Often sperm health was associated with having hot tubs, visiting saunas, cycling, working with computer in your lap... there is a reason why: heat. In humans and other mammals sperm production is in close connection with temperature in the testicles: when the testicles are approximately 2-4 oC below body temperature this is the time the sperm is produced. The organ that regulates the temperature in testicles is the scrotum: scrotum does that by expanding and contracting to facilitate heat transfer.
The heat exchange happens when blood flows into the testes via the arteries in the spermatic cord, and that flowing out of the testes via the veins in the spermatic cord. This is called also pre-cooling of blood entering the testes. This mechanism is often used by male bodies but pre-cooling mechanism can only regulate testicular temperature to a certain extent. For this reason, in situations of extreme heat the scrotum's natural cooling mechanisms may be insufficient to prevent a rise in testicular temperature.
Let's take a look how much damage exactly can situations of extreme heat cause. The study that was researching occupational drivers showed that the majority of unsafe heat exposure occurs in occupational situations: a study showed that men exposed to heat because of their occupation were 1.8 times more likely to have morphologically abnormal sperm, and they were also 1.8 times more likely to have been trying unsuccessfully to conceive for more than one year, comparing to men who had not been occupationally exposed to heat.
The other occupational study was comparing oven operators to other, non-heat exposed workers in a ceramic industry. The study showed that significantly bigger proportions of oven operators were childless: 7.6% of oven operators compared to 1.1% who don't work in that industry, 23% over operators reported difficulty conceiving compared to 12% of men not in that ceramic industry.
Another study that was conducted on bakers again showed correlation between health and problems conceiving. A significantly lower proportion of men in the baking profession conceived within three months (only 14%) or six months (29%) than men working in non-heat exposed professions (55% at three months, 74% at six months). Again a proof that exposure to extreme heat is the cause of infertility or at least problems of conceiving a child.