3.2.2. Inhalation

Since the MP concentration indoors is much higher than that outdoors, atmospheric MPs enter the body through human inhalation [31]. Indoor air is an essential source of human exposure to airborne MPs because people stay indoors for longer periods and dispersing machines are less capable of removing plastic particles [49]. Factors affecting MP behavior and transmission in indoor air are ventilation, airflow, and room spacing [67]. The concentration of MP particles indoors is higher than that outdoors, which may be influenced by textiles, furniture, building materials, and human activities [31]. Compared with MPs in other environments, MPs indoors are more easily inhaled directly and continuously and cause health risks [49,53].

One of the earliest criteria for determining airborne MPs is 0.3–1.5 particle/m<sup>3</sup> outdoors and 0.4–56.5 particles per cubic meter indoors (33% polymer) [31]. According to statistics, each person inhales between 26 and 130 MP particles from the air per day [49].

Based upon air samples taken from mannequins, men who exercise lightly can expect to inhale 272 particles/day [44]. Estimates vary depending on sampling methods and space use factors.

3.2.3. Gaps in and Prospective Research on the Destinations of Atmospheric MPs

Based on the above discussion, the studies on the fate of MPs in the atmosphere are limited. So far, only limited studies have traced the atmospheric migration paths of MPs; therefore, it is nearly impossible to quantify the various environmental factors and human activities affecting the behavior and transmission of atmospheric MPs. Further, the quantification and characterization of atmospheric MPs in different parts of the human body and associated health impacts is challenging. To this end, we propose the following for future research:

