Environmental and occupational exposures associated with male infertility

The association between occupational exposure and infertility is still neglected, and studies are rare and lacking systematic approach, even in developed countries. Occupational health protocols for many occupations in which it is known that technological processes may include exposure to EDCs do not include any medical examination or data collection on fertility status (15). A significant input, however, has been given by infertility clinics, which actively try to find the cause of infertility, as this might enhance the efficiency of ART.

Over the last few decades, they have identified several groups of compounds as potentially associated with male infertility, although life habits like smoking and alcohol may significantly mask or interact with the effects of these compounds present in occupational settings.

Studies of occupational exposure to benzene and its mixture with toluene and xylene have shown an association between benzene levels in blood and seminal plasma, significant genome damage, abnormal sperm count, motility, morphology (16, 17), disomy X and Y, and hyperhaploidy, even at low levels (≤1 ppm) (18).

A large Tunisian study (19) with 2122 men who visited infertility clinics and filled a detailed questionnaire showed that occupational exposure to pesticides was associated with a significantly higher risk of asthenozoospermia and necrozoospermia, while exposure to cement correlated with a higher risk of oligozoospermia. However, it found no association between semen impairment and exposure to solvents, excess heat, or mechanical vibrations. In contrast, one study (20) showed an association between male infertility and exposure to solvents and painting materials, lead, work with computers, shift work and work-related stress. A large Canadian national study (21) found a significant decrease in sperm motility in participants occupationally exposed to high levels of solvents (21), as did a recent meta-analysis of 272 exposed and 247 unexposed men (22).

Although the consumer use of formaldehyde at concentrations ≥0.1 % has been prohibited in the EU (23) due to its carcinogenicity, it is still omnipresent in the environment and widely used in resin production, construction, wood processing, textiles, hospitals, laboratories, and chemical industry and its sales are expected to reach US$ 34.8 billion by 2026. With this in mind, continuous efforts are made to limit/control its occupational exposure (24). In terms of male infertility, formaldehyde has been shown to significantly lower progressive and total sperm motility in a dose-dependent manner (25).

The major problem with pesticides is that exposure may be both environmental and occupational. It will depend on technology and timing of pesticide application and farming type. Pesticides as a group of chemicals which includes insecticides, herbicides, and fungicides express endocrine effects from xenoestrogens to aromatase inhibitors.

Sperm analysis in groundnut farmers in Myanmar who used chlorpyrifos and carbamates, often above recommended dosages, revealed that 74 % had oligozoospermia in the growing season, which was less pronounced (46 %) in the non-growing season (26). In abamectin-exposed farmworkers, significantly increased abamectin plasma levels were associated with significant genome damage and immaturity of spermatozoa, even though testosterone, LH, and FSH levels were normal (25). Similarly, exposure of farmworkers to organophosphate and carbamate pesticides, which was confirmed by elevated erythrocyte acetylcholinesterase ( A C h E ) and plasma butyrylcholinesterase (BuChE), was associated with lower sperm motility and sperm immaturity without significant deviations from normal serum testosterone, LH, and FSH levels (27).

The use of ethylene glycol-based chemicals is common in many commercial and industrial applications, including solvents, antifreeze, and coolants. One study (28) reported occupational exposure to be associated with low motile sperm count, but the risk diminished after adjustment for confounders. In vitro, ethylene glycol was reported to increase oestradiol levels in H295R cells (29). In rats and mice, ethylene glycol monomethyl ether (EGME) lowered the expression of spermatocyte-specific genes (30) and germ cell lineage (31), respectively. In cynomolgus monkeys, it damaged spermatocytes in the pachytene stage, implicating miR-34b-5p and miR-449a downregulation in the process (32).

