottles. At the same time, in order to have obvious toxicological consequences, the exposure concentrations of microplastics in animal experiments and human organoid experiments are often much higher than in the actual situation. Therefore, in our daily life, we do not need to be overly anxious and worry about the risks and harms brought by "plastic from the mouth" to our health.
4. How should we deal with microplastics?
In order to reduce the harm caused by microplastics to the ecological environment and human health as much as possible, we can start from two aspects: macro reduction of plastic use and micro adjustment of living habits. In terms of reducing the use of plastics, many countries' policies on plastic restrictions, taxes or paid use have effectively reduced the production and consumption of plastics. For example, the use of plastic bags in mainland China has decreased by 49% after the introduction of the plastic restriction order, while Botswana and Denmark have reduced the use of plastic bags by 50% and 66% respectively after the implementation of plastic bag taxes4. These examples illustrate how government policies can have a significant impact on reducing plastic consumption and mitigating microplastics in the environment.
On the other hand, we can minimize the exposure to microplastics by adjusting the following aspects of living habits: first, boiling water before drinking, boiling hard water can effectively remove at least 80% of microplastics, even in soft water, boiling can remove about 25% of microplastic particles; Microwave-heated plastic feeding bottles, but use glass or stainless steel bottles instead; Fourth, when buying ingredients, try to choose natural foods that have not been processed too much, such as fresh vegetables, fruits and meat, and try not to use plastic bags to directly hold food for direct consumption; fifth, try to choose clothes made of natural fibers (such as cotton, linen, silk) to avoid the release of microplastics from synthetic fiber fabrics during the washing and drying process; Sixth, in terms of personal care products, the National Development and Reform Commission issued the "Industrial Structure Adjustment Guidance Catalogue (2019 Edition)" It has been clearly stipulated: "The production of daily chemical products containing plastic microbeads will be prohibited by December 31, 2020, and the sale will be prohibited by December 31, 2022." "Nowadays, as long as we buy cosmetics or care products from regular channels, we can use them with confidence.
Science and technology are a double-edged sword, and through these ways, we can enjoy the convenience brought by plastics, while reducing its negative impact on us, and realize the harmonious coexistence of human beings and nature and the development of science and technology.
Bibliography:
1 Baseline report on plastic waste, UNEP and Basel Convention (2020) based on Geyer, 2020, New unpublished data. Geyer, R. (2020). Production, Chapter 2- Production, Use and Fate of Synthetic Polymers in Plastic Waste and Recycling. Letcher, T.M. (ed.). Cambridge, MA: Academic Press. pp. 13-22.
- 2Environmental Chemistry Letters, 04 Apr 2023, :1-41.https://doi.org/10.1007/s10311-023-01593-3
3 https://www.sohu.com/a/205971832_726549
4https://www.thaiheadlines.com/58167/。 2018《美国经济学杂志》