Published in Fertility and Sterility in 2024, researchers conducted a systematic review and meta-analysis specifically focused on fertile men (those who had fathered children) in the United States.

Across 58 studies and 11,787 men spanning 1970 to 2018, the unadjusted models showed a non-significant trend of 0.14 million sperm per mL per year. The authors concluded there was no clinically significant decline in sperm concentration among men known to be fertile.

This finding challenges the narrative that sperm counts are universally declining by showing that the decline may be concentrated in subfertile populations or driven by selection bias in study recruitment. If microplastics were causing a population-wide effect on sperm production, fertile men should show similar declines, but this analysis found they do not.

Published in Andrology in 2023, Italian researchers conducted an independent systematic review and meta-regression analyzing 65 studies from the USA, Scandinavia, and Central/Southern Europe that measured sperm parameters in unselected or fertile populations.

They found no statistically significant temporal trend in total sperm count for sperm donors, young unselected men, or fertile populations in any region. The residual heterogeneity (unexplained variation between studies) ranged from 96-97% across all models, meaning that differences in study methodology, laboratory techniques, and population selection explain far more variation than any temporal trend.

The authors argued that the extremely high heterogeneity undermines confident conclusions about decline from any meta-analysis, including the widely cited Levine et al. studies. They noted that changes in semen analysis methods, counting chamber technology, and abstinence time reporting over decades create systematic biases that can masquerade as biological trends.

Published in Toxicological Sciences in 2025, the National Toxicology Program''s Office of Health Assessment and Translation (OHAT) conducted a systematic review of all available mammalian studies examining microplastic reproductive and developmental toxicity.

All 24 studies that met inclusion criteria received a Tier III reliability rating (the lowest tier, indicating high risk of bias). Common problems included: lack of proper controls, failure to characterize microplastic contamination in control feed and water, unrealistic exposure routes, insufficient reporting of methods, and absence of dose-response analysis at environmentally relevant concentrations.

The review concluded that current evidence is "not reliable enough to identify causative hazards, establish meaningful toxicological thresholds, or guide regulatory decision-making." This does not prove microplastics are safe, but it establishes that the science has not yet met the evidentiary standards required to claim causation.

Published in BJOG: An International Journal of Obstetrics and Gynaecology in 2024, researchers conducted a systematic review following PRISMA guidelines to assess all available human evidence linking microplastic exposure to reproductive outcomes.

After screening thousands of papers, only 7 studies met inclusion criteria, involving a total of just 96 participants. The evidence was rated "unclassifiable" for birth outcomes and "low" to "very low" quality for semen parameters using GRADE criteria. The authors noted a "current absence of robust evidence about potential reproductive and developmental harms."

The tiny total sample size of 96 participants across all existing human studies is far below what would be needed to draw reliable conclusions about a population-level effect. For comparison, a single pharmaceutical drug trial typically requires hundreds to thousands of participants before regulators accept causal claims.

Published in Frontiers in Toxicology in 2024, this critical review examined the dose levels used in animal studies claiming microplastic reproductive harm and compared them to estimated real-world human exposures.

The review found that most animal studies administered microplastics at doses in the tens of millions of particles per kilogram of body weight. Estimated human exposure from food and water is orders of magnitude lower. Few studies examined chronic low-dose exposure or used environmentally relevant concentrations, and almost none studied transgenerational effects at realistic doses.

This dose gap is a fundamental problem for extrapolating animal findings to humans. Toxicology operates on the principle that "the dose makes the poison," meaning that effects observed at extremely high doses may not occur at real-world exposure levels. Until studies demonstrate reproductive harm at concentrations matching actual human intake, the animal evidence remains suggestive but not conclusive for human health.

Published in the Journal of Clinical Medicine in 2024, this review examined multiple lifestyle and environmental factors that could explain declining sperm parameters without requiring microplastic causation.

Global obesity has risen dramatically in parallel with reported sperm decline. Increased adipose (fat) tissue contains the enzyme aromatase, which converts testosterone to estrogen, creating a hormonal imbalance that directly impairs sperm production. Additionally, scrotal fat insulation raises testicular temperature, further reducing sperm quality.

The review also identified smoking, alcohol consumption, sedentary behavior, tight underwear, laptop heat exposure, and psychological stress as independently documented causes of reduced sperm parameters. Each of these factors has increased in modern populations and could account for sperm decline without invoking microplastics. The authors argued that attributing decline to any single cause is premature given the number of simultaneous changes in modern lifestyles.

Published in the International Journal of Environmental Research and Public Health in 2022, this meta-analysis examined the relationship between ambient temperature and sperm parameters across multiple studies.

Sperm concentration and total count were found to be up to 30% lower in summer months compared to winter, demonstrating that heat alone can produce large effects on sperm parameters. Separately, studies showed that seated office work raises scrotal temperature by approximately 3 degrees Celsius within 20 minutes, and laptop use adds another 2.5 degrees Celsius.

Since the shift toward sedentary desk-based work has accelerated dramatically since the 1970s (the same period over which sperm decline is reported), increased scrotal heat exposure from modern work habits could explain a significant portion of the observed decline. This hypothesis requires no novel toxicant and relies on a well-established mechanism: the testes are outside the body specifically because sperm production requires temperatures 2-4 degrees below core body temperature.

In expert reactions compiled by the Science Media Centre in 2017 following the Levine et al. meta-analysis, multiple reproductive biology specialists identified fundamental methodological limitations in sperm decline research.

Professor Allan Pacey (University of Sheffield, andrologist) noted that men in older studies were likely selected differently than men in newer studies. Earlier decades drew study participants primarily from fertile fathers or military recruits, while modern studies increasingly recruit from fertility clinic populations or men concerned about their fertility.

Professor Daniel Brison (University of Manchester) emphasized that semen analysis methods have changed substantially over 40 years, with newer WHO counting standards producing systematically different results than older methods. These measurement differences accumulate over time and can appear as biological trends when they are actually methodological artifacts. No meta-analysis can fully correct for these biases because the original data were collected under incompatible protocols.

The European Food Safety Authority (EFSA) announced in 2025 that it is conducting a comprehensive human health risk assessment of microplastics in food and water, with final results not expected until the end of 2027.

This means that as of mid-2026, no major regulatory authority in the world (including EFSA, the U.S. FDA, the U.S. EPA, or the WHO) has issued a formal determination that microplastics cause reproductive harm in humans at levels encountered in the environment. Regulatory risk assessments require a much higher standard of evidence than individual research papers, including demonstrated causation at relevant doses, dose-response characterization, and identification of susceptible populations.

The absence of a regulatory conclusion does not prove safety, but it indicates that the global scientific and regulatory community has not yet found the evidence sufficient to establish microplastics as a confirmed reproductive toxicant. This contrasts with substances like lead or certain pesticides, where regulatory determinations were made decades ago based on stronger evidence.