Degradation of Microplastics and Nanoplastics: An Underexplored Pathway Contributing to Atmospheric Pollutants

Microplastics (MPs) and nanoplastics (NPs) are widespread pollutants present across all environmental matrices, including the atmosphere. They originate anthropogenically from primary sources, like microbeads, glitters, industrial abrasives, etc., and from secondary sources through degradation of larger plastic products, textile fibers, tire wear, waste incineration, etc. Degradation processes, such as mechanical, photochemical, chemical, and microbial degradation, break down plastics into smaller particles and gaseous byproducts. Atmospheric degradation processes of MPs/NPs enhance their area/volume ratio and introduce oxygenated functional groups at the surface, which increases their hydrophilicity and interactions with other pollutants in the surroundings. Thus, MPs/NPs also act as great vectors for toxic substances, including heavy metals, polycyclic aromatic hydrocarbons, and persistent organic pollutants, amplifying their environmental and health risks. MPs/NPs have been detected in various human tissues and fluids. Being bio-inert, they cannot be metabolized and leave the body only through excretory routes. They not only interact with the human organs directly but also indirectly via releasing additives and adsorbed/absorbed pollutants and, thus, can exhibit higher toxicity compared to other atmospheric aerosols. Furthermore, atmospheric MPs/NPs influence radiative forcing and cloud formation, and their photodegradation also releases greenhouse gases, like CO<inf>2</inf>, CH<inf>4</inf>, and volatile organic compounds (precursors of ozone), linking plastic pollution to climate change. Despite their growing recognition, the study of atmospheric MPs and NPs remains in its infancy, with numerous uncertainties surrounding their behavior, fate, and effects. This review aims to highlight underexplored degradation pathways of atmospheric MPs/NPs that may be enhancing their environmental, health, and climatic implications. It also proposes the future directions for atmospheric MP/NP research.