Sperm counts for men in North America, Europe, Australia and New Zealand have declined by more than 50 per cent in the last four decades, a large research review has found.

"That's very worrisome," said the study's lead author, Dr. Hagai Levine, who works at the Hebrew University's Hadassah Braun School of Public Health and Community Medicine in Jerusalem.

Levine and his global team of researchers found no significant sperm count decline among men in South America, Asia and Africa, although they acknowledge far less information was available for those continents.

Dr. Hagai Levine, of the Hebrew University's Hadassah Braun School of Public Health and Community Medicine, is the lead author of the study. Levine says he wanted to get a definite answer to the question of whether or not sperm counts have declined. (Hebrew University )

The findings were published Tuesday in the journal Human Reproduction Update. They not only suggest a problem with male fertility in Western countries, Levine said, but also signal that men's health could be at risk, since previous research has shown that sperm count can be a predictor of wellness.

Although past research has also shown that sperm counts are decreasing, not everyone in the scientific community has taken the reports seriously because of concerns about their limitations and methodology, the authors say.

"I felt that this question of male fertility is a neglected question," Levine told CBC News on Tuesday, noting he wanted to use improved methods and "statistical power" to get a definite answer to the question of whether sperm counts have declined.

Environmental factors blamed

Levine and seven other researchers from Israel, the U.S., Denmark, Spain and Brazil performed a comprehensive review of more than 7,500 studies on human sperm count, eliminating those that didn't meet stringent criteria to ensure data consistency. With the 185 studies left, they did a meta-regression analysis — a method of making sense of multiple studies by accounting for variables to maximize the accuracy of the results.

In the end, their conclusions were based on sperm samples collected from 42,935 men around the world between 1973 and 2011. They found that over those 38 years, sperm concentration declined by 52 per cent, while total sperm count decreased by almost 60 per cent among men from North America, Europe, Australia and New Zealand.

The study did not delve into what's causing declining sperm counts in Western countries, and more specific research is needed, Levine said. But he said past studies have shown environmental factors play a role.

"Even though we did not study the causes, it's clearly not genetic causes," Levine said. "It must be the change in the modern lifestyle and the environment that we live."

That theory rings true for Dr. Keith Jarvi, chief of urology at Mount Sinai Hospital in Toronto and a male fertility expert. Jarvi was not associated with Levine's study. After reviewing it, Jarvi told CBC News he believes the findings are sound and raise important questions.

The black lines in this graphic from the study show the decline in sperm concentration and total sperm count among men in Western countries between 1973 and 2011. Researchers say there is no indication that the decrease has levelled off. (Hagai Levine et al/Human Reproduction Update)

"You have to ask why is this happening on a society level? Is this a reflection of something else that's happening, and this is just basically a marker for some other health concern that's going on with men?" Jarvi said.

There are a few likely lifestyle and environmental factors, he said.

"On average, men's weight is going up, so there's higher rates of obesity, and we also know that obese men have lower sperm counts," Jarvi said. "So you can start to correlate weight and reduction in sperm counts."

Environmental exposure to "estrogenics" — including female hormone compounds — can also reduce sperm counts, he said, noting that plastics are among the sources of estrogenics in the environment.

'Canary in the coal mine'

Both Jarvi and Levine say there are steps men can take individually to improve their fertility, including stopping smoking, exercising and maintaining a healthy weight.

But the study shows the need to deal with declining sperm count on a societal level, they say.

"It's costing money, because more and more people are needing IVF [in-vitro fertilization], so there's a fertility cost," Jarvi said. "You really got to wonder why this is happening."

Investment by governments and funding agencies into understanding the causes of low sperm count — and then preventing it — is critical, Levine said.

"We call it the canary in the coal mine," he said. "If the canary dies, the solution is not to replace the canary, you know, to get a sperm donor. That's not the solution. We need to see why this is happening and to solve it."