Disclosure: Bernard-Alex Gauzere, MD, has disclosed no relevant financial relationships.

Although a 2019 World Health Organization (WHO) report concluded that microplastics in drinking water posed no risk to human health, accumulating evidence is beginning to challenge these findings.

Since plastics became widely used in the mid-20th century, they have evolved from a novel substance to an essential component in countless applications, with global production reaching 368 million tons in 2019 and expected to double by 2039. The production and degradation of plastics involve physical, chemical, and biological processes, leading to the formation of tiny fragments known as microplastics (MPs) and nanoplastics (NPs), which accumulate in the environment. Beyond the well-documented environmental harms of MPs and NPs, growing evidence of their presence within the human body raises concerns about their potential to trigger various harmful biological processes. Their detection in the urinary tract and their potential links to kidney and bladder diseases, as shown in animal studies, are particularly alarming.

As the impact of plastic pollution becomes increasingly apparent, the need for standardized international definitions of MPs and NPs is pressing. Government publications reveal notable discrepancies between organizations in defining these fragmented plastics. The lack of consensus among regulatory bodies highlights the challenges in mitigating the environmental and health impacts of MPs and NPs. The International Organization for Standardization offers the most precise classification, defining MPs as solid, insoluble plastic particles ranging from 1 µm to 1 mm and NPs as particles smaller than 1 µm.

The intrusion of MPs and NPs into the human body, whether through inhalation, ingestion, or skin exposure (via wounds, hair follicles, or sweat glands), has been linked to harmful biological effects, including inflammation, alterations in cellular metabolism, physical cellular damage, and reduced cell viability.

The detection of MPs and NPs in the human urinary tract, combined with limited understanding of their effects, is a growing concern. An exploratory study published earlier this year aimed to systematically summarize the existing literature regarding the presence of MPs and NPs in the urinary tract and their potential consequences, guided by these research questions:

For this study, the "urinary tract" included the kidneys, bladder, ureter, urethra, and urine. By focusing on the urinary tract, the study aimed to consolidate current understanding of MPs and NPs, raise awareness of this emerging issue, and lay the groundwork for further research that could contribute to public health policies and clinical practice guidelines.

The researchers conducted a scoping literature review following the recommendations of the JBI [formerly known as the Joanna Briggs Institute). They systematically searched five databases -- PubMed, Scopus, CINAHL, Web of Science, and Embase -- as well as gray literature sources.

Eighteen articles were identified. The authors represent seven countries: Pakistan (n = 1), the Netherlands (n = 1), the US (n = 1), Taiwan (n = 1), Germany (n = 3), China (n = 5), and Italy (n = 6). Among these studies, six investigated and characterized the presence of MPs and NPs in the human urinary tract. MPs and NPs were detected in urine samples (n = 5), kidney cancer samples (n = 2), and bladder cancer samples (n = 1).

Additionally, 12 studies examined the effects of MPs and NPs on human urinary tract cell lines. Their findings suggest that MPs and NPs have cytotoxic effects, increase inflammation, reduce cell viability, and alter mitogen-activated protein kinase signaling pathways.

Raman spectroscopy was the primary method used to detect and characterize MPs and NPs in human samples (five out of six studies; 83%). Alternatively, pyrolysis-gas chromatography-mass spectrometry combined with direct laser infrared spectroscopy was used in one study.

This exploratory study underscores the urgent need for further research and policy development to address the challenges posed by microplastic contamination. It highlights the rapidly emerging threat of human urinary tract contamination by microplastics, questioning the WHO's claim that microplastics pose no public health risk. The documented cytotoxic effects of microplastics, and their ability to induce inflammation, reduce cell viability, and disrupt signaling pathways, raise significant public health concerns related to bladder cancer, chronic kidney disease, chronic urinary infections, and incontinence.