An increased risk of infertility has been reported in full-time firefighters (33), as their occupational setting is associated with exposure to a complex mixture of chemicals, including those known to be ED such as flame retardants, polyaromatic hydrocarbons (PAHs), as well as with hyperthermia, stress, and circadian rhythm disturbances due to night-shift work. Of special interest is exposure to per- and poly-fluorinated alkyl substances (PFAS) as the main constituent of aqueous film-forming firefighting foams (34). A human stem-cell-based model of spermatogenesis showed that PFAS lower the expression of markers for spermatogonia and primary spermatocytes without directly affecting germ cell viability, which suggests long-term effects on male fertility through the depletion of the spermatogonial stem cell pool and abnormalities in primary spermatocytes (35).

Exposure to phthalates, more specifically di-n-butyl and di-2-ethylhexyl phthalate, has been shown to lower free testosterone in workers producing unfoamed polyvinyl chloride flooring (36). It is interesting, however, that this study used construction sector workers for control, which may involve a significant bias, as they also may been exposed to phthalates. In a study with polyvinyl chloride (PVC) workers (37), lower sperm motility was associated with higher urinary phthalate metabolites, sperm apoptosis, and ROS generation. In a similar occupational setting, ambient air phthalate levels were positively associated with DNA fragmentation and negatively with sperm motility (38).

In epoxy resin workers occupationally exposed to bisphenol A exposure was associated with alterations of sperm LINE-1 hydroxymethylation (39, 40). In another study, bisphenol A exposure affected gene expression by affecting DNA hydroxymethylation, which partly depends on the trimethylation of histone H3 in human spermatogenesis (41).

Market globalisation and global mobility of goods and people have also globalised exposure through goods, canned or fresh food, and global transport of waste. In addition to common habits such as smoking and drinking, diet is also losing national characteristics, and distribution of potential food contaminants in canned or fresh food has become global. However, infertility issues associated with environmental exposure are still neglected by the media and educational programmes for the general population. In addition, environmental exposure has become so complex that it is almost impossible to reliably assess infertility risk. This is additionally challenging due to inter-individual differences in susceptibility, age, and unknown transplacental exposure with possible lifelong effects.

A large number of studies have associated smoking with poorer sperm quality (42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52) and suggested that quitting tobacco use and improving general lifestyle would improve male fertility. Sharma et al. (44) conducted a large meta-analysis of the effects of cigarette smoking and reported that moderate and heavy smoking affected sperm count, motility, and morphology. Another study (45) did not find any significant difference in the effects between those who smoked more and less cigarettes per day, which suggests that even mild smoking could impair spermatogenesis. Furthermore, heavy smoking has been reported to induce ferroptosis in seminal plasma (46) and to affect sperm motility (47). Among infertility clinic patients, smokers had lower ejaculate and seminal vesicle volume than non-smokers but not lower testosterone (48).

One predictor of genome damage through disease, environmental exposure and lifestyle is the aberrant methylation of repetitive elements such as LINE-1 (53). Normal methylation of different genomic elements is also important for fertility. One Chinese cohort study found that hypermethylation of LINE-1 and hypomethylation of the P16 gene were significantly associated with tobacco smoking in men with infertility risk (49).

Higher accumulation of ROS, such as superoxide anion and hydroxyl radicals, in the testes of tobacco smokers causes oxidative stress to which sperm is sensitive, as its plasma membrane is rich in polyunsaturated fatty acids (PUFA) and its cytoplasm deficient in scavenging enzymes (50). Oxidative stress may also render the acrosome membrane temporarily permeable to calcium ions that trigger premature acrosome reaction (51). Antoniassi et al. (52) have also associated smoking with lower sperm mitochondrial activity and increased pro-inflammatory response in accessory glands and the testes.

The testes are also susceptible to cadmium (Cd) and lead (Pb) from tobacco smoke absorbed by the lungs (54). Both act as metalloestrogens and have been reported to accumulate more in the blood and semen of smokers than non-smokers (55). Higher levels of Cd and Pb are associated with impaired spermatogenesis and elevated ROS (55). Moreover, Cd tends to replace zinc (Zn), which is crucial for normal spermatogenesis (56). In rats, Cd is implicated in age-dependent dysregulation of hypothalamus-pituitary-testicular axis, which means that Cd exposure can differently affect spermatogenesis with older age (57). Nicotine and its metabolite cotinine can also disrupt spermatozoa parameters in a dose-dependent manner (58). In an in vitro study (59) nicotine reduced the number of TM3 Leydig cells, most likely through autophagy, and, consequently, testosterone synthesis. In vivo nicotine decreased testosterone levels in serum and impaired the function of other sex hormones (FSH, LH and prolactin) in albino rats (60).

