Shanna Swan Interview: An Alarming Decline in Sperm Quality Could Threaten the Future of the Human Race, and the Chemicals Likely Responsible Are Everywhere


The opening of epidemiologist Shanna Swan’s new book sounds a bit like science fiction: We are half as fertile as our grandfathers were. And if the trend continues, we may very well reach a point where the human race is unable to reproduce itself. 

In Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race, Swan’s take on the procreative capabilities of the modern man is clear-eyed and terrifying. The data tells some of the tale. Sperm concentration—the number of sperm per milliliter of semen—has dropped more than 50% among men in Western countries in just under 40 years.

“Some of what we’ve been thinking of as fiction, from stories such as The Handmaid’s Tale and Children of Men, is rapidly becoming reality,” Swan, Ph.D., writes in Count Down. “I felt and remain genuinely scared by these findings on a personal level.”

The question of human fertility, and sperm counts in particular, is one with which Swan is well versed. An environmental and reproductive epidemiologist at the Icahn School of Medicine at Mount Sinai in New York, Swan was one of the lead authors of a meta-analysis, published in 2017, that examined semen from 42,935 men over a 38-year period. It found that the average man in places like the U.S. had 99 million per milliliter sperm in 1973; by 2011, that number had dropped to 47.1 million per milliliter. (For comparison’s sake, the World Health Organization deems 15 million per milliliter the lowest sperm concentration compatible with fertility.) The work of Swan and her colleagues received widespread attention. GQ even gave it the feature treatment at the time.

Her new book is a continuation of these earlier efforts. The question now is what explains the decrease in sperm counts. Well, a lot of things: obesity, smoking, alcohol use, lack of exercise, even a daily sauna.

Yet the more insidious and worrying cause of these changes is likely an omnipresent class of chemicals called endocrine disruptors, which interfere with the body’s production of the hormones testosterone and estrogen. Plastics have made many wonderful things possible, but, as we wrote in 2018, “it turns out that many of the compounds used to make plastic soft and flexible (like phthalates) or to make them harder and stronger (like Bisphenol A, or BPA) are consummate endocrine disruptors.” Men with excess phthalates in their bodies, for instance, will produce less testosterone and, as a result, fewer sperm.

So what should we do? And, more specifically, what should men interested in having children one day do to keep their sperm in top health? GQ put those questions to Swan. 

GQ: It’s maybe the hardest number to avoid in the book: Sperm counts in the West have dropped by 50 percent. I don’t mean to sound flippant, but should we be terrified? Are we doomed?

Shanna Swan: Doomed is kind of an emotional word. It’s not a scientific word, right? But let me tell you what I think. I think that sperm counts are really low in many places in the world, and people should be very concerned. Yes, I take it seriously. Am I panicking? No.

Why should we be concerned but not panicked?

The bottom line is that we can do things to improve our reproductive function. We wrote about one guy in the book. This man was having his sperm collected routinely at a sperm bank. He was one of their prime donors, and then suddenly he didn’t make the cut, and they said, “What’s up?” And he said, “Well, let’s see: I changed jobs; it’s more stressful. I have a new girlfriend, she smokes,” and so on. And so he went back and he cleaned up his act, and then after a couple of months, his sperm count returned.



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