Many articles published in the past have raised the serious issue that sperm counts are declining worldwide, in both animals and humans. Industrial and environmental pollutants such as estrogen, heat, various chemicals, and electronic and radiant energy are damaging to the reproductive system. The National Institute of Occupational Health (NIOSH) and the Health Registry of Toxic Effects of Chemical Substances list more than 104,000 physicochemical factors, 95 percent of which have not been studied for their effects on the reproductive system. Nevertheless, NIOSH lists 10 categories of national occupational diseases or occupational injuries that can cause infertility. Since little is known about these factors, it is even more important to study and avoid these potential germline injuries in the workplace.
Changes in the reproductive system caused by environmental factors can be divided into two categories: reproductive toxicity and developmental toxicity.
Reproductive toxicity: side effects that occur directly to the reproductive system are called germline toxicity. The male reproductive system can be affected either directly (e.g., testicular damage can directly lead to reduced or altered sperm production) or indirectly (e.g., through the neuroendocrine system that affects hormone balance and leads to testicular failure). These toxicities can manifest as changes in sexual behavior, infertility, or changes in pregnancy outcomes.
Developmental toxicity: The side effects that occur act on the developmental process, which may be due to parental exposure to these harmful factors before pregnancy or during prenatal and postnatal sexual maturation, are called developmental toxicity. The consequences of these side effects, which have been little studied and are difficult to prove, include, among others, miscarriage, increased genetic abnormalities and increased incidence of childhood malignancies. Classically thought to affect mainly females, it is now thought to have a relationship with males as well.
Description of the evidence
Determining exposure to a particular factor is very difficult. Often, we would like to know the duration of exposure and how long it lasted, but this information is not readily available. In addition, workers can be exposed to multiple factors at the same time, which makes it difficult to distinguish the effects of the various factors. Because these factors are harmful to humans, which makes prospective clinical trials impossible, trial design is best done using case studies and crossover trials or Cohort trials.
There are times when those published in the literature do not elaborate on clinical relevance. When retrospectively analyzing these data, the analyst must be aware of the inevitable one-sidedness of these studies. There are methodological problems with these analyses, including retrospective bias related to exposure and confounding with lifestyle-related factors (e.g., smoking versus alcohol consumption). In addition, statistical analyses of these harmful factors are often confounded and the results are unsatisfactory, so that researchers are often unable to examine a factor as a risk factor and derive a definitive result from a large body of historical information.
Although many occupational exposures have been shown to have effects on male fertility in animal studies, the application of these inferences to humans is limited due to racial differences in reproductive function and metabolism. To clarify the currently available information on environmental toxicants and environmental exposures, this review will focus on the categories of factors of interest in the occupational environment and its effects on the reproductive system.
Physical factors
Let’s start with physical factors such as heat, ionizing radiation, nonionizing radiation, electromagnetic fields (EMFs) and microwaves, noise, and whole-body vibration
In 1941, McLeod and Hotchkiss first discovered that elevated temperatures had a deleterious effect on spermatogenesis. In their initial data, these authors induced hyperthermia in six healthy men using a hot cabinet. They found a significant drop in sperm count in the subjects after 3 weeks, which lasted an average of 50 days. Thus, hot showers were considered as a possible cause of infertility. Some reports have also found that outdoor workers show a decrease in sperm density in the summer, which supports the theory that environmental heat can impair spermatogenesis as well as possibly affect epididymal function.
Men in hot occupations, such as bakers, cooks, welders, firefighters, potters, and foundry workers, can be exposed to high levels of heat radiation. Whether changes in spermatogenic function are due to exposure to heat or to testicular dysfunction under normal working conditions remains a controversial issue, but this view is often cited in different studies lacking controls.
Rachootin and Olsen performed a chart-controlled study with a fertility group and found that occupational exposure to thermal environments should be divided into two parts, e.g., welders were exposed not only to thermal radiation but also to toxic gases. To evaluate the separate effects of the thermal environment, Blonde had welders wear masks for 6 weeks and analyzed sperm changes and found significant and reversible changes in their sperm morphology. As the researchers suggest, the study design necessitates that the subjects be exposed to high levels of thermal radiation. It is also implied that the occupational environment of the welders in this study was warmer than usual, and therefore the results are not representative of the entire group of welders.
The results of a study conducted in a Scottish country where the father’s occupation was pottery and the increased incidence of low birth weight and preterm births should be of greater concern. kline and colleagues also found an increase in spontaneous abortion with increased exposure of the male partner to heat. Lindbohm and colleagues, however, found no significant association between moderate exposure of male partners to heat and miscarriage.
