The physiological and behavioral effect of micro/nano plastics on marine life has been documented. However, there is a severe lack of studies on the effect of micro/nano plastic on mammals. Recent studies show the toxicological effects as a result of micro/nano plastic ingestion in mammals. Future studies should address the effects of short and long term exposure of microplastic to animal models. The accumulation of micro/nano plastic occurs in mammals, but the long term consequences are yet to be known. This study explores the underlying cellular mechanisms involved in the toxic response to micro/nanoplastic.
As plastic production increases, it is important to observe their effect on the environment. Large plastics pose obvious environmental issues, while smaller plastics evade the public eye. Microplastics (MP) are smaller than 5 mm in diameter and are categorized into two groups. Primary MPs are manufactured to be that size, while secondary MPs become this small through degradation. Nanoplastics (NP) are smaller than 1 μm. Both MP and NP are potent pollutants and their exact long term effects are to be discovered.
Toxicity of MPs/NPs in Marine Life
There exists a large pool of marine life studied in regards to NP/NP toxicity. Despite this, all the studies generally show that MP/NP ingestion is a stress factor for marine vertebrates. For example, it was shown to cause behavioral issues for adults and larvae, reproductive issues, mother-offspring transfer of NPs, accumulation in various organs, inflammation, oxidative stress, disruption of gut microbiota, neurotoxicology, and various other health issues. The effects of MPs/NPs appear to increase as the concentration of plastics increases. Similarly, the toxicity is dependent upon the chemical toxicants attached to the plastic surface. These can be mercury, cadmium, or any pharmaceutical.
Toxicity of MPs/NPs in Mouse Models
MPs and NPs were found in the gut, liver, and kidney. The researchers noted several issues related to the gastrointestinal tract, inflammation of the liver, lipid accumulation, metabolism, and metabolic disorders. The effects seen were less severe than those in marine life most likely due to more complex barriers and less routes for plastic uptake.
Toxicity of MPs/NPs in Human Cells
There is very little data at this point, but one study used clean microplastic without any extra chemicals attached to the surfaces. This study showed little toxic effect, but did show plastic uptake. There are several studies that measure the effect of clean MPs on several different cell types, but none that measure effects of cells in a human body. The results generally show a moderately toxic effect on the cells often resulting in oxidative stress and inflammation.
The Cellular Mechanisms underlying MPs/NPs in temporary or chronic Toxicity in Mammals
High levels of MPs/NPs are toxic to cells. However, the surfactants (soaps) that often coat plastics may disrupt the cellular membrane and the processes inside. The NPs could enter the cell and cause disruption of the internal processes and damage cellular organelles. Even if the lysozyme was able to encapsulate the NPs they would not be digested. This would lead to the accumulation of byproducts in the cell and eventual cell death. NPs will at the very least cause cellular stress, which shows as the production of reactive oxygen species.
The Systematic Effects of MPs/NPs in Humans
Larger MPs would not be adsorbed, but smaller NPs could enter circulation. Only under certain conditions can the small concentration of MPs/NPs present in the typical human diet cause issues. However, this could change if the MPs/NPs have associated toxicants. If other chemicals are attached to the plastics, then this could easily lead to adsorption and other issues. The long term effect of smaller levels of MPs/NPs is currently unknown. When the NPs are absorbed, they enter the portal vein and head to the liver. If they accumulate in the liver, it can cause a variety of issues, such as inflammation and metabolic disorders. Since MPs/NPs are in the air humans breathe, it is vital that its effects on lung tissue be explored. Based on current studies, it appears MP/NP exposure could cause tissue damage in the lungs. Another important thing to consider is the effect of MPs/NPs on the gut microbiome. Disruption of this microbiome can lead to an array of issues ranging from kidney disease to cardiovascular disease and cancer. Finally, it has been shown that NPs can interact with proteins in the blood to form complexes. If these complexes aggregate and join together, it could block vessels.
Human uptake of MPs/NPs will only increase with time, and based on current evidence long-term health issues are unknown. More research is needed to address this.