Microplastics, defined as solid polymeric particles smaller than 5 millimeters, have become an emerging concern worldwide. They arise either from deliberately manufactured microscopic materials, so-called primary microplastics, or from the progressive fragmentation of larger plastic objects exposed to physical, chemical, and biological weathering, known as secondary microplastics. Wastewater treatment plants sit squarely on the front lines of this pollution pathway. They receive enormous loads of plastic fibers and fragments from household laundry, cosmetic products, and urban runoff, yet they also act as concentrators, retaining a significant fraction of the incoming plastic in the sludge line. When that sludge is reused on land or improperly disposed of, it can become a secondary source of contamination for soils and waterways. Understanding exactly how many particles flow through these facilities—and what they are made of—depends on analytical methods that can reliably pry plastic particles out of a sea of organic matter without destroying the evidence in the process. That is precisely the challenge the new research set out to solve.

Led by Ivanilson da Silva de Aquino of the University of Brasília, together with colleagues at the UnB Planaltina Faculty and the Federal University of Jequitinhonha and Mucuri Valleys, the team built its methodological framework from the ground up. The first step was to manufacture reference microplastics from the six commodity polymers that dominate urban plastic waste streams: polyethylene terephthalate, high-density polyethylene, low-density polyethylene, polyvinyl chloride, polypropylene, and polystyrene. These laboratory-made particles, produced as irregular fragments ranging from 0.1 to 3.0 millimeters, served as benchmarks against which every subsequent processing step could be tested. Before any chemistry was attempted, the researchers confirmed by attenuated total reflectance Fourier-transform infrared spectroscopy—ATR-FTIR for short—that each batch of reference particles displayed the characteristic absorption patterns of its parent polymer, ensuring that the starting materials were spectroscopically unambiguous.

The heart of the new protocol is a digestion step based on Fenton’s reagent, a mixture of hydrogen peroxide and iron catalyst that generates highly reactive hydroxyl radicals capable of shredding organic matter. Wastewater and sludge samples are so loaded with organic material, microbial biomass, and inorganic debris that microscopic plastic particles would otherwise be invisible. The catch is that the digestion must be aggressive enough to clear the matrix but gentle enough to leave the plastics themselves chemically intact. If the polymers are oxidized too severely, their infrared fingerprints can shift, complicating identification, and in some cases particles can fragment or lose mass entirely. The team therefore subjected its six reference polymers to five different Fenton protocols spanning temperatures from 30 to 80 degrees Celsius and various reaction durations, then scrutinized the consequences using FTIR and a metric known as the carbonyl index.

The carbonyl index tracks the relative abundance of carbon-oxygen double bonds on polymer surfaces, a hallmark of oxidative degradation. Because polyolefins such as polyethylene and polypropylene naturally lack carbonyl groups, even small increases in the index reveal genuine surface oxidation. The results were revealing. At 30 degrees Celsius the reaction was too sluggish to digest organic matter effectively. Above 80 degrees Celsius oxidation became severe, with polyethylene terephthalate and polypropylene showing the largest changes in carbonyl absorption. But in the moderate window between 40 and 60 degrees Celsius, with a reaction time of about two hours, the Fenton treatment delivered the best of both worlds: organic matter was substantially degraded, while the carbonyl index of all tested polymers showed no significant changes. FTIR spectra recorded before and after digestion showed no meaningful spectral deviations, meaning the polymeric fingerprints needed for reliable identification survived the chemical assault intact.

With digestion optimized, the researchers turned to the second major hurdle: separating buoyant plastic particles from the remaining debris. Density separation exploits the simple physical fact that most common polymers are less dense than suitable salt solutions, so they float while heavier mineral and organic particles sink. The team built a custom separation device and systematically compared separation media. Plain water, with a density of 1.0 grams per cubic centimeter, performed poorly because more than half of the target polymers are denser than water and simply sank. Saturated sodium chloride solution, the cheap and inert classic, reached about 1.2 grams per cubic centimeter and successfully floated roughly two-thirds of the studied plastics—but it excluded the high-density polymers polyvinyl chloride, at 1.10 to 1.45 grams per cubic centimeter, and polyethylene terephthalate, at 1.37 to 1.45 grams per cubic centimeter. Relying on sodium chloride alone, the authors caution, could systematically underestimate microplastic abundance.