These data suggest that high temperatures can alter spermatogenic function and that excessive exposure to heat should be avoided. More data are needed to clarify whether low birth weight, preterm birth and spontaneous abortion are directly related to increased exposure of male partners to heat (which implies that some genes of sperm are damaged).
Ionic radiation In the human body, the testis is one of the most sensitive tissues to radiation. Much information on this effect is available from patients who have received radiotherapy. Direct radiation to the testes at doses as low as 2 Gy (200 rad) is sufficient to impair spermatogenesis and requires 18 months to recover. When the dose is increased to 6 Gy, the recovery period takes 5 years. It seems that only high doses of radiation cause changes in Leydig and Sertoli, and that the degree of damage is correlated with the radiation dose and duration. Damage to the caudal microtubules of spermatozoa in rats irradiated with low doses of x-rays was also found in rat animal experiments.
Gardner and colleagues reported that men who worked in nuclear power plants and were exposed to external radiation had a higher number of offspring with leukemia than expected. In fact, if these fathers were exposed to external irradiation before conception, their offspring had seven to eight times the normal chance of developing leukemia. However, these results have not been confirmed by subsequent studies.
The Oxford Survey of Childhood Cancers failed to find an association between fathers’ exposure to ionizing radiation in the first 6 months of pregnancy and tumors in their offspring. Instead, they found a correlation between childhood tumors and possible exposure of the father to unsealed radionuclides in the work environment (e.g., chemists, nuclear power plant workers, etc.).
The association between paternal exposure to radiation and the incidence of spontaneous abortion and preterm birth was minimal. In addition, some studies have not found an association with increased incidence of birth defects. Radiation protection measures, modern radiation shields and monitoring techniques and measures have been effective in reducing the exposure of workers. New data on radiation exposure under modern protective measures are lacking.
Nonionic radiation Occupational exposure to nonionic radiation (e.g., microwaves, EMFs, etc.) has been suggested as a possible cause of reduced male fertility, but the evidence is controversial.
Radio frequency radiation acts through a cellular heating (cellular heating) mechanism. Testicular tissue is sensitive to such energetic and non-thermal biological effects, such as EMF effects and molecular excitation effects.
Electromagnetic fields and microwaves Exposure to electromagnetic fields (EMFs) is a common occurrence. The main sources of EMFs are radio, television, magnetic resonance imaging equipment, power lines, electric blankets, and electric hot water beds.
In 1965, the city of Drogchicina, by Lancranjan, reported a study of 1,000 cases exposed to EMFs for more than 5 years. They reported a significant decrease in libido and considered it as part of the manifestations of the debilitating syndrome. Also, some studies have noted a higher percentage of employees with decreased sperm density in radio companies compared to other industries, but there are some studies that do not confirm this relationship. Some case-control studies have suggested an association between EMF and childhood tumors, such as leukemia or central nervous system tumors. There is some bias in these studies due to different experimental designs, and therefore a conclusive result cannot be drawn from these studies.
Microwave is an electromagnetic radiation with a frequency spectrum between 300 and 30,000 MHz and is used not only for communication, but also in some rubber and plastic manufacturing industries, as well as in some pottery and leather processing industries. In 1975, Lancranjan and colleagues studied 31 cases of long-term exposure to microwaves in technicians, and found that 70% of the subjects had changes in libido, and 74% of the subjects had a mild decrease in sperm density and motility, but the morphology was normal. Semen indicators improved after 3 months of discontinued exposure to microwaves.
Several studies have attempted to determine the relationship between a father’s military radar work and an increased incidence of Down’s syndrome in his offspring, but ultimately failed. There is now no evidence that nonionic radiation affects male fertility and has harmful effects on offspring.
Noise and whole-body vibration Noise has been shown to cause many different somatic effects, including vasoconstriction and increased secretion of adrenal hormones (suggesting a stress response), which can induce increased secretion of adrenal hormone-releasing hormone and consequently decreased release of gonadotropins from the pituitary gland.
The working environment of tractor, truck, bus and other ground mobile vehicle drivers and helicopter pilots can lead to whole body vibration. Several studies have reported decreased semen quality in industrial and agricultural vehicle drivers, but a direct, separate relationship between decreased semen quality and mechanical vibration cannot be established because of a number of complex confounding factors (e.g., smoking and increased scrotal temperature). The Danish study mentioned earlier also found that men who reported a noisy work environment had a mildly elevated chance of infertility. Due to the limitations of the data, no conclusions can be drawn from this.