Environmental hazard of polypropylene microplastics from disposable medical masks: acute toxicity towards Daphnia magna and current knowledge on other polypropylene microplastics

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The COVID-19 pandemic has increased the use of disposable plastics, including medical masks, which have become a necessity in our daily lives. As these are often improperly disposed of, they represent an important potential source of microplastics in the environment. We prepared microplastics from polypropylene medical masks and characterised their size, shape, organic chemical leaching, and acute toxicity to the planktonic crustacean Daphnia magna. The three layers of the masks were separately milled and characterised. Each of the inner frontal, middle filtering, and outer layers yielded different types of microplastics: fibres were obtained from the inner and outer layer, but irregular fragments from the middle layer. The shape of the obtained microplastics differed from the initial fibrous structure of the intact medical mask layers, which indicates that the material is deformed during cryo-milling. The chemical compositions of plastics-associated chemicals also varied between the different layers. Typically, the inner layer contained more chemicals related to antimicrobial function and flavouring. The other two layers also contained antioxidants and their degradation products, plasticisers, cross-linking agents, antistatic agents, lubricants, and non-ionic surfactants. An acute study with D. magna showed that these microplastics do not cause immobility but do physically interact with the daphnids. Further long-term studies with these microplastics are needed using a suite of test organisms. Indeed, studies with other polypropylene microplastics have shown numerous adverse effects on other organisms at concentrations that have already been reported in the environment. Further efforts should be made to investigate the environmental hazards of polypropylene microplastics from medical masks and how to handle this new source of environmental burden.

Introduction

With the crisis of the global coronavirus pandemic (COVID-19), the consumption of single-use plastics, including personal protective equipment, has increased significantly. This has become essential to prevent the spread of infection among healthcare workers and the general public [1–3]. Among the protective equipment, disposable medical (face) masks (also called surgical masks) are most commonly used by the general public, as some governments have recommended or mandated their use indoors as well as outdoors [1].

The use of medical masks as an infection control measure was common in East and South-East Asia at the onset of the COVID-19 pandemic, and it then gained momentum in the rest of the world during 2020 and 2021 [4]. The production volume as well as the use of medical masks is already enormous, and is expected to continue to increase in the near future. For example, at the end of April 2020, China was producing about 450 million medical masks daily. Indeed, as the leading manufacturer, the annual production volume of medical masks in China increased from 5 billion in 2019 to 10 billion in 2020 [5].

There is a wide variety of medical masks on the market that are made of different plastics, such as polyurethane, polyacrylonite, polyester, polyethylene terephthalate and polypropylene. The last of these, polypropylene, remains by far the most common material and has a long history of use [6–8]. This was also confirmed by searching online medical devices catalogues (search term: surgical masks; Medical Expo, 2020), where all of the medical masks that indicated the polymer composition (about 20% of those available) were polypropylene. In addition, many companies online sell polypropylene as the raw material for making medical masks. Generally, medical masks consist of three main layers: the inner frontal layer, the middle filtering layer, and the outer layer, which is usually water repellent and coloured. The filter materials are produced by a ‘non-woven’ approach, which refers to the layers of the fibres as bonded together by physical entanglements or contact adhesion between the individual fibres [9, 10]. This approach includes different processes, such as for melt-blown and spun-bond fabrics, each of which results in different final fibre diameters. The most commonly reported approach to produce the middle filter layer of the medical mask material is melt blowing, while the spun-bond process is used to produce the inner and outer layers of the masks [9].

The major environmental concern associated with the increasing use of disposable medical masks by the general public is poor waste management [2, 6, 7]. Although it was suggested recently that a possible route for waste management would be thermo-chemical conversion of disposable medical masks into value-added products [11], improper disposal of masks in public spaces [2] and into the environment in general [6, 7], is widespread. This contributes to global plastics pollution, which has numerous negative impacts on the environment [12]. In addition, as identified recently, disposable medical masks might represent a significant new source of microplastics [2, 6, 7, 13, 14].

Similar to other plastic items, once medical masks are disposed of into the natural environment, they undergo weathering processes, including ultraviolet radiation, temperature fluctuations, increased humidity, biodegradation, physical abrasion and chemical oxidation. Weathering affects the physicochemical properties of plastics, and eventually leads to their fragmentation into microplastics and nanoplastics [15–19]. The degradation of plastics is highly dependent on the polymer type and any chemical additives [20, 21]. Polypropylene, the material most commonly used in the medical mask production, is susceptible to photodegradation [14, 20, 22], heat [23] and atmospheric oxygen [24]. Polypropylene is excellent in terms of water resistance, but inferior in terms of weathering resistance. Moisture accelerates the oxidative degradation of polypropylene, and consequently its melting point and thermal decomposition temperature are lowered, and its mechanical properties deteriorate [25]. In the external environment, conditions can be even more severe, such as lower (acidic) pH due to acid rain, higher (alkaline) pH due to fertilization with nitrogen compounds and accelerated decomposition of plastics, and the presence of electrolytes, due to road salting or along coastal areas. A number of studies have shown that polypropylene is susceptible to outdoor weathering [19, 26], although it is not readily biodegradable [27]. To some extent, shedding and peeling of microfibres from medical masks is also likely to occur, as has been shown for polyester textiles that release fibres into water and air during household washing and drying, and during their regular use [28]. However, the release of fibres is highly dependent on the textile structure, as there will be less release for textiles with a compact woven structure [29]. There are already some estimations on the amount of microplastics released from medical masks [13, 14]. Chen et al. [13] estimated the release of microplastics from 18 brands of medical masks into the water (shaking at 120 rpm, 24 h). In this way, from 159.80 ± 46.14 to 222.17 ± 98.79 particles/medical mask was released when the masks were new, but the numbers increased significantly when the masks were already used (1146.00 ± 307.60 to 1478.00 ± 265.80 particles/mask). Both fragments and fibres were found in water. The second study by Wang et al. [14] incubated the three layers of the medical mask separately in water with added quartz sand (shaking at 300 rpm, 24 h). The authors report that the release of microplastics depends on the UV weathering of the masks, addition of the sand as well as on the layer of the mask. They estimated that around 483,888 plastic particles could be released from one virgin disposable mask, and 1,566,560 particles from the weathered mask if the whole mask would disintegrate.

Over the past 15 years, tremendous research efforts have been made to understand the global occurrence, distribution and potential environmental hazards of microplastics and their associated chemicals [30, 31]. Research on the potential adverse effects of microplastics on organisms has been very intense over the past decade which has demonstrated physiological perturbations in organisms exposed to microplastics (e.g., alterations to behaviour, immune responses, energy allocation, life traits), and potential links to altered ecosystem function (for reviews see [32–34]).

The aim of this paper was to thoroughly characterise the microplastics obtained from commercially available polypropylene medical masks and to evaluate their acute toxicity to the planktonic crustacean Daphnia magna. The microplastics from the inner frontal, middle filtering, and outer layers of disposable medical masks were investigated separately in terms of their size, shape, organic chemical leaching and aquatic toxicity. We discuss the properties of polypropylene microplastics from medical masks in line with the literature reports on plastics-associated chemicals in other polypropylene products. We provide an extensive overview of currently available ecotoxicity data for microplastics from other polypropylene-based products, as currently very limited data for microplastics from medical masks exist. Finally, we identify the knowledge gaps to guide further research in the field.