**Abstract**

Currently, globally the demand and production of plastic items are increasing exorbitantly, generating a large amount of waste, and polluting the ecosystem, a site in which degradation processes are triggered, which give rise to smaller particles such as micro(nano)plastics (MNP). Continuous human exposure to these particles generates negative alterations in the host's health. Three routes of MNP exposure or contact have been established: inhalation, ingestion of particles, and dermal absorption. Recently, it has been pointed out that microplastics (MP) can even be found in the human placenta. This chapter aims to compile and provide information on their role as conveyor vectors of agents potentially toxic to humans, mechanisms by which they enter the human body, their bioaccumulation, and health human effects.

**Keywords:** microplastics, nanoplastics, conveyor, gut, microbiota, inflammation, health

### **1. Introduction**

At present, globally, there is an important and relevant environmental and public health problem; 8300 million metric tons of plastic were manufactured worldwide between 1950 and 2017, and this production continues to increase, reaching 390.7 million metric tons only in 2021, and is expected to increase to 34,000 million metric tons by 2050 [1, 2]. Degradation resistance is one of the most critical characteristics that initially gave an advantage to the use of plastics; however, now, it is a significant disadvantage since they are resistant to chemical, biological, and corrosive degradation; therefore, their durability in the environment is greater [3]. Plastics are widely used in various consumer products because of their low density and costs; there are about 30,000 types of plastics, being the most widely used polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polyurethane (PU), polyvinyl chloride (PVC), and polycarbonate (PC) [4]. Plastic residues are transported by rivers, storms, and strong winds or are discarded directly into

terrestrial or aquatic ecosystems. Plastic residue pieces are classified according to their size into mesoplastics (50–200 mm), macroplastics (200–1000 mm), and mega-plastics (>1000 mm) [5]. These residues undergo physical changes due to environmental interactions, such as fragmentation, and changes in their physicochemical properties, generating new types of micropollutants such as microplastics (MP (less than 5 mm)) and nanoplastics (NP (<1 μm)) [6]. These tiny plastic particles are ubiquitous worldwide and generate great concern for environmental and human health damage. The impact on human health may be due to their small size, specific surface, and high biological penetrability [7].

Micro(nano)plastics absorbs and transports external toxic pollutants; these plastic particles harm human and wild health, altering the physiological functions of immunity and metabolism and modifying the intestinal microbiota, thereby facilitating exposure to pathogens [8]. In fact, according to morphology, size, and concentration, MP can trigger ecotoxicological problems in different organisms [6]. Exposure to MNP causes local inflammation, oxidative stress, metabolic alteration, gastrointestinal toxicity, hepatotoxicity, reproductive disorders, and neurotoxic effects [9]. Humans are subjected to prolonged exposure to this type of particle in low concentrations; however, these effects on the organism need to be profoundly and widely studied, mainly in the case of NP [9]. So far, eradicating plastic waste remains challenging, and its impact on health is becoming increasingly evident. Despite the increase in recent research on plastic waste, it is still in the early stage; therefore, more research is required. This chapter presents and discusses the role of these particles as carriers of different molecules or microorganisms that directly impact health. Besides, it presents the current knowledge about MNP human exposure pathways and the toxicological effects on the intestinal microbiota-immunity, reproductive and neurotoxic.
