Sperm counts are declining. Scientists believe they have pinpointed the main causes why

Men’s reproductive capacity has fallen drastically in recent decades - and a new analysis of thousands of studies has revealed the factors that pose the biggest risk to sperm quality.

Sperm counts around the world have halved over the past 50 years, with the pace of decline more than doubling since 2000, according to recent research on male fertility.

Researchers at Semmelweis University in Hungary have dug deeper into the findings of nearly 27,000 studies to determine the biggest causes of deterioration of sperm cells, and found pollution, smoking, age, and certain health conditions to have the greatest effect.

Their findings were published in the journal Reproductive Biology and Endocrinology.

The functionality of sperm cells is tested by “DNA fragmentation analysis,” currently the only evidence-based test to make the determination, explained Dr Zsolt Kopa, head of the Andrology Centre at the Department of Urology at Semmelweis University.

"It examines their DNA content, namely the proportion of intact or fragmented genetic material in the sperm. The more fragmented the DNA, the less the sperm’s ability to fertilise; also, it can increase the risk of miscarriage," he said.

The research comes amid growing concern about the dramatic fall in male reproductive capacity. Research published late last year in the journal Human Reproduction Update showed that sperm counts worldwide have halved over the past five decades.

Professor Hagai Levine, one of the researchers behind that particular study, called it a "canary in a coal mine," adding that "we have a serious problem on our hands that, if not mitigated, could threaten mankind’s survival".

The Semmelweis scientists searched three international databases for previously published