Effects of Microplastic Exposure on Human Digestive, Reproductive, and Respiratory Health: A Rapid Systematic Review
December 18, 2024 | Environmental Science and Technology
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Abstract
Microplastics are ubiquitous environmental contaminants for which there are documented human exposures, but there is a paucity of research evaluating their impacts on human health. We conducted a rapid systematic review using the “Navigation Guide” systematic review method. We searched four databases in July 2022 and April 2024 with no restriction on the date. We included studies using predefined eligibility criteria that quantitatively examined the association of microplastic exposure with any health outcomes. We amended the eligibility criteria after screening studies and prioritized digestive, reproductive, and respiratory outcomes for further evaluation. We included three human observational studies examining reproductive (n = 2) and respiratory (n = 1) outcomes and 28 animal studies examining reproductive (n = 11), respiratory (n = 7), and digestive (n = 10) outcomes. For reproductive outcomes (sperm quality) and digestive outcomes (immunosuppresion) we rated overall body evidence as “high” quality and concluded microplastic exposure is “suspected” to adversely impact them. For reproductive outcomes (female follicles and reproductive hormones), digestive outcomes (gross or microanatomic colon/small intestine effects, alters cell proliferation and cell death, and chronic inflammation), and respiratory outcomes (pulmonary function, lung injury, chronic inflammation, and oxidative stress) we rated the overall body of evidence as “moderate” quality and concluded microplastic exposure is “suspected” to adversely impact them. We concluded that exposure to microplastics is “unclassifiable” for birth outcomes and gestational age in humans on the basis of the “low” and “very low” quality of the evidence. We concluded that microplastics are “suspected” to harm human reproductive, digestive, and respiratory health, with a suggested link to colon and lung cancer. Future research on microplastics should investigate additional health outcomes impacted by microplastic exposure and identify strategies to reduce exposure.
Introduction
In 2019, 460 million metric tons of plastic were produced, (1) with estimates that production will triple by 2060. (1,2) The largest proportion of plastic production comes from single-use plastics, and 98% of single-use plastics are derived from fossil fuels. (3) Fossil fuels are used to make petrochemicals, a broad and diverse group of chemicals that are the feedstock for the production of plastics. (4) The petrochemical industry is pivoting to ramp up the production of plastics given expectations that the sales of oil and gas will decrease. (5,6) This has raised concern, as the production of plastics also contributes to greenhouse gases across their life cycle from cradle to grave. (3,7) In addition, there is well-established evidence from authoritative or systematic reviews on the human health effects of plasticizers and plastics-related chemicals. (8) For example, phthalates can increase the risk of preterm birth (9) and adverse male reproductive effects (10) and bisphenol A (BPA) exposure is likely or very likely to be a hazard for immunotoxicity, metabolic effects, neurotoxicity and developmental toxicity, female reproductive toxicity, male reproductive toxicity, and carcinogenicity. (11)
Microplastics are defined as plastic particles that are <5000 μm in size and can be further classified as primary or secondary depending on their source. (12) Primary microplastics are those that are intentionally produced to serve a specific function, for example, as microbeads used for exfoliation in cosmetic products. (13) Secondary microplastics, in contrast, are the breakdown products of larger plastic debris and can be generated by physical, chemical, or biological processes. Secondary microplastics are more prevalent in the environment and can include, for example, the microfibers that degrade from car tires, plastic bottles, and clothing. (14) Like bulk plastic, microplastics can also be a variety of polymers with different physical and chemical properties. (15)
Microplastics are widespread and mobile in the environment, being detected in air, surface water, costal beaches, sediment, and food. (14,16,17) They have been discovered in remote and pristine locations, including the Antarctic, (18) deep ocean trenches, (19) and Arctic sea ice. (20) Due to their small size, microplastics more easily enter and are distributed in the human body in comparison to larger particles; (21) microplastics have been measured in human placenta, (22) breastmilk, (23) and liver. (24) It has been estimated that humans consume a “credit card worth” of microplastics every week. (25,26) Due to ubiquitous exposure (23) and bioaccumulative characteristics of microplastics, (17) the extent of human health impacts due to microplastic exposure is of great concern.
Research on microplastics and their health effects on humans is still in its infancy. A growing body of evidence exists, however, indicating the adverse health effects of microplastic exposure on living organisms. (16) For example, microplastics increase the susceptibility of fish and seabirds to infections. (27,28) Microplastics have also been shown to accumulate in organs and lead to biological changes, including oxidative stress and inflammation in human cell lines, (29,30) and exposure to microplastics has been linked to poor cardiovascular and respiratory outcomes, metabolic disorders, gastrointestinal effects, reproductive effects, and cancer in humans. (29−36)
Evaluations of the human health effects of microplastics have been narrative nonsystematic reviews, not systematic reviews that assess both the quality and strength of the existing evidence, using rigorous, predefined, transparent methods that minimize bias and provide a bottom line summary of the evidence. (29−33,37) These narrative reviews, therefore, are able to speculate about only the association between microplastic exposure and human health outcomes as they do not follow prespecified, consistently applied, and transparent rules like those utilized by systematic reviews. Systematic reviews are thus needed to provide more confidence in the evaluation of the relationship between microplastic exposure and health effects and to provide a conclusive statement regarding the implications for human toxicity.
Given the growing body of evidence, as well as the urgent need to better characterize the effects of microplastic exposure on human health, we were therefore asked to conduct a rapid systematic review of the evidence to assess the association of microplastic exposures on human health outcomes for policymakers in the State of California (details in Materials and Methods). The primary objectives of this rapid systematic review were to evaluate the human and animal evidence assessing microplastic exposure to any adverse human health outcome,a rate the quality and strength of the human and animal evidence, integrate the human and animal evidence streams and develop a final bottom line statement regarding the health effects of microplastics.
