NEW YORK — The sperm count of men in Western countries has been declining precipitously with no signs of “leveling off,’’ according to new research, bolstering a school of thought that male health in the modern world is at risk, possibly threatening fertility.

By examining thousands of studies and conducting a meta-analysis of 185 — the most comprehensive effort to date — an international team of researchers ultimately looked at semen samples from 42,935 men from 50 countries from 1973 to 2011.

They found that sperm concentration — the number of sperm per milliliter of semen — had declined each year, amounting to a 52.4 percent total decline, in men from North America, Europe, Australia, and New Zealand.

Total sperm count among the same group also tumbled each year for a total decline of 59.3 percent over the nearly 40-year period.

Decreasing sperm count was first reported a quarter century ago, but the new analysis shows that “this decline is strong and continuing,’’ said Dr. Shanna H. Swan, one of the study’s authors and a professor at the Icahn School of Medicine at Mount Sinai in New York.

And while this paper offers the most data yet on a subject of lengthy debate, it is far from settled, as the cause and the impact on fertility — and whether it has any real-life consequence — remain unknown.

Allan Pacey, a professor of andrology at the University of Sheffield in Britain, has been skeptical of studies that claim a recent decline in sperm count. He noted that a 52.4 percent decline in concentration “may sound a lot,’’ but it represents a change from “normal (99 million sperm per milliliter) to normal (47 million sperm per milliliter).’’

Still, Pacey conceded in a recent interview that the new paper piqued his interest.

The survey’s authors pointed to existing research that showed that exposure to cigarette smoke, alcohol, and chemicals while in utero, as well as stress, obesity, and age, were factors in the drop.

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