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, Professor Pacey conceded in a recent interview that the new paper piqued his interest and represented “a step forward in the clarity of the data, which might ultimately allow us to define better studies to examine this issue.”
That the downtrend in sperm count is seen in Western countries suggests that “chemicals in commerce” are playing a role, Dr. Swan said.
While this survey did not focus on the causes of these declines, its 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.
“If the mother smokes, her son’s sperm count is decreased — that’s been shown in multiple studies,” she said.
A 2005 study, Dr. Swan said, showed that prenatal exposure to phthalates, also called plasticizers, affected the development of sons.
Phthalates are a group of chemicals used to make plastics more flexible and harder to break. In several studies over the last two decades, they have been shown to disrupt the operation of male hormones like testosterone and have been linked to genital birth defects in male infants.
Dr. Swan, who conducted a 2008 study about phthalate exposure, said that scientists have had the ability to measure exposure to plasticizers only since about 2000, via urine. That has led to a 20-year lag in the process since researchers cannot enroll men to produce sperm until they are in their 20s.
That evidence is the “missing piece of the puzzle,” she said.
Professor Pacey cautions that while the changes in data may be driven by “greater exposure of pregnant women or adult men to more man-made chemicals,” it is too soon draw a conclusion.
No trend studies were performed in the first half of the 20th century, said Niels Skakkebaek, a reproduction researcher at the University of Copenhagen, but in the 1940s, fertility doctors claimed that men should have at least 60 million sperm per milliliter to be considered normal and that many had more that 100 million per milliliter.
“Nowadays, average young men have 40-50 per milliliter,” he said.
Professor Skakkebaek, an author of a 1992 study that suggested chemicals play a role in the steady decline in semen quality, has since indicated that a rise in abnormal male reproductive systems may be linked to exposures to endocrine-disrupting chemicals.
“We must find out which ones are to blame for the problems with male reproduction, including male infertility and testicular cancer,” Professor Skakkebaek said.
The website for the Centers for Disease Control and Prevention says the effects from low-level exposure to phthalates are unknown, but it acknowledges that some types of phthalates have affected the reproductive system of laboratory animals and that more research is needed. The agency declined to offer further comment.
The National Institutes of Health also declined to comment on the research. The American Society for Reproductive Medicine did not respond to a request for comment.
The practical effects
Professor Skakkebaek pointed to Denmark and Germany as examples of how this course is playing out.
“In Denmark, 8 percent of all children are now born after assisted reproduction,” he said. “In spite of this activity, the birthrates have for 40 years been significantly below replacement level.”
“The number of young Germans have already declined 50 percent since the 1960s,” he said, adding that a similar pattern has been seen in Japan, which while not a Western country, is a developed one.
In the United States, he said, the fertility rate among white people is “below the levels where the population can be sustained.”
Data about assisted pregnancies has been linked to women having children later in life, and in developed nations, statistics have shown that more women are choosing to have fewer (or no) babies, which may also contribute to the fluctuations.
In the recently released research, no significant decline in sperm quality was seen in men from non-Western countries, but this segment made up only about a quarter of the results.
Dr. Hagai Levine, the head of the Environmental Health Track at the Hebrew University of Jerusalem, who led the team, said that one of the differences between Western and non-Western countries is that man-made chemicals like phthalates “became widespread much earlier in time” in developed nations.
Professor Skakkebaek said that reproductive issues among African men were less common: “It is already known that Africans have significantly lower rates of another testicular problem: testicular cancer.”
A study published last fall that looked at samples from just over 30,600 Chinese men asserted that semen quality and sperm count in the men had declined over a 15-year period ending in 2015 — with the percentage of qualified donors at a Hunan clinic falling from 55.8 percent to 17.8 percent in that time. To qualify, donors need to meet acceptable semen parameters including sperm concentration, sperm motility and semen volume.
“We urgently need international research collaboration to detect the causes,” Professor Skakkebaek said.
Continue reading the main story