Microplastics are easily ingested due to their micro-level sizes. They also move easily through the food chain and persist in the environment since they are refractory to biodegradation. In addition, as microplastics exist in micro-level to nano-level sizes, they are virtually impossible to remove once released into the environment. Due to these characteristics, microplastics pose potential hazards to humans and the environment. As a representative example of the risk posed by microplastics, they can cause physical and mechanical harm (e.g., cause abnormalities in internal organs) to marine organisms when they mistakenly ingest microplastics. Ecotoxicity may be caused by the polymer itself, unreacted monomers, impurities (e.g., residual catalysts or reaction by-products), additives (e.g., stabilizers), or other substances in the polymer matrix (e.g., dyes, lubricants, or plasticizers). In addition, microplastics can enter the human body when they are not filtered out during sewage-treatment processes, or they can flow into the sea, thereby posing risks for the ecosystem and humans. Various examples of damage caused by microplastics have been reported, such as microplastic accumulation in the bodies of marine and aquatic organisms (leading to malnutrition), inflammation, reduced fertility, and mortality. The threats that microplastics present to the human body have not yet been clearly identified. However, previous reports have shown that ultrafine microplastic absorption resulted in complex toxicity in zebrafish and that microplastics under 100 nm in size can reach almost all organs after entering the human body. Therefore, concerns exist regarding the negative effects of continuous microplastic accumulation in the human body.