Infertility has emerged as a critical global health challenge, with approximately 17.5% of the adult population suffers from infertility affecting about one in six couples worldwide. Recent epidemiological surveys indicate that male factors contribute to 40–50% of infertility cases (1). Over the past four decades, accumulating evidence from industrialized nations has suggested a concerning decline in human semen quality, particularly characterized by reduced sperm concentration and total sperm count (2). However, significant controversies persist regarding the generalizability of these trends, particularly in Asian populations and under standardized laboratory conditions.

Differences in study populations—ranging from fertile volunteers to military recruits and sperm bank donors—as well as heterogeneity in semen collection and analytical methods may introduce bias. Since its first edition in 1980, the World Health Organization (WHO) Laboratory Manual for the Examination and Processing of Human Semen has undergone multiple revisions (3), each refining protocols for sample collection, analysis of sperm concentration, motility, morphology, and quality-control procedures. Over the years, our team has been working on the promotion and dissemination of the WHO manual in China, and has been carrying out the construction and quality control of Chinese male laboratories (4, 5). The interpretation of longitudinal semen parameter changes remains complicated by methodological inconsistencies across studies. Notably, the transition from WHO 4th to 5th edition introduced critical modifications including updated reference limits, standardized quality control measures, and revised classification of teratozoospermia (5, 6). These methodological shifts may introduce systematic biases when comparing historical data, potentially confounding true biological trends with technical artifacts. In order to provide more valid data on changes in male fertility, we used a single-center retrospective survey of possible changes in semen quality, using the same criteria and based on a large fixed population.

China presents a unique epidemiological landscape for investigating male fertility trends. Rapid urbanization, environmental exposures, and lifestyle transitions have created distinct pressures on reproductive health (7). Nevertheless, existing Chinese studies exhibit marked heterogeneity in cohort selection criteria and laboratory methodologies (8, 9). Notably, the study period coincided with the implementation of large-scale national environmental interventions, particularly the Air Pollution Prevention and Control Action Plan launched in 2013, providing a unique policy-relevant context to explore population-level reproductive health trends. And to avoid the effect of COVID-19 that outbroke in 2019, we analyzed the semen data, in our lab from 2011 to 2018 to discover the trend of sperm quality. By leveraging a large, well-characterized cohort and standardized laboratory procedures, we aim to provide robust evidence on modern semen quality in a major urban Chinese population, clarify environmental determinants of male reproductive health, and inform future surveillance and intervention strategies. Our findings aim to resolve current controversies regarding semen quality trends while providing novel insights into environmental determinants of male reproductive health.

This study conducted a large-scale retrospective analysis of 5,886 semen samples from healthy sperm donors in the Beijing area (2011–2018). Revealing significant improvements in key semen parameters: sperm concentration (SC) and total sperm count (TSC) increased by 12.3 and 18.7%, respectively, from 2011 to 2018, contrary to the previously reported global decline trend (2). Levine et al.’s (2) comprehensive review and meta-regression of 244 estimates from 1973 to 2011, found that sperm concentration declined by approximately 0.70 million/mL per year—an overall drop of more than 50%—and total sperm count by 2.23 million per year (about 0.75% annually), trends that were most pronounced among unselected Western men. But the Levine et al.’s (2) meta-regression captured four decades and multiple WHO guideline revisions, weighting studies by standard error and adjusting for numerous covariates, yet remained susceptible to between-center and between-era variability in lab practices. Our findings were rigorously standardized according to the fifth edition of the World Health Organization (WHO) criteria, eliminating potential biases caused by differences in testing methods. This study accurately assessed the annual trends in semen parameters over the past decade in a fixed population, providing a reliable basis for understanding changes in population fertility. Cargnelutti et al. (11) in Rome, conducted a retrospective analysis of 3,329 men—divided between idiopathic infertility patients and healthy controls—whose semen was assessed from 2010 to 2019 using consistent WHO 2010 protocols and personnel. They observed no significant change in total sperm number over that ten-year span, and identified body mass index, smoking, and infertility history (but not calendar year) as the principal drivers of inter-individual variability (11). The study thereby minimizing analytical heterogeneity but perhaps missing longer-term environmental influences. Our donors were young (20–45 years old), healthy, and non-smokers, minimizing common confounding factors such as age and comorbidities often encountered in infertility clinical studies. Several methodological and contextual factors likely account for these divergent outcomes.

