**8. Conclusions**

While, microplastics and nanoplastics are widely studied in the context of the marine environment, we have only recently recognized the potential human exposure pathways. Following exposure, uptake is plausible via ingestion and/or inhalation. The toxicity assessments of micro- and nanoplastics on human are mainly focusing on gastrointestinal and pulmonary toxicity, which involve oxidative stress, inflammatory reactions, and metabolism disorders.

Based on the findings of recent studies, further research is needed to investigate the potential mechanisms of micro- and nanoplastics toxicity in human. Moreover, it is important to understand whether microplastics and nanoplastics can be further degraded after ingestion under the acidic conditions in the gu<sup>t</sup> or inside the lysosomes of the cells. Hence, the long-term fate of the ingested microplastics and nanoplastics in human body warrant further investigation.

Unfortunately, the accurate assessment of human exposure to nanoplastics remains a scientific challenge due to the lack of validated methods, certified reference materials, and standardization across the analytical procedures used [161,162]. Notably, most of the reported studies were conducted using polystyrene due to its ease in synthesis and processing into nanoparticles, while the most common commercial used of plastics are polyolefins (e.g., polyethylene and polypropylene), polyesters, and polyurethanes. Given the large variety in particle size, shape and chemical composition of plastics, the potentially hazardous effects of different types of micro- and nanoplastics to human health remain largely unknown [163]. Therefore, we recommend that future research should focus on the understanding of the potential hazards and risks of chronic exposure to diverse micro- and nanoplastics at relevant concentrations.

**Author Contributions:** Writing—original draft preparation, M.S.-L.Y., S.-F.W., Y.-Y.K., B.-K.T., C.-Y.W.; writing—review and editing, L.-W.H., C.K.L., W.-M.L., C.-O.L.; visualization, L.-W.H., C.K.L., W.- M.L.; supervision, C.-O.L.; project administration, C.-O.L.; funding acquisition, B.-K.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the MINISTRY OF HIGHER EDUCATION, MALAYSIA (FRGS/1/2020/SKK06/IMU/01/1 to COL, SFW, BKT, and MSLY).

**Acknowledgments:** All figures in this review paper were created with BioRender.com (accessed on 24 January 2021) by the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
