Nanoplastics – when size matters

The smaller the plastic particles, the more likely they are to cross the blood-brain barrier, reach organs and tissues and accumulate in them. And this increases the risk of their negative effects on our health. Thanks to the development of increasingly sophisticated analytical methods, researchers are able to see the scale of the previously overlooked problem of the smallest microplastics – the so-called nanoplastics.

What are nanoplastics?

Nanoplastics are defined as plastic particles ranging in size from 1 to 100 nanometers (sometimes up to 1,000 nanometers) [1]. Like microplastics, they enter the environment from waste and everyday products – paints, cosmetics, biomedical products, detergents and fabric softeners, 3D printing and many others. Nanoplastics are distinctly different from microplastics, both in their physicochemical properties and biological reactivity. It is estimated that the abundance of nanoplastics in the environment is several orders of magnitude higher than microplastics. There may be more than 300 million tons of them in the global environment [1, 2].

In addition, growing evidence suggests that ingested nanoplastics cross the intestinal barrier and travel to secondary tissues. It is worth mentioning that microplastics are largely physically retained by the gut [1]. Therefore, researchers suggest that smaller particles pose a greater threat to ecosystems and human health. However, data on the concentrations of nanoplastics in human bodies are limited. It is difficult to identify them in tissues, even after the death of the carrier. Representative nanoplastics, made of polyethylene, polypropylene, polystyrene, polyvinyl chloride or polyethylene terephthalate, have a density similar to water (0.88-1.50 ^g/cm3), which favors their transport in suspension rather than their deposition [1].

In addition, the tiny particles have a strong tendency to aggregate, i.e. they quickly attach to larger colloidal systems consisting of organic or inorganic molecules. These include. natural clay materials, humic acids, polysaccharides, but also biological particles – bacteria, fungi or chemical pollutants from industry and consumption (those originally found in plastics, as well as those sorbed from the environment).

And during the degradation and decomposition of polyethylene, among other things, peroxides are formed (which are cytotoxic, meaning they have toxic effects on living cells). Like microplastics, nanomaterials also interact with their surrounding environment, so their physical and chemical parameters can change. Because of this, we are more often exposed to nanoplastics, whose properties differ significantly from the pristine plastics used in consumer products (and often in scientific research). There is still little research on nano-contaminants of environmental origin [1-3].

When does a nanoplastic become toxic?

The toxicity of nanoplastics largely depends on their transformation at the nano-bio interface, as a result of physical, chemical and biological interactions with various biomolecules (e.g., proteins, lipids, carbohydrates) in membranes, organelles, cells, tissues or physiological fluids [1]. The eco-corona and protein corona formed as a result of transformation significantly affect their physicochemical properties and biological activity, and facilitate the induction of negative health effects, because their composition is crucial in the transport of nanoplastics around living organisms, including blood plasma.

In studies with crowned polystyrene nanoplastics (13-135 nanometers), researchers have confirmed their higher genotoxicity and cytotoxicity in human blood than virgin plastics of these sizes [1, 2]. Degradation of microplastics to nanoplastics probably also occurs in the human body, but it is unclear to what extent nanoplastics can be degraded in the acidic environment of the stomach and neutral environment of the intestines [3].

Researchers also still disagree on how nanoplastics can cross the blood-brain barrier. The discrepancies in research results are probably the result of individual differences in the properties of the plastics. Perhaps even a subtle change in research methods yields different experimental results.

The main route for micro- and nanoplastics to enter the human body is through ingestion, oral exposure and through the gastrointestinal tract. But nanoplastics can also enter the body through inhalation and absorption through the skin. Previous studies show that only a small fraction of micro- and nanoplastics are able to pass through the alveolar wall into the capillaries and eventually into the bloodstream. The situation is similar in the case of skin – there is insufficient evidence for the penetration of particles through damaged skin, sweat glands or hair follicles [3, 4].

Much more research is being done on the interaction between nanoplastics and the digestive system, and on their accumulation in selected organs. Most microplastics from the digestive tract are removed naturally. However, studies show that plastic particles smaller than 100 nanometers can cross the intestinal barrier. After crossing it, they enter the circulatory system. The largest blood vessel in the human body, the aorta, is about 25,000 µm in diameter, and the smallest capillary is about 8 µm, allowing nanoplastics to be carried, circulate in the blood and eventually accumulate in organs, tissues and body fluids. But it also raises concerns about the effects of nanoplastics on the circulatory system [1, 4].

