Microplastic content of over-the-counter toothpastes - a systematic review

Introduction

A future without plastics is becoming more and more difficult for the younger generation to imagine. Because of their durability and lack of biodegradability, plastic materials are both more appealing to humans and harmful to the environment. Environmental experts today are extremely concerned about microplastics. Microplastics are defined by the United Nations Environment Programme as, solid phase materials, less than 5mm in size, water insoluble, nondegradable and made of plastic.¹ They are divided into primary and secondary microplastics based on the manner of development. While secondary microplastics are produced when large plastics are broken up into tiny detritus, primary microplastics are plastics designed to have a microscopic size.² There are various types of primary microplastics such as polypropylene, polymethyl methacrylate, polyethylene terephthalate, polymethyl methacrylate, nylon, and polyethylene.³

Microplastic particles that are manufactured for their use as ingredients in personal care products are called microbeads or cosmetic microplastics and although they are characterized by a size less than 0.8 mm, most of the microbeads are less than 0.1 mm.⁴ As a result, since traditional wastewater treatment systems are not built to remove microplastics, they bypass these treatment facilities and are subsequently transferred into river and sea waters, where they persist forever because of their non-biodegradable nature.⁵ Presence of microplastics in sea water, freshwater, fruits and bottled water have been found across countries.^6–9

Where do these microplastics in the environment come from? It was in the 1990’s that it was first recognized that personal care products such as face washes/scrubs, shower gels and toothpastes formed one of the main sources of microplastic pollution, especially in the marine environment.¹⁰ These microplastics when subjected to ultraviolet radiation, gets degraded and absorb persistent organic pollutants (POPs) like polychlorinated biphenyls (PCBs), becoming more toxic in the long-term.¹¹ Once these microplastics make their way into the marine environment, they are ingested by marine organisms at the base of the food chain.² Studies have reported toxic effects, such as structural changes to the gills, necrosis in other tissues as well as a heightened immune response in mussels that were exposed to microplastics in water bodies.^12,13

In addition to being a potential pollutant to the environment,^3,14 the transfer of microplastics to humans via their engulfment by aquatic animals can become a severe threat to public health.¹⁵ Studies have not only identified microplastics in human placenta in utero¹⁶ but have also shown evidence of interactions between microplastics and gut epithelium leading to oxidative impairments and disturbance in the inflammatory intestinal balance, impacting the epithelial permeability of the gut and toxicity of immune cells.¹⁷ It has been reported¹⁸ that prolonged use of toothpaste containing microbeads aggravates the inflammatory process of the gingiva and that the risk of these being unintentionally ingested on a day-to- day basis could also be potentially dangerous to health.³

Although many studies have confirmed the presence of microbeads in face washes/scrubs in significant quantities,^11,19–21 very few evidence exists^22–24 with respect to the presence of microbeads and their characteristics in toothpaste.

Lebreton and Andrady²⁵ have reported that 60–99 million metric tonnes of mismanaged plastic waste were produced worldwide in 2015 and have predicted that the world’s microplastic emission would triple by 2060 to reach up to 170–270 million metric tons/year, thus portraying a bleak picture. Considering these glaring evidence, the United Nations Environment Program has recommended an eventual phase-out and ban of microplastics in personal care products and cosmetics.¹

However, is it even achievable? Microplastics were originally used in personal care, cosmetics, and cleaning products (PCCPs) as abrasives for their scrubbing effects. But now they are believed to carry out several other crucial functions, including acting as binders, bulking agents, emulsifiers, exfoliants, film formers, viscosity regulators, opacifying agents, glitters, skin conditioners, tooth polishing agents, moisturisers, and stabilizers.²⁶

This systematic review was conceptualized to identify evidence for the presence of microplastics in toothpaste formulations, with the hope that this evidence might help in understanding where we stand in our goal of phasing out and eliminating microplastics from toothpaste formulations all over the world. The findings would have implications for research as well as policy implications too.

Methods

Results

Study selection

A total of 905 articles were obtained, of which, 695 were from Google Scholar, 128 from Springer Link, 23 from Embase, 22 from PubMed, 20 from Scopus and 17 from Web of Science.