Alcohol consumption is another lifestyle factor associated with male infertility but with inconsistent findings, as reviewed by La Vignera et al. (61). One large cross-sectional study with 8344 European and USA participants (62) reported that men who drank alcohol more frequently had higher serum testosterone levels and that moderate alcohol intake did not significantly correlate with semen quality. However, this study did not exclude confounder effects. The authors concluded that elevated testosterone may be related to changed metabolism in the liver and that moderate drinking may not affect men fertility potential. Hoyer et al. (63) also investigated the relationship between male alcohol consumption and couple fertility in an internet cohort study on Danish and North Americans (63). They found no significant association, except for slightly lower conception rates among couples in which men consumed more alcohol.

One Vietnamese study (64), however, reported a significant correlation between alcohol consumption and impaired sperm DNA integrity, which was, in turn, associated with poorer semen quality in infertile men. Several other studies reported similar findings for drinking and abnormal semen parameters, especially morphology, but in combination with smoking (65, 66, 67). Smoking and obesity are confounding factors that often come in combination with moderate to heavy alcohol consumption (59) and make alcohol effects impossible to separate unless the confounding factors are excluded. One such study with non-smoking chronic alcoholics (68) reported higher serum FSH and LH and lower testosterone levels and semen quality. Studies on alcohol effects on male infertility should also take into account ethnic differences in response to alcohol and other EDs (69). Even so, current knowledge seems to suggest that male infertility is more likely in heavy drinkers, especially if they are smokers and/or obese.

Diet is another important factor that can affect sperm quality. Overweight and obesity are usually the consequence of nutrient-poor and sugar and saturated fat-rich diet in combination with little to no physical activity. The magnitude of this issue has been clearly illustrated by the Global Burden of Disease Study of 2013 (70) claiming that overweight and obesity causes 3.4 million deaths and its prevalence is in rise in the last few decades.

One study based on a food frequency questionnaire and nutritional indices (71), associated healthy eating in men with better sperm quality. Several other studies (72, 73, 74) established an association between Western animal-based, high-energy diet with lower quality of sperm. Meanwhile, Mediterranean diet rich in fruits, vegetables, and whole grains does not necessarily correlate with better sperm quality, as reports are somewhat conflicting (74, 75, 76, 77).

Disregarding the type of diet, the Longitudinal Investigation of Fertility and Environment (LIFE) study (78) has associated increased body mass index (BMI) and waist circumference with poorer sperm quality in men with unknown fertility status, whereas physical activity had limited effect.

Because testicular function and spermatogenesis are under tight hormonal control, they are subject to every shift in hormonal balance, which, in turn depends on the regulation of hypothalamus-pituitary-testis (HPT) axis, proven to be affected by overweight and obesity (79). One of the mechanisms by which adipose tissue can downregulate spermatogenesis is aromatase overexpression. Aromatase converts testosterone to oestrogen, and the amount of aromatase is proportional to body fat. In other words, body fat reduces testosterone levels in circulation and the testes. Oestrogen can further affect the HPT axis by negative regulatory feedback that downregulates LH and FSH (80).

Furthermore, testicular function may be affected by other hormones released in response to body fat, such as leptin released by adipocytes. One study with an interesting design comparing two obese male groups, one fertile and one infertile, showed that infertile men had significantly higher BMI and leptin levels than the fertile group (81). In contrast, another study (82) did not find any significant association between leptin and infertility, but this may have to do with the fact that infertile study participants were not exclusively obese men. One study (83) also did not find a correlation between leptin and the usual sperm parameters but did find a negative correlation with testosterone, which may point to its adverse effects on testicular function through testosterone-dependent deregulation of Leydig cell steroidogenesis.

