Humans are constantly exposed to a range of chemicals and toxins in their environment which can affect their health, including their reproductive health: this is the case for women, and this is the case with men. For men, a range of environmental exposures are often associated with reduced sperm concentration, more common known as low sperm count, and is also often associated with sperm quality. It is logical: if a man has sub-optimal in either quantity or quality, he is less likely to conceive, and will, on average, take longer to conceive, should they manage to successfully conceive at all.
Infertility among men is on the rise: 50 years ago males were twice as fertile as they are today. Perhaps this is in connection with environmental chemicals and toxins are found in many environments, and that wasn't so present in our environment 50 years ago. These chemicals are most commonly found at dangerous concentrations in occupational environments, such as chemicals used in agricultural and manufacturing sector workplaces. Other environmental factors which have evidence of an association with male infertility, include heat, radiation, chemicals and trauma besides the environmental factors mentioned above. Heat exposure is often a problem because sperm production in humans and other mammals is dependent on the temperature of the testicles, and the increased testicular temperature may affect both the quality and quantity of sperm produced.
Some men take hot baths, but some only sit for long hours: long periods of sitting can increase testicular heat, as the testicles are insulated by the thighs whilst a man is seated. Another study showed that comparing oven operators to other, non-heat exposed workers in a ceramic factory reported that significantly greater proportions of oven operators were childless; for example 7.6% of oven operators compared to 1.1% who don't work in that industry, and they also reported difficulty conceiving (23% of oven operators compared to 12% of men not in that industry). After the analysis the semen of the oven operators contained significantly less motile sperm (7% motile) than that of the other workers in the ceramic factory (who had 28% motile sperm).