Insights into the toxic effects of micro-nano-plastics on the human brain and their relationship with the onset of neurological diseases: A narrative review.

The intensive production and use of plastics, poor biodegradability and inadequate recycling have caused excessive and alarming environmental pollution. This has led to the inevitable intake by humans, through different routes, of small plastic particles, the micro and nano-plastics (MNPs) with sizes ranging from nanometers (<1000 nm) to micrometers (from 5 mm to 1 µm). MNPs can cause harmful effects in human tissues and organs, contributing to the early onset of aging and various age-related diseases. A growing body of evidence supports this toxic role of MNPs. In this regard, it has been shown that their different chemical and physical properties, including different chemical composition with different additives, different size, shape, solubility and ability to interact with metals and microbial agents, as well as the duration of multiple exposures, modulate their toxic action. In the brain, as documented mainly by studies conducted on brain tissues of deceased individuals, nanosized nanoparticles (NPs) of mostly 50 nm or smaller, made of polyethylene, bioaccumulate, causing damage. The mechanisms involved do not seem to be fully understood. However, studies on animal models and human cell cultures using plastic particles made of synthetic polystyrene, of slightly larger dimensions, partially clarify this aspect. They demonstrated that these particles have the unique ability to cross the blood-brain barrier and evoke neurotoxicity, through the activation of pathways that determine oxidative stress, inflammation, apoptosis, altered synthesis of neurotransmitters, endocrine molecules and key enzymes related to nerve conduction, and able to influence the gut-brain axis. Despite the paucity of studies conducted directly in humans, this review collects a growing body of evidence demonstrating that exposure to MNPs, and essentially NPs, can damage neurons. This could lead to alterations in learning, memory and behaviour, and could evoke additional potential negative impacts, contributing to amplifying neuroinflammation and the onset of neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Preventive approaches and measures to limit their use and human exposure, as well as potential therapeutic strategies, are also suggested.