Challenges in Interpreting Reported Urine Microplastic and Nanoplastic Levels: Mass- vs. Particle-Based Measurements

Abstract

The peer-reviewed literature on microplastics and nanoplastics (MNPs) in human urine remains limited, with approximately 70–100 samples reported across major studies. Reported concentrations vary by multiple orders of magnitude depending on analytical methods, sample preparation procedures, and reporting units. When normalized to common units such as particles per liter, the resulting values can be difficult to reconcile biologically and analytically. This technical note reviews major published studies and discusses challenges associated with comparing mass-based and particle-based measurements across differing analytical workflows. This paper is also available at: https://doi.org/10.5281/zenodo.20576841

Figure 1. Comparison of Reported Urinary Microplastic and Nanoplastic Concentrations Across Published Studies.

Approximate urinary microplastic and nanoplastic (MNP) concentrations reported in selected published studies after normalization to particles per liter and displayed on a logarithmic (log₁₀) scale. Reported values span approximately 9–10 orders of magnitude, ranging from a few particles per liter in direct-counting studies (Pironti et al., Massardo et al., Song et al.) to approximately 2 × 10¹⁰ particles per liter in estimates derived from mass-based measurements using particle-size assumptions (Ji et al.). Values are shown for illustrative comparison only and should not be interpreted as directly equivalent measurements. Differences may reflect variations in sample preparation, storage conditions, analytical methodologies, size detection limits, reporting frameworks, and assumptions used for mass-to-particle conversion, in addition to potential biological variability. This figure highlights the current challenges in harmonizing urinary MNP measurements across studies and underscores the need for standardized analytical and reporting approaches.

1. Introduction

Human urine has emerged as a non-invasive matrix for investigating environmental exposure to microplastics and nanoplastics. However, published studies report highly variable concentrations, ranging from a few particles per liter to billions of particles per liter following mass-to-particle conversion. This variability reflects differences in sample collection, storage conditions, processing methods, analytical technologies, size detection limits, and reporting frameworks.

As interest in urinary MNP biomonitoring grows, careful interpretation of reported values is necessary to avoid direct comparisons between studies that employ fundamentally different methodologies.

2. Overview of Major Urine MNP Studies (n = 70—100 individuals)

Study	Year	Samples	Method	Reported Result	Approximate Normalized (particles/L)	Notes
Pironti et al.	2022	6	Raman microspectroscopy	7 fragments total	Few MP/L	Sizes 4–15 µm
Massardo et al.	2024	10	MicroRaman	Mean 1.28 fragments/sample	~4–5 MP/L	Sizes 3–13 µm
Song et al.	2024	12	Py-GC/MS + LDIR	~1.5 mg/L and ~15 particles/L	~15 particles/L	Mass and count reported separately
Rotchell et al.	2024	38	μFTIR	Presence confirmed	Low / not quantified	Endometriosis cohort
Ji et al.	2025	18	Proteinase K + Py-GC/MS	0.268 µg/mL (mass)	~20 billion particles/L*	Back-calculated estimate

*Approximate value derived using commonly applied particle-size assumptions.

Total samples across major published studies remain approximately 70–100 individuals.

3. Core Challenge: Incompatible Units and Back-Calculation

Published studies report MNP concentrations using fundamentally different metrics.

Some studies report direct particle counts, while others report total recovered polymer mass.

Converting mass into particle numbers requires multiple assumptions, including:

• Spherical particle geometry

• Uniform particle size

• Uniform polymer density

• Complete recovery of analyzed material

The volume of a spherical particle is:

The number of particles is then calculated as:

Because particle volume scales with the cube of particle radius, small differences in assumed particle size can produce extremely large differences in estimated particle counts.

For example, the same recovered mass may correspond to billions of particles when a diameter of 300 nm is assumed, but only a small number of particles if larger particle diameters are assumed.

Consequently, particle-number estimates derived from mass measurements should be interpreted within the context of the assumptions used for conversion.

