Urgent action is needed to investigate disturbing trends in men's reproductive health, argues an expert in The BMJ today.

Professor Niels Skakkebaek at the University of Copenhagen points to research showing significant declines in sperm counts among men in the Western world, and says we must act now to find out what could be causing such disturbing trends.

The research, published in July, reviewed thousands of studies and concluded that sperm concentration had fallen by 52 percent among men in Western countries between 1973 and 2011.

Should we be concerned? Is male reproductive health really at risk, asks Skakkebaek?

He suggests that data on the quality of semen should be interpreted alongside trends in testicular cancers, as reports suggest that countries with a high incidence of this cancer have generally lower semen quality and vice versa.

Worryingly, incidence of this cancer has risen substantially over the past few decades, particularly in young men, he writes.

What could be causing such disturbing trends, he asks? The short answer is that we do not know. However, data showing that the incidence of testicular cancer has more than doubled in recent decades leaves little doubt that we should look into environmental causes - including lifestyle effects, he suggests.

Studies suggest that modern lifestyles are associated with increased exposure to various endocrine disrupting chemicals such as pesticides that together may be harmful to wildlife and humans even though exposure to individual chemicals is low, he explains. However, little has been done to explore their potential effects on semen quality and testicular cancer.

Should we be worried about our future ability to reproduce ourselves?

He points out that more and more children are now born after fertility treatments. However, despite increased use of assisted reproduction, fertility rates in many countries remain well below the replacement rate of an average of 2.1 children per woman.

In many European countries, including Germany, Japan, and Singapore, fertility rates range between 1.0 and 1.5, and fertility has become important in political and economic debates.

In order to help future generations "we must act now to prioritise new basic and clinical research programmes in reproductive medicine," he writes.

Medical researchers cannot do it alone, he adds. "We need health and research authorities that can see the urgent need for research in reproductive medicine, not just more infertility treatments, which are a short term solution for individuals not for the fertility of future generations."