LAUSR

M (<5 mm in diameter, including <1 μm nanosized plastics) have been ubiquitously detected in marine, freshwater, terrestrial, and atmospheric systems since 2004. Thereafter, understanding the biological consequence of these plastic particles has become of interest to many areas of science. A growing body of evidence indicates that microplastics are detrimental to a wide range of living organisms, including plankton, fish, microorganisms, plants, and rodents. Newer data suggest that microplastics are moving from the environment to the human body, as demonstrated by their presence in human lungs, placenta, and stool, which has raised significant concerns about their hazards and human health implications. Ingestion and inhalation are considered the major routes of human exposure to microplastics. Although there have been more toxicological studies regarding the biotoxicity of microplastics in recent years, there is a paucity of research for interrogating the toxicological effects of airborne microplastics on humans via inhalation. According to a survey conducted by Cox et al., humans may consume 74000−121000 microplastic particles per year, where inhalation contributes approximately half of the annual exposure estimates. In line with this finding, researchers in the United Kingdom compared human consumption of microplastics via ingestion of contaminated mussels to that of household fiber fallout during a meal. Their results indicated the latter poses a greater risk. Additionally, there is compelling evidence that workers processing nylon, polyester, and polyamide fibers in the United States, Canada, and The Netherlands exhibited a higher prevalence of respiratory irritation, with severe symptoms being coughing, dyspnea, occupational asthma, and interstitial lung disease, implying the health hazards of high-dose exposure of inhaled microplastics. Our recent study also demonstrated the role of the size of nanoscale plastics in the internalization in and mitochondrial damage to human respiratory cells. However, much information is still lacking about the toxicological effects and risks of low-level exposure of inhaled microplastics as occurs in the general population under realistic exposure scenarios. Unlike other air quality indices, such as PM2.5 and PM10, the widespread use and continuous abrasion of plastic-related products seems to have eliminated the air pollution inequality of airborne microplastics between developing and developed countries, where high levels of microplastics in the atmosphere have been found in London (England), Paris (France), Tehran (Iran), Shanghai (China), and Hamburg (Germany), which poses a threat to global health and imposes a significant regulatory burden. In view of these facts, we thus call for extensive research to interrogate the health hazards and molecular determinants of inhaled microplastics by using either canonical or new toxicological models. There are some obstacles and challenges to overcome in seeking answers about inhaled microplastic-induced toxicity. Available data suggest that various physicochemical properties of microplastics, such as polymer types, sizes, surface areas, and functional groups, have an inevitable impact on their biotoxicity. The situation becomes more complicated when plastic particles enter the atmospheric environment, which may undergo a variety of degradation and erosion processes, such as hydrolysis, photodegradation, and mechanical disintegration, resulting in large variability in their morphology and surface characteristics (Figure 1). Parallel evidence has indicated that the abundance and types of airborne microplastics might vary over urban, suburban, and remote areas. Consequently, these confounding factors lead to a striking discrepancy between microplastic particles found in nature and made in the laboratory; the latter have been predominantly used in toxicological studies. Another dimension of toxicological concern is that atmospheric microplastics may act as vectors to transfer toxicants in the air, such as heavy metals and organic chemicals. These toxic hazards, along with the artificial and natural additives of plastic itself (e.g., stabilizers, plasticizers, flame retardants, and antimicrobial agents), may lead to complex chemical interaction in a variety of ways, which may magnify or mitigate the biological effects of inhaled