For years, researchers have warned that the male sperm count has been in a substantial decline. And for years, skeptics have waved those researchers away as alarmists.

But a new study published Tuesday in the journal Human Reproduction Update shows that it’s finally time to take the problem seriously. The study — the largest meta-analysis ever done on the issue to date — examined data from 185 studies between 1973 and 2011, covering nearly 43,000 men. The results are stunning: During that period, the concentration of sperm per milliliter of semen among men from North America, Europe, Australia and New Zealand was cut in half. Total sperm count fell by almost 60 percent.

This should be alarming for several reasons: First, the study suggests that there is no indication the trend is leveling off, so the portion of men with subfertility or infertility will likely increase, meaning more couples will have trouble conceiving. But more pressing, lower sperm counts predict shorter life expectancy and are associated with cancer and disorders of sexual development.

Skeptics have long considered the trend of falling sperm counts a myth, often arguing that previous research is based on skewed data samples because men are more likely to get their sperm examined if they already fear they are infertile. This new study addresses that critique, as it looks primarily at young men who had never conceived and had no knowledge of their fertility status.

Shanna H. Swan, a professor at Icahn School of Medicine at Mount Sinai and co-author of the study, started off as one of those skeptics when she first started studying the issue in the late 1990s. Back then, she was tasked with replicating a shocking 1992 study published in the British Medical Journal — the first to claim that sperm count has been in peril over the past few decades.

“I went into that really unconvinced,” Swan said. “I looked at every factor raised: I looked at the sperm count method; I looked at all kinds of men; I looked at smoking rates. By putting them into the model, I tried to make [the trend] go away … After 25 years of trying to do that, I haven’t been able to.”

What Swan’s new paper doesn’t tell us is why this trend is happening. Public health advocates fear that chemicals such as those in pesticides, flame retardants, cosmetics and plastics are to blame. These substances, known as “endocrine disruptors” because they can interfere with the body’s hormone systems, are ubiquitous — especially in developed countries.

China has given observers an excellent natural experiment, given its rapid development over the past two decades. During that time, sperm quality among Chinese men quickly deteriorated, according to a study that looked at more than 30,000 sperm donors in the Hunan Province. In 2001, more than half of those men gave sperm that were healthy enough for donation. By 2015, that rate declined to 18 percent. The study’s authors speculated that environmental pollution could be to blame.

But the causal relationship between these chemicals and fertility has never been conclusively established. We know that exposure to these chemicals can cause temporary harm when exposed in large doses to adult men. The bigger, unanswered question is whether exposing fetuses to these chemicals during a mother’s pregnancy can cause lasting damage to male reproductive systems later in life. This is difficult to prove, especially since there’s no way to conduct randomized studies on pregnant mothers.

We do already have a massive literature demonstrating how these chemicals can severely disrupt reproductive organs, especially among animals. Herbicides have disrupted sexual development among male frogs and turned them into females. And the insecticide DDT, now banned in many nations, has caused alligators to be born with smaller genitalia and eggshell thinning among ospreys.

Establishing such causality in humans, however, will require more research — as well as time and scientific resources. Given the implications of those questions, policymakers and research institutions should support that science.

Declining sperm count is a public health issue that will affect everyone, especially future generations. With this new evidence, it cannot be ignored any longer.

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