A dramatic decline in sperm counts is occurring around the world — more than 50 percent over five decades. This means a man today likely has only half the number of sperm that his grandfather did. As one of the world’s leading environmental and reproductive epidemiologists, Shanna Swan has been sounding the alarm about the human infertility crisis for more than twenty-five years. An expert in environmental threats to human reproductive health, she researches the causes of low sperm counts and other fertility problems, such as the rate of miscarriages, which has risen 1 percent each year for two decades now.

Swan is especially concerned about endocrine-disrupting chemicals — human-made substances that our bodies can confuse with our natural hormones. One such class of chemicals, the phthalates, which are found in various plastics and personal-care products, have been proven to affect sperm counts and quality. She is the coauthor, with journalist Stacey Colino, of Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race. The New York Times Book Review calls it “an important book for anyone concerned about the environment, pollution, successful childbearing, or declining health of the human species.”

Swan did not set out to be a scientist. As a young girl growing up in New York City, she was introduced to acting at the 92nd Street Y and became a child actress on radio and TV. She went on to enroll in the High School of Performing Arts but later transferred to the Bronx High School of Science in search of a more rigorous academic experience. (She’d also been told she was too short to be an actress.) She majored in math at the City College of New York and went on to earn a master’s in biostatistics at Columbia University and a PhD in statistics from the University of California, Berkeley.

It was a job at Kaiser Permanente, where she investigated the health effects of oral contraceptives, that led Swan into the field of epidemiology. She was involved in the formation of California’s environmental-health program and worked in the state’s Department of Health Services for seventeen years. Her first assignment was to determine whether a spill of toxic chemicals at a semiconductor plant in Santa Clara County was harming human health through drinking water. The study, completed in 1985, found that the rate of miscarriages and birth defects was two to three times higher in the affected neighborhood than in the county as a whole. The experience turned her into a public-health scientist. Public health “has remained my overarching concern,” Swan says. “I don’t believe it makes sense to do science in a vacuum.”

Swan has served as Professor of Environmental Medicine and Public Health at the Icahn School of Medicine at Mount Sinai in New York City since 2011. She has published more than two hundred papers. Her research has shown that baby boys are more likely to have undescended testicles and small penises if their mothers were exposed to high levels of phthalates during pregnancy, and that phthalate exposure in utero affects the brains of young boys and alters their play behavior. Since 1998 she has served as principal investigator of the Study for Future Families, which examines reproductive-health outcomes in different parts of the country.

I have a long-standing interest in how chemical exposures affect people’s health, so I was eager to speak with Swan. We connected over video chat, and the conversation flowed easily from declining sperm counts, to reproductive problems in wildlife, to the dangers of pesticide exposure.


A photograph of Shanna Swan.


Frisch: In the mid-1990s you read a paper about a decline in sperm counts and didn’t quite believe it was accurate, so you spent quite a while with colleagues doing an analysis.

Swan: I’d been appointed by the National Academy of Sciences to a committee on endocrine-disrupting chemicals. [The endocrine system produces the body’s hormones, which convey information through the bloodstream and regulate bodily functions like metabolism, growth, and reproduction. — Ed.] We were asked to determine whether chemicals that disrupt that system posed a threat to human health. It was a new question to me because I hadn’t worked in that area before.

A few years earlier scientists from Denmark had published a paper claiming that endocrine disruptors had caused sperm counts to decline dramatically over the prior fifty years. The committee asked me to review the paper to determine whether we should consider it in our deliberations. When I first looked at it, I was skeptical. That’s my general stance on everything: I’m a skeptic. I thought there could be many factors explaining the decline. So I spent six months examining the underlying sixty-one studies, trying to determine what else might explain it: Maybe the method of counting sperm had changed. Or maybe the men who were included in studies had changed — for example, if they were using only men who were already concerned about their sperm counts.

Two colleagues and I extracted all of these variables and put them into a model, trying to explain away the decline in sperm counts. We were surprised to find that our results were not substantially different from the results of the original paper.

