The current study investigated the effects of air pollutants on the reproductive health of men. Our interest in the topic was fueled by the limited studies and controversies on the effects of toxic ambient particles on the reproductive system.
As air pollutants are hazardous to humans, it is infeasible and unethical to conduct an interventional study on patients. Most studies on the effects of ambient pollutants on humans were retrospective or observational and only focused on the effects of pollutants on semen quality.
Zhou et al. reported that urban areas had high SO2 and NO2 concentrations and that they were associated with proportionately few sperms with normal morphology. Liu et al analyzed exposure–response relationships of 2841 semen samples and found that SO2 exposure was associated with a decreased semen concentration and total sperm count.17, 18 In addition, Broggia et al. reported that workers occupationally exposed to NO2 had significantly lower total sperm motility than those not exposed to NO2.19 However, Zhang et al. found that SO2, NO2, and PM2.5 were not associated with altered semen quality.20
The effect of ozone on the reproductive health of men was relatively higher than that of other ambient pollutants. Zhang et al. obtained the air pollutant data from the national real-time platform for city air quality monitoring and found high ozone exposure to be an independent factor predicting low semen concentrations.20 Farhat et al. reported compromised semen quality after exposure to a high O3 level in 28 patients with systemic lupus erythematosus.10 Echoing the findings of the aforementioned studies, Sokol et al. analyzed 5134 semen samples and found a negative association between ambient O3 exposure and sperm concentration.21 In addition, Wdowiak et al. found that ozone reduces the percentage of sperms with normal morphology.22 Nevertheless, Hansen et al. found no significant effect of O3 on sperm quality in fertile men.23
Our study found that NO2 exposure was negatively associated with sperm concentration and sperm motility, which is consistent with the finding of Broggia.19 Conversely, O3, SO2, and PM2.5 had no statistically significant effects on semen quality.
The detailed mechanism in which the ambient pollutants interfere with the male reproductive system was not well elucidated. One possible mechanism is induction of oxidative stress in the reproductive system. After exposing a mice model to a high PM2.5 concentration, a decrease was observed in superoxide dismutase (SOD), which is one of the crucial enzymes for protecting cells from reactive oxygen species (ROS). Furthermore, an increase in heme oxygenase (HO) was observed. HO plays a role in metabolizing ROS, serving as an indicator of oxidative stress. These changes were observed in testicular biopsy, signifying that ambient pollutants can alter the oxidative balance in the testicular microenvironment.13 Mirowsky et al. isolated primary human bronchial epithelial cells in a culture medium and exposed the cells to different levels of NO2 and O3. The expression of oxidative-stress-related gene, especially heme oxygenase (decycling) 1 (HMOX1), which encodes HO, significantly increased in a dose-dependent manner. Additionally, the expression of proinflammatory-related gene, including interleukin-6 and interleukin-8, increased with NO2 and O3 exposure, which supports the pro-oxidative stress role of those ambient pollutants.24
Furthermore, SO2 plays a role in altering oxidative stress. Meng et al. demonstrated that after exposing mice to a high concentration of SO2, the level of antioxidant enzymes, including glutathione peroxidase (GPx) and SOD, decreased significantly compared with the control group.11 Moreover, ozone plays a role in increasing oxidative stress. Exposing a rat model to O3 and other concentrated ambient particles increased the lactate dehydrogenase level and increased the activities of antioxidant enzymes including SOD and catalase in the cardiopulmonary system, altogether signifying increased oxidative stress. Moreover, ozone penetrates the blood–gas barrier, and ROS circulates around different organ systems, including the reproductive system.25, 26
Spermatozoon is a cell type that generates ROS on its own, which is crucial in the acrosome reaction. However, if the balance between oxidative stress, ROS, and antioxidative capacity is disturbed, it will lead to reduced sperm quality. Excessive ROS attacks the fluidity of the sperm plasma membrane and induces DNA damage in the sperm nucleus.25, 26 In addition, spermatozoa are more vulnerable to oxidative stress than oocytes as they lack downstream enzymes that participate in base excision repair.11, 27 Testicular biopsy of a mice model showed an altered expression of HO and SOD after exposure to ambient pollutants, signifying that increased oxidative stress within the reproductive system could be induced by ambient pollutants. HO plays a role in metabolizing ROS, serving as an indicator of oxidative stress.28, 29
Human testicular development mainly occurs during puberty, and different cell types, including Sertoli cells, spermatogonia, and spermatocytes, play a role. During childhood, testicles grow mainly due to the elongation of seminiferous tubules. During puberty, testicles enlarge due to an increase in the diameter of the seminiferous tubules.30, 31 Later, the exponential growth of germ cells during spermatogenesis constitutes the increase in the testicular volume.30, 32
The effect of increased oxidative stress on the testicles has been studied in mice models. Li et al. exposed rat models to oxidative stress caused by crotonaldehyde, which is used to produce sorbic acid, a food preservative. The experimental group had significantly lower testicular and epididymal weights than the controls. Furthermore, in the exposed group, the concentration of malondialdehyde (MDA), a direct indicator of lipid peroxidation-induced injury caused by ROS, significantly increased, whereas GPx and SOD activities decreased.33 Farsani et al. induced oxidative stress within the mouse testicles by using doxorubicin. The MDA level decreased with an increase in GPx and SOD activities. Moreover, after 8 weeks, the testicular volume, epididymal sperm count, and seminiferous tubule diameter decreased in the exposed group compared with the control group.34, 35 Balanced ROS within the testicular environment is crucial. Overloaded oxidative stress and excessive lipid peroxidation can lead to germ cell damage and testicular degeneration. Koksal et al. evaluated the association between oxidative stress measured using MDA and the testicular biopsy obtained from infertile men. Thus, the high MDA level was associated with mild to moderate hypospermatogenesis and maturation arrest.35, 36
To our knowledge, this is the first study to evaluate the effects of ambient pollutants on the testicular volume of infertile men. Nevertheless, the current study has several limitations. First, we considered the exposure variation on the basis of daily average exposure concentrations for a specific period (2 years), while ignoring the effects of the peak and exposure of the pollutant within a day. Second, our study was retrospective and observatory in design, as intentionally exposing participants to air pollutants is not ethical. Moreover, our study population was relatively small, and some data were missing. Therefore, the statistical power to detect the effects of pollutants on the reproductive system and sex hormone profile may be weakened. Furthermore, selection bias was inevitable, as most of our patients were from northern Taiwan. Recruiting patients from other parts of Taiwan and future cross-institute research are recommended to strengthen the statistical power. In addition, other potential confounders may exist that are not included in this study. Third, using the residential address of the study population may not correctly represent the exposure level as the exposure level of candidates to pollution when they commute to other places is not considered.