Our Beijing cohort, analyzed under a single-center, highly standardized WHO 5th-edition framework, spans a period of intensive air and water pollution control measures—most notably the 2013–2017 “Air Pollution Prevention and Control Action Plan”—which coincides with measurable gains in sperm concentration, count, and motility. This is consistent with the findings of Zhang et al. (9), who observed a temporary improvement in semen quality during the period of reduced air pollution before and after the pollution control period in 2017–2018. However, due to an unexpected rise in ozone (O₃) levels during this period, the improvement was short-lived. Notably, Zhang et al. (9) found a significant inverse relationship between O₃ and sperm concentration. This finding underscores the need to pay particular attention to ozone levels when formulating pollution control policies, as changes in atmospheric conditions may have unintended consequences for reproductive health. In contrast, Liu et al. (12) found that semen quality in Henan Province continued to decline from 2009 to 2019, a trend that may reflect long-term exposure to pollutants without significant mitigation measures. Their study suggests that factors such as environmental pollutants, obesity, and lifestyle changes may be contributing to the decline in semen quality. The decline in sperm concentration reported in their study (from 62 million/mL in 2009 to 32 million/mL in 2019) is comparable to other reports from Shandong and global studies, indicating that deteriorating sperm quality is a broader regional and international trend.

Airborne particulate matter (PM10) and heavy metals may disrupt spermatogenesis homeostasis. Particulate matter (PM10) is rich in cadmium (Cd) and lead (Pb), which accumulate in testicular tissue and generate hydroxyl radicals (•OH) through Fenton-type reactions, leading to double-strand DNA breaks (13). In animal models, cadmium exposure reduces spermatogonia proliferation by over 40%, and similar damage aligns with the negative correlation we observed between PM10 and total sperm count. PM10 also disrupts the blood-testis barrier by upregulating matrix metalloproteinase-9, degrading tight junction proteins such as tight junction protein-5 and claudin-3 (14). Blood-testis barrier dysfunction facilitates toxin penetration, explaining the decline in TSC during high PM exposure. Previous studies have shown that gaseous pollutants such as sulfur dioxide (SO₂) and nitrogen dioxide (NO₂) impair sperm motility by damaging mitochondrial function: SO₂-derived sulfites inhibit cytochrome c oxidase (complex IV), thereby inhibiting ATP production, which is crucial for flagellar movement (15). Our findings also support this: SO₂ levels were negatively correlated with motility (ρ = −0.904, p = 0.002). Additionally, SO₂ increases lipid peroxidation by oxidatively depleting glutathione (GSH), thereby damaging sperm membranes. NO₂ induces abnormal DNA methylation at imprinting sites through nitrosative stress, disrupting post-meiotic sperm development (16). Elevated ammonia nitrogen (AN) concentrations in water are associated with reduced semen volume, potentially through inhibition of aquaporin-9 expression in epididymal cells, impairing fluid reabsorption (17). Additionally, high chemical oxygen demand (COD) in wastewater indicates the presence of estrogen-like endocrine disruptors (e.g., alkylphenols), which antagonize androgen receptor signaling in Sertoli cells, thereby affecting lactate production—a critical energy source for germ cell maturation (18).

This study has several limitations. First, due to the retrospective ecological design, associations between annual semen parameters and population-level environmental indicators cannot be interpreted as causal relationships at the individual level. Second, the environmental correlation analysis was based on limited annual observational data, which may reduce statistical power and increase sensitivity to outliers; therefore, these results should be interpreted with caution. Third, although the donor cohort underwent rigorous screening for major confounding factors, several individual-level factors—including body mass index, lifestyle characteristics, occupational exposures, abstinence duration, and seasonal effects—could not be adjusted for. Finally, as this study was conducted at a single center in Beijing, its conclusions may have limited generalizability to other populations. Future multicenter prospective studies will provide more robust evidence regarding the relationship between male semen quality and environmental factors.

While global meta-analyses report alarming declines in sperm counts, our study reveals a 12.3–18.7% increase in key semen parameters among Beijing sperm donors (2011–2018). This paradox is particularly noteworthy given the temporal overlap with China’s aggressive environmental policy interventions.