Zinc chloride proved to be the decisive upgrade. At concentrations reaching 1.6 and 1.8 grams per cubic centimeter, zinc chloride solutions kept every polymer tested in suspension, allowing effective recovery of even the heaviest particles. In the validation experiments, the densest solution ensured that all target polymers remained in the floating fraction, minimizing losses during sample processing. The researchers note, however, that the analytical superiority of zinc chloride comes with trade-offs: its toxicity, higher operational cost, and disposal requirements must be weighed when designing routine monitoring programs, and reuse of the solution can help mitigate cost and waste.

The true test came when the optimized protocol was unleashed on real samples from a full-scale facility, the Brasília North wastewater treatment plant. The researchers deliberately chose two matrices representing opposite ends of the processing complexity spectrum: raw wastewater, with a chemical oxygen demand averaging 882.7 milligrams of oxygen per liter, and dewatered sewage sludge, whose chemical oxygen demand averaged a staggering 30,600 milligrams of oxygen per liter—roughly thirty-five times higher. Applying the optimized Fenton protocol cut chemical oxygen demand by 81.3 percent in the raw wastewater and 82.3 percent in the sludge, leaving values of 165.4 and 5,420 milligrams of oxygen per liter respectively. Visually, the transformation was dramatic: dark, turbid samples became noticeably clearer and more translucent after treatment, although sludge samples, given their enormous organic load, sometimes required a second digestion cycle to remove residual suspended material.

Following digestion, density separation with zinc chloride at 1.8 grams per cubic centimeter produced striking stratification in the custom unit, with a floating fraction enriched in microplastics accumulating at the top and residual organic matter and denser inorganic particles settling at the bottom. Spiked reference particles of every polymer type were successfully recovered from both matrices, including the notoriously difficult polyethylene terephthalate and polyvinyl chloride. The researchers then applied the validated workflow to hunt for native microplastics—particles that arrived in the samples from the real world rather than from the laboratory. Stereomicroscopy revealed a rich menagerie of fragments, films, and fibers varying widely in color, size, and surface texture, exactly the heterogeneity expected from the diverse plastic sources entering urban sewer networks.

ATR-FTIR analysis of the recovered native particles confirmed the method’s analytical power. Polyethylene emerged as the dominant polymer in both wastewater and sludge, identified through its characteristic absorption bands near 2915, 2845, 1462, and 717 wavenumbers, corresponding to carbon-hydrogen stretching, methylene bending, and rocking vibrations. A subtle band near 1377 wavenumbers even allowed the team to distinguish low-density from high-density polyethylene. Crucially, the spectra of environmental particles retained all the features needed for identification, confirming that the upstream pretreatment had not compromised the spectroscopic evidence. The predominance of polyethylene aligns with the ubiquity of packaging films and containers in urban life and echoes findings from previous studies around the world.

The significance of this work extends well beyond a single treatment plant in the Brazilian capital. By demonstrating that Fenton digestion, zinc chloride density separation, stereomicroscopy, and ATR-FTIR can operate as a coherent, validated pipeline across both liquid and solid waste streams, the study provides exactly the kind of methodological scaffolding that the field has lacked. The authors emphasize that their framework can support future monitoring programs aimed at evaluating the occurrence, transport, and fate of microplastics throughout wastewater treatment systems, and that it contributes to the ongoing effort to establish standardized protocols for microplastic analysis in complex environmental matrices. As regulators and researchers worldwide push to quantify how much plastic flows from drains to rivers to oceans—and how much lingers in the sludge spread across farmland—reliable, reproducible measurements will be the foundation of every policy decision. This new protocol offers a tested blueprint for generating them.

Microplastics are tiny plastic particles measuring less than five millimeters. These pollutants can result from the breakdown of larger plastic debris or be deliberately manufactured for specific applications, such as cosmetics or textiles. Once released into the environment, microplastics infiltrate almost every ecosystem, including oceans, rivers, and even the soil. The pervasiveness of these particles suggests that they may come into contact with nearly all organisms, including humans, raising alarming questions about their potential health implications.

