Nanoplastics as endocrine disruptors

Introduction

Plastics are present in our everyday life and widely used. Strategies to reuse or recycle plastic waste are developing but only 21–26% of the plastic waste are appropriately recycled and incinerated (Geyer et al., 2017). The rest is incinerated in open pits or discarded to the environment, leading to plastic pollution of water, air, soil…

Degradation of plastic waste generates small particles that are defined as micro- or nanoplastics: “particles unintentionally produced (i.e. from the degradation and the manufacturing of the plastic objects) and presenting a colloidal behaviour, within the size range from 1 to 1000 nm” (Gigault et al, 2018).

Nanoplastics get into the environment by being degraded from bigger particles but they can also come from industries making and using nanoplastics in the first place. Researches conducted worldwide resulted in nanoplastics being found in the sea, sediments, air, rivers…being detected in all oceans led to particles being “absorbed and bio-accumulated by marine animals” (Jiang et al, 2020). They usually then contaminate animals by getting into the food chain in various ways.

More and more studies seem to find that nanoplastics have a direct effect on our health, therefore, raising concerns across the globe.

Why do nanoplastics are said to disrupt the endocrine system ?

Where are they found ?

According to the WHO (World Health Organization) the main sources of micro and nano plastics are:

• Road traffic: via tire abrasion, braking and road markings
• Plastic particles from the cosmetic industry and personal care products
• Paints used in construction work, i.e., marine coating
• Washing of synthetic textiles
• City dust:
  • Artificial grass
  • Industry: i.e., sandblasting to reduce and replace sand
  • Improper disposal of waste
  • Plastic packaging, plastic bottle, cigarette release into the environment

Figure 1 : Global releases of micro and nano-plastics to environment (WHO)

Therefore, this environmental contamination by nanoplastics can be found worldwide, in both marine and terrestrial ecosystems (Jiang et al, 2020). As plastics are said to account nearly eighty per cent of all waste found in our oceans. (...?)

We can distinguish the primary nanoplastics, which are plastic particles commonly added to personal care products, and secondary nanoplastics resulting from the degradation of large pieces due to biodegradation, UV light and/or physical wear (Jiang et al, 2020). On a global level, primary micro and nano plastics contribute only a little to environmental pollution, in reality, the majority of micro and nano plastics in the environment are secondary.

Studies have found out that a lot of nanoplastics are getting into the food chain through water consumption, especially in bottled water: “an individual who only ingest bottled water is potentially consuming an extra 90,000 particles in comparison to people who only drink tap water, who will ingest only 4000 extra particles.” (Cox et al, 2019). Indeed, plastic particles have a very long life-span. Plastics break down very slowly and are transported over long distances. As a result, microplastics are found in waterways, soil and air and because of its long life-span, it accumulates in the environment, sometimes in large quantities.

Figure 2 : Degradation pathways of Plastics to Microplastics & Nano-plastics particles

How do nanoplastics disrupt the endocrine system ?

Their effect on animals

The danger of large pieces of plastic is obvious. If animals mistake these pieces of plastic with potential food, they ingest it, leading to potential abrasion, occlusions and abscesses in the intestinal tract.

Nanoplastics and microplastics are largely found in marine environments as researches conducted worldwide resulted in nanoplastics being found in the sea, sediments, air, rivers… and detected in all oceans. It is not surprising to find these plastic particles in animal’s organisms living in this biotope:“Perturbation of organismal physiology and behaviour by micro- and nanoplastics have been widely documented for marine invertebrates. Some of these effects are also manifested by larger marine vertebrates such as fishes” (Yong et al, 2020).

Numerous studies focus on the health impacts of micro-/nanoplastics on non-mammalian marine animals as oceans can represent the ultimate repository for plastic waste. Moreover, they are an important food source for humans, and represent one pathway by which humans may be exposed directly to plastic particles. But nanoplastics are quite difficult to distinguish from other organic materials; there is a lack of research on these very small particles due to the inability of analytical techniques to be used for nano-sized particles.

Recent studies have shown that feeding micro and/or nanoplastics to fish would result in some degree of toxicological and/or pathological effects, toxicological responses arising particularly when exposed to smaller plastic particles that others: Larger plastic particles at around 100 µm or above were shown not to have any significant effect in a number of studies (Yong et al, 2020).