What does a nanoplastic do?

The number of studies on the effects of micro- and nanoplastics on living organisms continues to grow, but only a small fraction of them report on the effects they can have directly in humans. The particles are transported to various organs, including the liver, so they are sometimes suspected of having hepatotoxic effects [2], as well as inflammatory response, oxidative stress and cellular dysfunction, leading to potential liver damage and dysfunction. In addition, these molecules react with other toxins or substances present in the body, exacerbating their negative effects on this organ [1, 2].

Preliminary studies have confirmed that micro- and nanoplastics inhibit lipid digestion and reduce vitamin D3 absorption. The main reason is that they can agglomerate nutrients and reduce their bioavailability or affect the activity of relevant enzymes. In addition, a stable gut microbiome is essential for human health, and micro- and nanoplastics can cause an imbalance of this microbiome. In vivo exposure experiments on model animals have shown that plastic particles alter bacterial counts in the gut of mice. In addition, higher levels of mitochondrial depolarization have been demonstrated on human colonic epithelial cells, resulting in intestinal barrier dysfunction, metabolic dysfunction, inflammation, and may eventually lead to the development of related diseases [1-3].

Experiments on animals or cells have shown that micro- and nanoplastics can lead to increased secretion of pro-inflammatory cytokines, disrupting immune homeostasis and ultimately leading to immune system disorders and causing autoimmune diseases [2].

Studies have confirmed that nanoplastics, by interacting with proteins found in the brain, may be responsible for changes associated with some types of dementia and Parkinson’s disease [5], which is now being called the fastest growing neurological disease in the world. Many data suggest that environmental factors may have a significant role in this, although most have not been identified [5].

Studies in wild and model organisms have linked exposure to micro- and nanoplastics to infertility, inflammation and cancer, but the actual broad health effects in humans are currently unknown [1, 6, 7]. Most human health research has focused on in vitro studies using human and other mammalian cell lines. Experiments show that nanoplastics, once absorbed, can be mistaken by immune cells for viruses, inducing an inflammatory response. Hence, effects such as oxidative stress, immune reactions, genotoxicity, DNA damage, neurotoxicity and reproductive impairment are observed [3, 7].

However, the findings should be interpreted with caution, as cancer cells show marked differences in metabolism and reactive oxidative stress, and the magnitude and type of exposure to nanoplastics varies strongly and depends on lifestyle and diet, location, presence of other contaminants, type of plastics, their origin, age and many other factors

Dr.-Ing. Edyta Łaskawiec – water and wastewater technologist, scientist at the Zabrze Institute of Fuel and Energy Technology, science popularizer, author of an educational profile on Instagram platform: wastewater_based.doctor. Nominated in the POP SCIENCE Science Popularizer Contest of the Silesian Science Festival Katowice 2024.

In the article, I used, among others. z:

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[2] E.D. Tsochatzis et al, Microplastics and nanoplastics: Exposure and toxicological effects require important analysis considerations, Heliyon, Volume 10, Issue 11, 2024, e32261, https://doi.org/10.1016/j.heliyon.2024.e32261

[3] Y. Feng et al, A systematic review of the impacts of exposure to micro- and nano-plastics on human tissue accumulation and health, Eco-Environment & Health, Volume 2, Issue 4, 2023, 195-207, https://doi.org/10.1016/j.eehl.2023.08.002

[4] P.A. Stapleton, Microplastic and nanoplastic transfer, accumulation, and toxicity in humans, Current Opinion in Toxicology, Volume 28, 2021, 62-69, https://doi.org/10.1016/j.cotox.2021.10.001

[5] Z. Liu et al, Anionic nanoplastic contaminants promote Parkinson’s disease-associated α-synuclein aggregation, Science Advances, Volume 9, Issue 46, 2023, 1-20, https://www.science.org/doi/10.1126/sciadv.adi8716

[6] M.B. et al, Beyond microplastics – investigation on health impacts of submicron and nanoplastic particles after oral uptake in vitro, Microplastics & Nanoplastics, 2, 16, 2022, 1-19, https://doi.org/10.1186/s43591-022-00036-0

[7] Chi-Yun Chen, Zhoumeng Lin, Exploring the potential and challenges of developing physiologically-based toxicokinetic models to support human health risk assessment of microplastic and nanoplastic particles, Environment International, Volume 186, 2024, 108617, https://doi.org/10.1016/j.envint.2024.108617