As the production and use of masks continuously increase, the large number of masks entering the environment is inevitable, which has been a large source of microplastics [25]. Therefore, mask-associated microplastic pollution is a crucial issue that requires further research. We investigated the long-term microplastic release kinetics of four commonly used masks under a simulated natural aquatic environment scenario. We found that all four masks could persist release a large number of microplastics in a time-dependent manner, among which particles/fibers seized < 20 µm as the dominant type. Uncovering the long-term kinetic characteristics of released microplastics of masks in the aquatic environment, which could provide a scientific basis for accurately assessing the microplastic pollution and adverse health risks that originated from masks.
We designed an experimental condition that maximumly mimics natural water forces and climatic conditions to reveal the long-term release dynamics of microplastics from masks. In past studies, Liang et al., Sullivan et al., Chen et al., and Morgana et al. explored the release of microplastics from masks in water environments under short-term high-intensity mechanical forces on masks [11, 26–28]. The experimental condition and periods in the above studies are not according to the degradation process of masks in the natural environment. Thus, these data failed to truly reflect the microplastic release of masks in the virtual water environment. To simulate the natural environment closer to the degradation process of masks in their natural state, such as mimicing12:12-h light/dark cycle, making the mask float in the water in a large enough container, and gently shaking the water body for 300 seconds every day. Our experiment condition considers the sunshine and water stream acting force on mask degradation, which was all neglected in previous mask degradation studies. Therefore, our study broadly provides a more realistic condition to accurately assess the release kinetics of microplastics from the mask into the aquatic environment.
Under our experimental condition that mimics actual aquatic environments, we found that the number of microplastics released by all masks increased gradually over time after exposure for 3, 6, 9, and 12 months. The size of released microplastics from the four masks was mostly under 100 µm, with an order of magnitude of about 105/L. Still, the number of microplastics sized > 20 µm did not increase significantly. Liang et al. found that the released particle size of masks was longer, including 500 ~ 1000 µm, 1000 ~ 2000 µm, and larger than 2000 µm. The magnitude of microplastics released is about 103/m3[11]. This discrepancy results may be attributed to the larger microplastics released into water, which will be further degraded into smaller particle-sized microplastic fragments, even nanoscale microplastics [29]. Chen et al. found that masks can release microplastics in the form of fibers and fragments, most of which are distributed between 100 µm and 500 µm, followed by less than 100 µm [28]. Similarly, Morgana et al. found that microplastic particles ranging from 0.1µm to 100 µm were released by the mask under mechanical stress, with an order of about 1010/m3 [26]. However, none of these studies can truly reflect the release of microplastics from masks during natural degradation due to their experimental condition, which may explain the disparity regarding the count of released microplastic particles/fibers. In conclusion, these results indicated that face masks continuously release a vast amount of micron/nanometer scaled microplastics in the aquatic environment.
The released microplastic particles in aquatic environments can persist for a long time due to their difficulty of complete degradation property may present another perspective to explain a large amount of microplastic particles/fibers in aquatic media. It is noteworthy that micro/nano-sized plastics have extensively infiltrated our daily life and accumulated in the food chain/web, which could threaten human health [30–32]. The environmental and health risks of microplastics released from masks in water environments deserve serious attention, as microplastics with small particle sizes have multiple adverse effects on environmental and biological health [33].
We also examined the changes in the structure of masks exposed to the aquatic environment. Some loose plastic fibers could be observed attached to the fabric surface among the microstructure of the four masks, which might be formed by melting during the manufacture of masks. Chen et al. found that loose plastic fibers on the surface of mask fibers are an important source of microplastics released from masks [28]. These loose plastic fibers were released into the water after entering the aquatic environment, which may partially mechanistically explain the released microplastics by masks. Thus, the melt spray technique should not be advanced to diminish microplastic release from the masks. Furthermore, the damage to the mask in the aquatic environment increased over time, evidenced by the surface of the fabric that makes up the mask beginning to deform. The subsequent breakage of some fabric fibers and friction between the fractured fibers promotes the release of microplastics into the aquatic environment. Therefore, mask damage may be an important mechanism for the released microplastics.
Under simulating natural water environments during the degradation process of masks, our results showed that masks could continuously release large amounts of micrometer plastic into the water environment in a time-dependent manner. Previously revealed kinetic characteristics of released microplastics by masks could provide a scientific basis for preciously assessing the environmental risks of masks. As a country with a large population, the production and use of masks during COVID-19 exceeded 100 million in China. Therefore, urgent action is needed to limit the number of masks entering the water environment to a minimum and control its generated adverse health effects.