New Review Highlights Human Health Risks from Microplastic Exposure

I was happy to join coauthors from UCSF’s Program on Reproductive Health and the Environment on a recently published review that evaluated the evidence linking exposure to microplastics to human health harms. This new study, published in Environmental Science & Technology in December 2024 extends upon earlier work the team conducted as part of the California State Policy Evidence Consortium (CalSPEC) report that was published in January 2023.
What are microplastics?
Microplastics are tiny pieces of plastic that are less than five millimeters long in at least three dimensions. Microplastics can be diverse in terms of their origin, shape, color, size, and chemical composition. Some microplastics are intentionally produced and added to products, for example to provide an abrasive property to the product. Other microplastics shed from synthetic textiles or tires or otherwise result from the breakdown of larger pieces of plastic.
Microplastics are widespread environmental contaminants, polluting the food we eat, the water we drink, and the air we breathe. They are even polluting us—microplastics have been detected throughout human bodies, including in human blood, lung, vascular, colon, liver, placental, penile, testis, and amniotic fluid samples. And though microplastics move through our bodies, as evidenced by their presence in breastmilk, sputum, saliva, semen, and stool samples, the rate at which they leave our bodies is currently unknown.
What did we conclude?
Our new paper evaluated the evidence linking exposure to microplastics to harmful health effects, specifically those impacting the digestive, reproductive, and respiratory systems. We concluded the following:
Exposure to microplastics is suspected to be a digestive hazard to humans, including a suspected link to colon cancer.
This conclusion was based on 10 animal studies that investigated six outcomes related to the small or large intestines.
Exposure to microplastics is suspected to be a reproductive hazard to humans.
This conclusion was based on 2 human studies that evaluated outcomes related to infant birth weight and growth and to gestational age as well as 11 animal studies that evaluated 10 outcomes related to hormone level changes, impact to ovarian follicles, and sperm and testicular damage.
Exposure to microplastics is suspected to be a hazard to the human respiratory system.
This conclusion is based on 1 human study and 7 animal studies that evaluated five outcomes related to lung function, injury, or inflammation.
About the review
We conducted a rapid systematic review of any studies that investigated health effects related to microplastics exposure. In brief, we searched scientific databases for relevant studies in July 2022 and April 2024, systematically reviewed the results of the database searches, and identified all of the available human and rodent studies that investigated digestive, reproductive, or respiratory health effects related to microplastics exposure. We used the Key Characteristics of Carcinogens and Key Characteristics of Female and Male Reproductive Toxicants frameworks to organize the analysis of biological changes reported in the included studies. We also used established methods to critically appraise each study and to integrate the available evidence from animal and human studies to reach a hazard identification conclusion that considered both the strength of the body of evidence and the quality of the body of evidence.
After reviewing over 2,800 studies retrieved from the database searches, we ultimately identified 28 rodent studies and 3 human studies for inclusion in our review. The included human studies were conducted in Turkey, Iran, and China and evaluated between 40-80 subjects each. Of note, most of the animal studies (79%) exposed animals to spherical polystyrene microplastics, with fewer studies evaluating polyethylene or polypropylene. Only one study evaluated the impacts of secondary microplastics, namely tire wear particles.
In summary, this review used state of the art and transparent methodology to evaluate the evidence linking microplastics exposure to human health effects. The growing body of evidence indicates that our exposure to microplastics is suspected to cause harm to our reproductive, digestive, and respiratory systems. Though most of the research to-date has been conducted in animal models, it is suspected that the conclusions also apply to humans.
Recommendations
Our review reiterates that microplastics are a growing human health concern. Moving forward, we need policies that reduce the loading of microplastics in the environment, including banning the use of intentionally added microplastics and policies that reduce the use of unnecessary and avoidable plastics in general. Although we have enough evidence to take these actions now, we will also benefit from a better understanding of exposures to microplastics and their impacts to human and environmental health. More research should be funded to explore these areas.
“Effects of Microplastic Exposure on Human Digestive, Reproductive, and Respiratory Health: A Rapid Systematic Review.” (free download here). Coauthors: Nicholas Chartres, Courtney B. Cooper, Garret Bland, Katherine E. Pelch, Sheiphali A. Gandhi, Abena BakenRa, and Tracey J. Woodruff.
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