Environmental toxins can impair sperm production. Oil refineries are amongst the most implicated work places. Excessive heat exposure can reduce sperm counts in firemen and welders. Other work place exposures may have an impact but they are difficult to characterize.
Cadmium, lead, mercury, vinyl chloride, ethylene oxide, and radiation exposure all have known detrimental impacts on sperm production. There is debate about whether there are large scale, measurable declines in sperm counts around the globe. Some toxins may act as endocrine disrupters, which suppress the normal action of testosterone and these are thought to be present in many plastics. Eliminating a known hazardous exposure may allow improvement over time if there has not been permanent damage.