There is also the issue of type 2 diabetes and insulin resistance in obese men. A recent longitudinal study (84) investigated a potential bidirectional association between insulin resistance and testosterone levels and showed that baseline lower testosterone could predict insulin resistance but not vice versa after adjustment for body composition, alcohol intake, smoking, hypertension, and diabetes. Other studies showed that obesity, metabolic syndrome, and type 2 diabetes can lead to lower testosterone levels (85, 86, 87).

Obesity can also enhance negative effects of environmental toxicants such as bisphenol A (BPA) on spermatogenesis (88). Even though conflicting results have been reported on the relation between obesity and male infertility, it is certain that obese men are more prone to mechanisms that can disrupt spermatogenesis than men with a normal weight range. Moreover, obesity causes hormonal deregulation and increases the risk of many other diseases that may affect male fertility.

Bretveld et al. (89) have given us a very useful and comprehensive overview of the mechanisms of pesticide-induced reproductive toxicity in men. A number of studies have established associations between exposure to some pesticides in everyday life and sperm parameters. This particularly concerns pyrethroids, used on massive scales because they are highly toxic to insects and barely toxic to mammals, for which reason the popular belief is that they are safe. However, several studies have reported an association between pyrethroid metabolites in non-occupationally exposed men and abnormal sperm morphology, lower sperm counts, and testosterone levels (90, 91). Several metabolites were also associated with disrupted sperm DNA integrity, and higher rates of sex chromosome aneuploidy (90, 91, 92, 93, 94). In contrast, two studies with Japanese student population found no significant association between pyrethroid metabolites and sperm quality or reproductive hormones (95, 96).

Another group of pesticides that has been associated with reproductive toxicity in men are the organochlorine ones, massively used in the mid-20^th century, some of which were later banned but are still present in the environment due to their chemical persistency (97). Environmental exposure to organochlorines p,p’-DDE and lindane was reported to be associated with lower sperm motility, higher DNA fragmentation, higher ROS production, and depolarised mitochondria (98). One study (99) reported the association between non-occupational exposure to and p,p’-DDT and lower sperm motility and viability. Another (99) associated it with oligozoospermia and asthenozoospermia. Lower sperm count was also associated with higher organochlorine hexachlorocyclohexane (HCH) levels found in semen, especially in azoospermic men with deletions in the AZF region (100).

Organophosphates are another widely used group of pesticides that degrade faster in the environment and are neurotoxic to humans (101). Several classes of organophosphate pesticides found in urine have been associated with lower sperm concentration, count, and motility. One study (102) reported a positive correlation between urinary organophosphate metabolites and FSH and LH were in men with elevated gonadotropins. Another study (103) reported correlation of different organophosphate metabolites in urine with increased sex chromosome disomy.

As organophosphates are also used as flame retardants or plasticisers, their effects were investigated in a large cohort study (104), which reported no or weak correlation between their urinary levels and sperm parameters. In another study (105), only bis(1,3-dichloro-2-propyl) phosphate showed a stronger association with increased sperm count in men attending infertility clinic, but the authors warned that the associations were inconsistent overall.

Bisphenol A (BPA) is a precursor in plastic industry that acts as an endocrine disruptor and xenoestrogen, which can be found in plastic water bottles, food containers, can coatings, tooth fillings, cash receipts, and plastic toys (106). BPA binds to oestrogen receptors less efficiently than oestradiol but its circulating concentration can be higher than that of oestradiol. BPA also has an antiandrogen effect, because it binds to androgen receptors and inhibits normal androgen response (107). Because of its detrimental effects on male hormonal pathways, several studies investigated BPA impact on sperm parameters. Li et al. (108) found a dose-dependent correlation between higher urinary BPA levels and lower semen quality. In seminal plasma BPA levels were also negatively associated with sperm concentration, count, and morphology in men attending fertility clinic (109). However, several other studies found no or only weak, clinically insignificant associations (110, 111). One study (112) indicated higher BPA levels in Italian men living in a metropolitan area than in those living in a rural area. Furthermore, there were differences between fertile and infertile men but without BPA presenting risk factor for infertility (112). BPA certainly has an impact on the male reproductive system, but its clinical significance is unclear.

