Over the last century the male sperm count has been gradually falling.

Pesticides
Smoking
Caffeine
Water Pollutions
Air Pollution
Fast Foods
Obesity
Sea Food
Other Factors
Pesticides :
Pesticides have a toxic effect on the testicles and other sex glands according to a French study. The problem is that the semen quality once affected by pesticides may not return to normal even with treatment.
Smoking :
Smoking causes to the sperm to move sluggishly and the sperms are unable to reach the egg or not able to penetrate it as they become weak. Once smoking is given up the sperm do return to normal in a few weeks.
Besides this smoking also reduces blood supply to organs such as heart and the sex organs causing difficulty with erections.
Smoking cannabis or grass or marijuana also cause sluggish sperms and impair them.
Caffeine :
Although one is not entirely sure about caffeine research has shown that caffeine appear to make sperm sluggish. But conflicting research has also found that caffeine actually increases the ability of sperm to reach the egg at the right time. A moderate amount may do no harm (couple of mugs a day).
Water Pollutions :
Water is now getting slowly polluted more so by women taking oral contraceptive pills. These are excreted in the urine and increase the level of estrogen in the water. This in turn when ingested increases its level in the blood and causes problem with male fertility. The synthetic contraceptives are more difficult to get rid of compared to the natural ones. Environmentalists, including representatives of Friends of the Earth, have constantly been warning about this for over a decade, alerting men that they may find themselves 'firing blanks.'
Air Pollution :
Scientists are of the opinion that Nitrogen di oxide and Lead from the exhaust fumes of vehicles maybe the main culprit causing infertility in men. A few studies are underway on this aspect.
Fast Foods :
The fast foods usually contain Soy and this mimics estrogen or the female hormone and causes imbalances in the hormones in men.
Obesity :
Overweight causes increase in estrogen and lead to infertility due again to imbalance of hormones.
Sea Food :
Sea Food consumption can increase the level of mercury in the blood and cause infertility.
Other Factors:
The temperature of the testicles is lower than the body temperature and any factor that increases the temperature can cause problems with sperm count and their motility. These include:
• Tight Pants And Hot Tubs
• Driving For Long Time
• Laptops
• Nappies
• Mobile Phones Especially If Kept In Pant Pockets.
The lowering of male sperm count was first brought to the notice by a Danish study in 1991.