Identification of duplicates was done, resulting in the removal of 46 articles and 859 articles were selected for level 1 title screening. After title screening, 808 articles were excluded since they did not confirm with our inclusion criteria and 16 articles were found to be eligible for the next process of full text review. When the full text review was carried out, ten articles were excluded and finally six studies were included for systematic review (Figure 1).

Figure 1. Flow chart of steps in literature search.

Discussion

Microplastics in water are susceptible to physical forces, fluctuating temperatures, ultraviolet radiation, oxidation, and salinity. They also become coated with bacterial biofilm. Since they settle to the bottom and have altered surface morphologies and higher densities, these weathered or conditioned microplastics are more readily available to a wide range of marine organisms.³¹ However, polyethylene is a type of microplastic that floats on the water’s surface due to its specific density of <1,³² and therefore it is available to a range of planktonic species as well as fish and seabirds that eat at the water’s surface.

Although evidence shows that microplastics are contaminating our environment in great proportion and constitute an environmental hazard leading to health impacts, they are still being incorporated in personal care products including toothpastes. Our focus was on microplastics in toothpastes, and we found a relative dearth of studies in this area. Only six studies fulfilled our inclusion criteria and were included in this systematic review, showing evidence only from a few countries like China,^3,29 Malaysia,²² Turkey,²³ Vietnam,²⁹ Myanmar,²⁹ UAE³⁰ and India,²⁴ out of which 3 studies^22–24 showed evidence of their presence in toothpaste.

The existence of microplastics was reported in three investigations, two^22,23 of which identified polyethylene as the microplastic while the third, an Indian study,²⁴ identified cellophane, polypropylene, polyvinyl chloride, and polyamide. Interestingly, the samples from India did not include polyethylene, a common polymer found in toothpastes in other studies.

The size of the microplastic particles found in the selected studies ranged from 3.5 μm to 145 μm. Because of their small size, microplastics flow through wastewater systems and are probably not detected by wastewater treatment facilities. Microplastics are available to microscopic species at the base of the food chain because of their limited size range and resistance to degradation in the environment. As a result, they enter the marine environment where they are consumed by marine life and then they enter the human body.¹¹

Regarding the shape of the microplastics present in toothpastes, studies reported them to be granular,²² irregular in appearance²³ and as fragments and fibers.²⁴ Evidence³³ shows that these microplastics are not always spherical but have a variety of irregular shapes. Despite the fact that the word “bead” refers to spherical particles and despite the widespread misconception that microbeads are the coloured spherical microparticles found in toothpastes,¹¹ microbeads are typically of irregular form to improve abrasion.^4,34 In reality, PCCPs may contain multiple types of bead shapes, and irregular shapes contribute to larger specific surface areas and therefore more friction.³⁵

Regarding the color of the microplastics present in toothpastes, one study²³ reported opaque appearance and another²⁴ as colorless. Microbeads have been shown to have different colors such as white, transparent, opaque, blue, red and orange.¹¹

Three studies^22–24 also did the environmental risk assessment for microplastics released through the toothpaste. Praveena et al.²² reported that five facial cleansers and five toothpastes released 0.199 trillion microbeads per year in Malaysia. Ustabasi and Baysal²³ reported that an estimated average of 871 million grams of microplastics are released every year from toothpastes in Turkey. According to Madhumitha et al.,²⁴ in India, 1.4 billion grams of microplastic particles are emitted from toothpaste each year on an average. Using toothpaste might carry a significant danger of increasing the amount of microplastics that are added to the environment.³⁶

The quality assessment of the selected articles showed that all the studies were classified under medium risk of bias. None of the articles gave any detailed explanation of the sample size calculation and the sampling technique. The criteria of outcome assessor details and blinding was also not specified in all the six selected articles. This highlights the need for maintaining scientific rigor in every research irrespective of where they stand in the hierarchy of evidence.

Conclusions

The result of this systematic review shows that toothpastes, at least in some regions of the world, do contain microplastics and there is a significant danger that using these toothpastes will increase the amount of microplastics added to the environment. Even if it is begun to take steps now, to fully outlaw microbeads internationally, the environment would continue to contain microplastics for a very long time. It is therefore high time to initiate urgent action to curb the menace of microplastics in the environment by eliminating its presence in personal care, cosmetics and cleaning products globally.