Scientists are alarmed by a significant drop in sperm counts since the 1970s in North America, Europe, and Australia.

Sperm counts are on the decline for many men in the most developed countries in the world, according to a new study.

The study published today in the Human Reproduction Update medical journal, found that sperm counts and concentration have declined year after year, dropping by more than 50 percent for many men living in Western countries.

The researchers, based at the Hebrew University-Hadassah Braun School of Public Health and Community Medicine, and the Icahn School of Medicine at Mount Sinai, reported a drop in both sperm concentration and total sperm count in their findings.

Dr. Hagai Levine, lead author of the study and head of the Environmental Health Track at the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Jerusalem, said the results were extremely alarming.

“This is a definite answer to the question; sperm counts have declined,” Levine said.

It’s “obvious” for Western men, he added.

The researchers analyzed information from 185 studies consisting of 42,935 men.

They looked at men from primarily developed Western parts of the world, including the United States, Australia, and Europe, as well as other areas that included Asia, South America, and Africa.

The groups were divided into “unselected” men, such as college students or military recruits, who were unlikely to be aware of their fertility, and “fertile” men, who were known to have had children or impregnated women.

The data revealed that from 1973 to 2011 “unselected” men from Western countries had a 52 percent drop in sperm concentration — from 99 million per milliliter to 47 million per milliliter.

In addition, there was a 59 percent decrease in total sperm count, dropping from 337 million to 137 million.

The drop was so steep for Western men that the study authors wrote that new research is “urgently needed” to address the possible public health threat.

For fertile men from Western countries, and for both “unselected” and “fertile” men from non-Western countries, the drop was much less severe or nonexistent.

Levine said the study highlighted the need for more research on men’s issues with fertility. He pointed out that unlike other areas of medicine, men were not as well-studied as women for fertility problems.

“In fertility, men are neglected. This is a problem that we must prevent,” he said.

He pointed out there’s little concrete advice doctors can give to men with low sperm counts.

“We also need to put much more attention in this problem,” Levine said. “Especially when studies have shown that low sperm count is a strong predictor of morbidity and mortality.”

While the study did not examine possible causes for the dropping sperm counts, the authors did highlight a few possible reasons.

These included chemicals that disrupt hormones, exposure to certain pesticides, maternal smoking during pregnancy, and overall lifestyle factors.

Dr. Avner Hershlag, chief of fertility at Northwell Health in New York, said the medical community may focus on genetics, environmental hazards, or even global warming to try and find a reason for the decline.

“Are we exposing ourselves to … environmental components that are toxic to sperm?” Hershlag questioned while speaking with Healthline.

He pointed out that there has been concern about plastic materials containing BPA-disrupting hormones.

Hershlag also wondered, “What are the effects of industrial pollution?”

Since the drop was seen in Western countries and not in Asia, Africa, or South America, Shanna H. Swan, PhD, co-author of the study and a professor in the Department of Environmental Medicine and Public Health at the Icahn School of Medicine at Mount Sinai in New York, said the medical community will need to examine if exposure to chemicals could be one reason for this change.

“This definitive study shows, for the first time, that this decline is strong and continuing,” Swan said in a statement. “The fact that the decline is seen in Western countries strongly suggests that chemicals in commerce are playing a causal role in this trend.”

Dr. James Goldfarb, division chief of Reproductive Endocrinology and Infertility at UH Cleveland Medical Center, said the current sperm concentration and sperm count numbers documented in the study would not be considered in the “danger zone” for fertility issues.

However, if the numbers continue to fall it could be a problem for fertility in the coming decades.

“If you look at this over the next 20 or 30 years, the average sperm count could drop to the point where people are troubled,” Goldfarb told Healthline.

Additionally, Goldfarb said he’s concerned that the drop in sperm counts could be a sign that young men today are at increased risk for unknown health conditions in the future.

“The way they said it is this is the canary in the coal mine,” Goldfarb said. “Is this the harbinger that other things are going on that will impact men’s health?”