4. Biological Plausibility Considerations

Published mass concentrations ranging from approximately 0.268 mg/L to 1.5 mg/L imply measurable daily excretion of plastic-associated material. Depending on assumptions regarding urine output and particle size distributions, back-calculated particle counts may span several orders of magnitude.

Direct-counting studies generally report only a small number of visible microplastic fragments per sample, whereas some mass-based approaches can yield particle-number estimates in the billions per liter following conversion.

These differences may reflect methodological factors including sample preparation workflows, analytical recovery, lower size detection limits, assumptions used for particle-number calculations, contamination control procedures, and differences in the particle populations being measured.

5. Methodological Factors

Several methodological variables may contribute to inter-study variability:

• Sample storage at 4°C or frozen conditions

• Alkaline digestion and enzymatic digestion workflows

• Filtration and membrane selection

• Centrifugation and concentration procedures

• Polymer recovery efficiency

• Laboratory contamination controls

• Instrument-specific size detection limits

Many spectroscopic techniques have practical lower detection limits in the low-micrometer range and may incompletely characterize nanoplastics. Conversely, mass-based approaches may detect polymer mass without directly enumerating individual particles.

These methodological differences complicate direct comparisons between studies.

6. Need for Standardization

Future studies may benefit from more standardized reporting practices, including:

• Sample volume analyzed

• Storage conditions

• Processing workflows

• Polymer identification methods

• Particle size distributions

• Mass concentrations

• Particle concentrations

• Limits of detection and quantification

• Recovery efficiency estimates

Reporting both mass-based and particle-based metrics, when feasible, may improve comparability across studies.

7. Clinical Interpretation Limitations

At present, the available urinary MNP literature remains too limited to support the use of urinary microplastic or nanoplastic concentrations as clinical risk indicators, diagnostic biomarkers, prognostic markers, or treatment-monitoring endpoints.

Published studies are characterized by small sample sizes, heterogeneous methodologies, differing reporting units, and limited longitudinal follow-up. Consequently, there is currently insufficient evidence to establish population reference ranges, clinically meaningful thresholds, or exposure levels associated with specific health outcomes.

Urinary MNP measurements may provide useful information regarding environmental exposure assessment and biomonitoring; however, their clinical significance remains under active investigation.

Conclusion

The current literature on microplastics and nanoplastics in human urine remains limited and highly heterogeneous. Differences in analytical methods, reporting frameworks, storage conditions, processing workflows, and particle-number conversion assumptions make direct comparison across studies challenging.

This technical note is intended as a methodological review and does not invalidate any individual study. Reported values should be interpreted within the context of each study's analytical workflow and reporting framework.

Additional standardization, larger population studies, and complementary analytical approaches may improve understanding of urinary MNP measurements and their potential role in environmental exposure assessment.

References

1. Pironti C, Ricciardi M, Motta O, et al. First evidence of microplastics in human urine, a preliminary study of intake in the human body. Toxics. 2023;11(1):40. doi:10.3390/toxics11010040

2. Rotchell JM, Austin C, Chapman E, et al. Microplastics in human urine: characterisation using μFTIR and sampling challenges using healthy donors and endometriosis participants. Ecotoxicol Environ Saf. 2024;274:116208. doi:10.1016/j.ecoenv.2024.116208

3. Song X, Chen Y, Feng L, et al. Micro(nano)plastics in human urine: a surprising contrast between Chongqing's urban and rural regions. Sci Total Environ. 2024;912:170455. doi:10.1016/j.scitotenv.2024.170455

4. Ji S, et al. Quantitative detection of micro- and nanoplastics (≥300 nm) in human urine using double-shot Py-GC/MS with internal standard calibration. Toxics. 2025;13(6):452. doi:10.3390/toxics13060452

5. O’Callaghan L, et al. Plastic induced urinary tract disease and dysfunction: a scoping review. J Expo Sci Environ Epidemiol. 2025. doi:10.1038/s41370-024-00709-3

6. Massardo S, Verzola D, et al. MicroRaman spectroscopy detects the presence of microplastics in human urine and kidney tissue. Environ Int. 2024.