Phthalate esters can migrate from plastics (such as polyvinyl chloride plastics, food packaging, and toys) into cosmetics, solvents, and medicine film coatings (113). Data on phthalate exposure effects on male infertility are most often collected at infertility clinics. The disadvantage of such data collection is that it is not always clear whether exposure has been environmental or occupational.

Even so, evidence of the association is rather convincing, such as that between phthalate metabolite levels and decreased sperm quality (114, 115). Findings reported for general population are not as consistent. Those reporting phthalate effects on sperm quality but not on reproductive hormone levels suggest that phthalates deteriorate semen quality via ROS production, lipid peroxidation, and mitochondrial dysfunction (116, 117, 118, 119, 120, 121). There where hormonal effects were reported, more specifically changes in testosterone to oestradiol ratio, the authors suspected possible effects on aromatase suppression (122, 123).

One study investigating the effects of environmental exposure to complex chemical mixtures reported an interesting finding of a synergistic effect of polychlorinated biphenyls and monobutyl phthalate on sperm damage (124).

Antimicrobial preservatives such as parabens and triclosan are common additives in consumer goods. In the US, 58–99 % of general population have their detectable concentrations in the urine. As both have endocrine disrupting properties, both have been investigated for adverse effects on male fertility. Hydroxylated paraben metabolites and triclosan have shown significant association with some semen quality parameters (115, 125). Additionally, urinary paraben concentrations were reported to be significantly associated with abnormal sperm morphology, lower motility and testosterone levels, and XY18 and chromosome 13 disomy (126, 127). Similar to other endocrine disruptors, parabens have also been suggested to play a role in mitochondrial dysfunction (128).

Environmental exposure to electromagnetic fields from cell phone use, telecommunication towers, and radars has been a source of public controversy for decades. Research of its influence on male fertility has also produced some controversial findings, most likely because studies have greatly differed in design and experimental conditions. One recent study (129) has shown a significant association between living close to a telecommunication tower and lower sperm volume and between carrying mobile phone in trouser pockets and lower sperm motility. Similar results were reported in a meta-analysis of ten in vitro and in vivo observational studies (130). One study (131) reported a significant association between phone use duration and lower sperm count and concentration. Some studies explain these effects on sperm quality with disturbances in mitochondrial function caused by extremely low frequency electromagnetic fields and the resulting elevated ROS production (132, 133).

The risk of male infertility is of particular concern among adolescents who are using cell phones more than 10 h a day (134). However, continuous changes in technology and application of cell phones make it extremely difficult to conduct comparable studies on their effects, especially if we take into consideration the effects of many other electronic devices used from early childhood and the specific biological effects of sex and age, all of which create a complex exposure, much similar to exposure to complex chemical mixtures, which is why it is difficult to clearly identify causality between a certain exposure type and its biological effect.

Table 1 summarises our literature review of current occupational and environmental risks for male fertility. Male infertility is a serious health condition that affects a large number of men especially in developed countries. A large number of chemicals with endocrine disrupting potential are present in occupational and environmental settings, usually in the form of complex mixtures. Phthalates, bisphenols, pesticides, and flame retardants as the main chemical groups associated with male infertility risk stand out, but their combined (omni)presence in food, cosmetics, air, and water, poses a great challenge to researchers to establish clear causality and association between exposure to a specific group and male infertility. Poor sperm quality seems to be accompanied by an imbalance in the levels of reproductive hormones and high levels of endocrine disruptors, DNA damage, and epigenetic and mitochondrial disturbances. However, due to the diversity of investigated populations, differences in epidemiological models, and uneven data collection, some of the results seem to be controversial. Yet, there is enough solid evidence for concern. Occupational and environmental exposures significantly contribute to male infertility, but many more studies with large numbers of participants are needed to obtain a data corpus reliable enough to determine effective preventive and/or protective measures and make infertility part of risk assessment in occupational health surveillance programmes.