# Microplastics in Water Resources: Threats and Challenges
## 1 Introduction  
At the beginning of the 20 th century (1907-1909), [PERSON] made a breakthrough discovery, that is, he developed technology to produce a material based on phenol-formaldehyde resin (so-called bakielite) [1]. Since the invention of Bakelite (probably one of the first polymeric materials to be produced on a large scale), plastics have become a common and integral part of human life [2]. Convenient and consumptive human lifestyles account for a significant share of the increasing amount of plastic waste in the environment. Examples of this phenomenon include the widespread use of single-use plastic products [3]. The COVID-19 pandemic also played a significant role in the global scale of this problem, during which the use of disposable personal protective equipment (masks, gloves, etc.) increased significantly [4]. Unfortunately, despite many years that have passed, the issues related to mass plastic waste, which still ends up in the environment in large amounts, have not been effectively resolved.  
Plastics exposed to atmospheric conditions undergo mechanical, physical, and chemical degradation. The natural environment gradually transforms plastic waste into microplastics (MPs) through a series of factors [5]. Examples include solar radiation (photodegradation), sand abrasion, mechanical damage from contact with transportation, anddecomposition by microorganisms [6]. This degradation process leads to the formation of microplastics, which are defined as plastic particles smaller than 5 mm. Due to their small size and persistence, MPs exhibit unique environmental behaviors and impacts compared to larger plastic debris [7]. The continuously increasing amount of microplastics in the environment poses one of the main threats both to the natural environment and human health. Once formed, MPs are highly mobile and can be transported over long distances by wind, water, and biological vectors. Their small size and hydrophobic nature make them prone to adsorb harmful pollutants, such as heavy metals and persistent organic pollutants, which can further exacerbate their environmental and health risks [8,9]. Consequently, as early as the 1970s, attention was drawn to polymeric micropollutants [10]. Over time, evidence has emerged that microplastics accumulate in soil [11] and bottom sediments [12] and are readily bioaccumulated (including in the tissues of plants and animals) [13]. This bioaccumulation can lead to trophic transfer, where MPs and their associated pollutants move through the food chain, ultimately posing risks to human health through the consumption of contaminated food and water [14].  
Despite extensive research on MPs, inconsistencies in their definitions and size classifications persist, complicating comparative studies and regulatory frameworks. Establishing standardized terminology and size ranges is crucial to ensure consistency in MP research and to facilitate more reliable assessments of its environmental impact. Therefore, this study aims to address these discrepancies by reviewing existing classifications and proposing a more unified framework. Given the fundamental role of water in human life, this review focuses on the presence of MPs in water resources, examining their sources, associated risks, and the analytical challenges in their detection. By addressing these knowledge gaps, this study contributes the ongoing efforts to standardize MP definitions and size classifications, ultimately supporting the development of more accurate and comparable research methodologies.