Sperm counts in men from America, Europe, Australia and New Zealand have dropped by more than 50 percent in less than 40 years, researchers said on Tuesday.

They also said the rate of decline is not slowing. Both findings -- in a meta-analysis bringing together various studies -- pointed to a potential decline in male health and fertility.

"This study is an urgent wake-up call for researchers and health authorities around the world to investigate the causes of the sharp ongoing drop in sperm count," said Hagai Levine, who co-led the work at the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Jerusalem.

The analysis did not explore reasons for the decline, but researchers said falling sperm counts have previously been linked to various factors such as exposure to certain chemicals and pesticides, smoking, stress and obesity.

This suggests measures of sperm quality may reflect the impact of modern living on male health and act as a "canary in the coal mine" signalling broader health risks, they said.

Studies have reported declines in sperm count since the early 1990s, but many of those have been questioned because they did not account for potentially major confounding factors such as age, sexual activity and the types of men involved.

Working with a team of researchers in the United States, Brazil, Denmark, Israel and Spain, Levine screened and brought together the findings of 185 sperm count studies from 1973 to 2011 and then conducted a so-called meta-regression analysis.

The results, published in the journal Human Reproduction Update, showed a 52.4 percent decline in sperm concentration and a 59.3 percent decline in total sperm count among North American, European, Australian and New Zealand men.

The former measures the concentration of semen in a man's ejaculation, while the latter is semen concentration multiplied by volume.

In contrast, no significant decline was seen in South America, Asia and Africa. The researches noted, however, that far fewer studies have been conducted in these regions.

Experts asked to comment on the work said it was a comprehensive and well-conducted analysis and did a good job of adjusting for confounders that could have skewed its findings.

Daniel Brison, a specialist in embryology and stem cell biology at Britain's Manchester University, said the findings had "major implications not just for fertility but for male health and wider public health".

"An unanswered question is whether the impact of whatever is causing declining sperm counts will be seen in future generations of children via epigenetic (gene modifications) or other mechanisms operating in sperm," he said in an emailed comment.

Richard Sharpe at Edinburgh University added: "Given that we still do not know what lifestyle, dietary or chemical exposures might have caused this decrease, research efforts to identify (them) need to be redoubled and to be non-presumptive as to cause."

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