Fertility clinics found that the number of moving sperm – or ‘swimmers’ – in men’s samples has dropped by 1.8% each year.
Meanwhile, another study found that male fertility is declining in five out of six US cities.
Its authors say the findings could be a public health warning and that junk food, lack of exercise and pollution may be fuelling the crisis.
Researchers led by the Sidney Kimmel Medical College in Philadelphia and fertility clinic IVIRMA monitored almost 120,000 men seeking treatment for fertility problems in Spain and the US from 2002 to 2017.
Among American men in the most fertile group, who had more than 15 million moving sperm, this went down by 1.8% for each year of the study.
Between 2002 and 2005, 84.7% of men were in the most fertile group, but this fell to 79.1% between 2014 and 2017.
At the same time, the proportion of the least fertile men rose. Those with poor fertility, from five million swimming sperm to none, increased from less than 9% of the total to 11.6%.
The second study, led by Icahn School of Medicine at Mount Sinai, New York, involved more than 2,500 sperm donors.
It found that fertility declined over 11 years in Los Angeles, Palo Alto, Boston, Houston and Indianapolis. New York was the only one of the six cities studied to buck the trend.
Sedentary lifestyles are believed to lower sperm production, while cheap and saturated fats found in junk food are known to harm sperm counts.
The chemical bisphenol A (BPA), widely used in plastic wrappers and containers, has been found to be toxic to sperm.