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dcyphr | Plastic and Human Health: A Micro Issue?

Abstract 

    Humans have exposure to microplastics through a variety of sources on a daily basis. The health effects are unknown, and this study looks at all pertinent studies published up to September of 2016 in relation to health effects. Microplastics could accumulate and produce a toxicity that is localized to the spot where they gather. This could even induce an immune response. There is also risk in the toxicity that comes from chronic exposure and the chemicals added to the plastic. The toxicity is expected to increase as the amount of exposure to microplastics increases.



Introduction 

    As plastic production increases, so does the creation of micro and nanoplastics. 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. They can be found around the world from the equator to both of the poles. MP and NP are hydrophobic and, therefore, absorb a variety of organic toxicants and metals such as cadmium, nickel, and lead. When the microplastics enter the body, the chemicals attached to the surface may leach out. There is evidence that suggests human exposure to microplastic by ingestion and inhalation. However more data is needed. 


Evidence for Dietary Exposure Pathway 

    There is evidence suggesting the exposure and ingestion of microplastics for fish, bivalves, and other foods. MP fibers have been found in honey, sugar, beer, salt and several other items. The MP either came from environmental depositing or from the production process. However, the discovery of MP fibers in the honey and on flowers supports the idea of airborne microplastics. 


Evidence for an Inhalation Exposure Pathway 

    MP that remain on the top of the water in the ocean could aerosolize MP just as it does to sea salt. However, this theory has yet to be tested. Wastewater treatment plants (WWTP) sludge byproducts contain synthetic clothing fibers. This byproduct is often used as fertilizer for farms. After application to agricultural land the MP fibers remain in the soil for 5 years and can be detected 15 years later. This byproduct fertilizer could dry and then enter the air column. MP in the atmosphere has been confirmed by several studies and is a worldwide phenomenon. The origin of these air-borne MP is unknown, but several hypotheses are being tested. 


Microplastic Uptake and Translocation 

    Studies on factory workers that deal with high levels of nylon, polyester, and acrylic have shown they have no increase in risk of cancer. However, they do have respiratory irritation. The workers had shortness of breath, coughing, reduced lung capacity, and increased mucus production. 

    Analysis of lung tissue from these same high exposure workers showed scarring and lesions in the lung tissue. Studies show that MP fibers were able to penetrate the tissue lining of the lungs and become absorbed. In a study on rats, the MP fibers were taken up by macrophages present in the alveoli of the lungs. The MP were then retained in these cells for 29 days where they caused inflammation. The toxicity of the MP could potentially increase as more toxicants bind to the surface of the MP and then leach out once inside the body. 

    The human body has been exposed to non-digestible micro particles before, and has developed clearing mechanisms to rid our body of them. Depending upon the shape, size, charge, and other factors, it may be able to bypass our clearing systems. The inhaled microplastics are taken up by the body via a mixture of endocytosis or diffusion. If ingested, it is theorized that nearly 60% of NP was taken up by a portion of the small intestine known as Peyer's Patch. A study using dogs and a high dose of a MP found the MP in the urine, bile, and cerebrospinal fluid. The absorption, transportation in the body, and elimination are likely to dependent upon the type, size, charge, hydrophobicity, shape, and associated chemicals of the MP/NP. 


Potential Human Health Risks of Microplastics 

    The shape, size, accumulation, associated chemicals, type, and charge all will affect the toxicity of the MP/NP to the human body. MP are likely to have reactive oxygen species (ROS) due to their breakdown and manufacturing process. These ROS will cause cellular damage and inflammation. Plastic particles also have the ability to travel throughout the body and are often found to gather in the lymphatic tissue

The plastics could also leach the chemicals added in the manufacturing to give the plastic certain characteristics or the organic toxicants it picked up in the environment. MP/NP are also found to cause damage to the microbiome present in the lungs and the gastrointestinal tract. This could lead to decreased immunity and increased inflammation.