Is the impact of atmospheric microplastics on human health underestimated? Uncertainty in risk assessment: A case study of urban atmosphere in Xi'an, Northwest China

Microplastic (MP) exposure in the environment has been commonly demonstrated to have adverse effects on human health. The majority of studies on MP were related to the aquatic and terrestrial systems, its potential risk for ecosystem and human health when exposed to the atmosphere is not well-understood. The presented study, taking Xi'an, a megacity in Northwest China, as an example, first estimated the possibility of local residents bearing MPs pollution. The results figured out an average abundance of MPs in TSP, PM, and PM was 12.5, 3.5 and 0.8 particles/L, respectively. A total of 15 polymer types of MPs were identified in the atmosphere. Although a species sensitivity distribution (SSD) approach is acknowledged to be useful to estimate the potential risk of pollutants, the result of SSD when used to evaluate the risk of MPs is debatable. In this study, SSD-based risk assessment showed that the atmospheric MP pollution in Xi'an had not yet reached the level of threatening human. However, unlike chemicals, it is unreliable to assess risk using the relationship of dose-response for MPs because toxic effects of MPs can be influenced by not only the abundance but also the characteristics, e.g., morphological size, shape and oxidative potential. Since insufficient mechanistic understanding regarding the relative relationship between MP characteristics and their toxic effects and limitation of the quality and relevance of toxicity data, the uncertainty of risk assessment of the atmospheric MPs is inevitable and the risk of the atmospheric MPs was tended to be underestimated. This poses a challenge to manufacturers and public health authorities, as well as researchers alike, however, we are already being exposed to the atmospheric MPs.