The research categorizes microplastics into two distinct types: primary microplastics, which are directly produced as micro-sized particles for commercial use, and secondary microplastics, which result from the degradation of larger plastic items. This distinction is essential because it sets the stage for understanding the chemical composition and behavior of these particles once they enter biological systems. It’s within these systems that microplastics interact with cells, potentially initiating a cascade of biological processes that could lead to cancer.

One critical aspect explored in the review is how microplastics can serve as vectors for toxic substances. These particles can adsorb numerous environmental pollutants, including heavy metals and persistent organic pollutants, increasing their concentration in aquatic and terrestrial environments. When ingested by marine life or humans, these toxins may disrupt cellular functions, leading to oxidative stress, inflammation, and ultimately, DNA damage—hallmarks of oncogenic processes.

Furthermore, the review highlights how microplastics may provoke chronic inflammation, a well-established risk factor for several cancer types. The body’s immune response to foreign particles involves a complex interplay of immune cells. When microplastics are introduced into living organisms, they can stimulate an immune reaction, resulting in chronic inflammation that may promote the proliferation of cancerous cells over time. This connection creates an imperative for deeper research into the long-term exposure effects of microplastics on human health.

Perhaps one of the more insidious features of microplastics is their ability to translocate across biological barriers. Emerging studies have shown that microplastics can penetrate cellular membranes, leading to potential cytotoxic effects. This switch in perception—that microplastics are not merely ingested but can actively invade cellular environments—underscores the urgency of comprehending their link to cancer development.

The review also discusses the implications of microplastics in the human diet. With evidence suggesting that microplastics are contaminating food sources, the discussion around dietary exposure is increasingly pertinent. Seafood, particularly, has been scrutinized as a major conduit for microplastics. Consuming contaminated food may not only expose individuals to microplastics but may also introduce myriad other associated toxins, collectively heightening the cancer risk profile.

In addition to dietary exposure, the study delves into occupational exposure scenarios, particularly in industries heavily involving plastics. Workers in manufacturing and recycling sectors may experience higher exposure to microplastics and associated hazards, necessitating stringent regulations and protective measures. This warrants an urgent reevaluation of workplace safety standards and comprehensive health assessments for those frequently encountering microplastics.

Notably, the researchers advocate for a multidisciplinary approach to tackle the implications of microplastics on human health. This should encompass environmental science, toxicology, epidemiology, and cancer research, intertwining insights to create a holistic understanding of the problem. As our knowledge evolves, so too must our strategies for surveillance and public health interventions aimed at mitigating exposure.

The investigation concludes by emphasizing the need for regulatory frameworks that address not only the environmental impact of plastics but also their health implications. Awareness campaigns must educate the public about the potential dangers associated with microplastics, as knowledge is the first step toward prevention.

In summary, the evidence presented in this review underscores that microplastics represent not just an environmental hazard but a pressing public health concern. Their ability to interact with biological systems and serve as vectors for toxicants may enhance the risk of oncogenic transformations in human cells. As further research unfolds, it becomes increasingly critical to prioritize microplastics in the discourse surrounding cancer prevention and environmental health.

As we move forward, collective action from governments, industries, and communities will be imperative to develop strategies that minimize plastic usage and improve waste management systems. With timely intervention, we can mitigate the risks posed by these micro-pollutants and safeguard future generations from their harmful effects. Environmental health is intrinsically tied to human health, and the momentum generated by studies such as this has the potential to catalyze significant change in how we manage our plastic footprint.

Subject of Research: Microplastics and their role in cancer initiation and progression.

Article Title: From exposure to oncogenesis: a review on the multifaceted roles of microplastics in tumor initiation and progression.

Article References:

He, Z., Shen, Z., Zhang, H. et al. From exposure to oncogenesis: a review on the multifaceted roles of microplastics in tumor initiation and progression. J Transl Med (2025). https://doi.org/10.1186/s12967-025-07553-5

Image Credits: AI Generated

DOI: 10.1186/s12967-025-07553-5

Keywords: microplastics, cancer, oncogenesis, inflammation, environmental health, exposure, toxins, dietary risks, occupational hazards, public health.