A good example of endocrine disruption was seen when the Medaka fish was given nanoplastics, the particles quickly accumulated inside the intestine, gill and liver and the observed effect was a reproductive endocrine disruption in a sex-dependent manner (Cong et al, 2019). According to different studies conducted by various scientist, nanoplastics exposures can cause health problems in different species and in various forms: such particles can for example cause reproductive toxicity in oysters, reduce the number of follicles and sperm motility in oysters as well as the production and development of offspring larvae (Sussarellu et al, 2016), liver toxicity in zebrafish (Lu et al, 2016) and tissue bioaccumulation with potential organ toxicity in mice (Jiang et al, 2020).

But the conclusion that could be drawn from such results are still to be taken into consideration cautiously as we poorly understand the mechanisms behind the said toxicity. Moreover, “The toxicity of any substance is determined by its concentration and diameter and other physical parameters.” (Jiang et al, 2020), which means that the size of the plastic particles and different concentrations will cause inconsistency in their toxic effects. Chae et al (2019) suggested that “the size of microplastics is closely related to the corresponding biological effects, and that nanoplastics may cause more serious biological toxicity”. More sophisticated and targeted evaluations are necessary to determine the health impacts of these particles on marine animals.

Based on the results obtained from mammalian animal models, it is reasonable to assume that plastic particles can possibly accumulate and affect human health.

Their effect on humans

On the other hand, we currently know very little about the effect of microparticles and nanoparticles of plastics consumed by humans. The effect of microplastics and nanoplastics particles has to be connected with the effect of associated substances. In fact, in addition to plastics, we should be concerned about the toxic substances such as softeners or chemicals associated with plastic. In summary, we should be concerned about the “cocktail” effect of the particles ingested.

According to the WHO (World Health Organization), micro and nano plastics are potentially responsible for three major problems in humans:

• Physical damage in the human body (intestinal tract)
• Chemical damage from additives (i.e. softerners)
• Endocrine disruptors

Another problem starting to raise concern but not in our discussion today is the carcinogens damage of plastic particles.

Recently, researchers have begun to use mammalian animal models to predict the potentially harmful impact of micro-/nanoplastics on human health. However, it is not yet proven that the micro-/nanoplastic particles absorbed by the human body enter the internal circulation through the gastrointestinal tract and ultimately cause organ damages. Also, there are currently no accurate datas to determine the daily exposure and intake of micro-/nanoplastics. Indeed, for macro plastic particles, the endocrine perturbations have been shown with studies on the pregnant ewe (Viguié et al, 2012). However, current studies are still running for micro and nanoparticles and their potential harmful effects as endocrine disruptors. Another example is the study of Deng et al (2017), which suggests that microplastics can cause lipid metabolism disorders and liver inflammation in mice, as lipid droplets were detected in the liver.

Humans are exposed to micro and nanoplastics largely through ingestion, either directly from food or with the packaging. “The pathophysiological consequences of acute and chronic micro- and nanoplastics exposure in the mammalian system, particularly humans, are yet unclear” (Yong et al, 2020). The diversity of plastic’s components leads to having a large list of substances capable of causing harm for humans. We already know thousands harmful substances and manufactured chemicals (called EDCs) and known EDCs that leach from plastics include bisphenol A and related chemicals, flame retardants, phthalates, per- and polyfluoroalkyl substances (PFAS), dioxins, UV-stabilizers, and toxic metals such as lead and cadmium. "Many of the plastics we use every day at home and work are exposing us to a harmful cocktail of endocrine-disrupting chemicals" said Jodi Flaws, Ph.D., of the University of Illinois at Urbana-Champaign in Urbana. For example, mercury and microplastics seem to be able to bioaccumulate together, they were observed in Dicentrarchus labrax, the sea bass. The analysis in the brain and muscle tissues showed significant interactions between mercury and microplastics (Barboza et al, 2018).

It appears that a combination of microplastics and their adsorbed chemicals can be more toxic than either counterpart on its own, especially heavy metals are a potential hazard to both wildlife animals and humans.

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