Frisch: What are some endocrine-disrupting chemicals, and how are people exposed to them?

Swan: I study phthalates primarily and also bisphenol A, commonly known as BPA. I’ve recently begun working on some pesticides and medications.

Phthalates have a couple of major industrial functions. They make plastic soft and flexible, so they’re in PVC and myriad plastic products in our homes. They’re also in soft water bottles. They’re in squishy toys, shower curtains, and food containers. Phthalates are not chemically bound, which means they come off easily, particularly when the product is heated. Heat results in more molecular action. Since the molecules aren’t tightly bound, they hop out and enter whatever they’re in contact with.

Frisch: Does this happen when they’re microwaved?

Swan: Yes, microwaving plastic and food together is bad because it directly conveys phthalates into the food. If you leave a soft water bottle in your car, it will heat up, and you’ll get a dose of these chemicals in your water.

Anything with fragrance contains phthalates. So an air freshener in your car will off-gas phthalates when the car is sitting in the sun. When you put scented personal-care products on your warm body, the phthalates enter through the skin.

Water hoses contain phthalates, so when you water your lawn or garden using a hose that’s been sitting in the sun, the phthalates will go into the water and onto your plants. If you’re growing vegetables, they’re going to be in your food. Phthalates can also get into foods through pesticide products.

We absorb BPA through foods, among other things. I think pretty much everyone knows by now that most tin cans are lined with BPA. We don’t typically heat food in the can, but BPA does get into the food in measurable amounts. It’s also in cash-register receipts. We increase BPA absorption on our hands if we use hand cream and if our hands are warm. It’s in recycled pizza boxes, which are kept warm. Warmth is bad in terms of exposure.

There’s also an inhalation danger when you use hair spray or nail polish that contains bisphenols, phthalates, or other chemicals. If you can smell it, it’s likely to contain these chemicals. I try to avoid products with scent, whether it’s laundry detergent, cleaning products, or personal-care products. The less scent, the better.

We are now learning, pretty alarmingly, about exposure through our clothing, particularly when the clothing is made from recycled plastics. Polyester can be made from recycled water bottles, for example.

Microplastics are also released into the air by tires on roads. They’re everywhere. Recent studies have found microplastics in the placenta and in the stool of newborn infants. Though we don’t yet understand the health implications of microplastics, they have the potential to interfere with nutrition. But that’s a story yet to be written.

Frisch: Aren’t we finding microplastics in the actual flesh of fish?

Swan: Yes. But, as I said, we’re not sure what they actually do to us.

Frisch: So when you had been appointed to that National Academy of Sciences committee, you knew nothing about endocrine-disrupting chemicals?

Swan: That’s right. These committees are always made up of people who have views on both sides of the issue, and also neutral people in the middle. I was a neutral participant. Over several years, however, I became more and more convinced endocrine disruptors are a real problem. Some other public-health-oriented scientists — Lou Guillette, Ana Soto, and Fred vom Saal — agreed with me. We became known, in the words of one business journalist, as the “endocrine-disruptor crybabies.” We were also called “truth disruptors.” As you can see, it got fairly adversarial. The committee generated a majority report and a minority report, and I signed the minority report.

Frisch: And you and your colleagues in the minority have continued to study the dangers of endocrine disruptors.

Swan: Fred and Ana and I are still fighting the battle. Unfortunately Lou Guillette died. He was well-known for his work with alligators in Lake Apopka in Florida, where he actually went out and wrestled these alligators into a boat and measured their genitals. The alligators’ penises were smaller in Lake Apopka, where the water was contaminated by pesticide runoff, than in Lake Woodruff, which was the control. The Apopka alligators suffered not only from smaller penises but also from smaller litters. Their health was very poor. It was an early sign that runoffs from agricultural pesticides can severely affect wildlife.

Frisch: Why was there such antagonism to your warnings about endocrine disruptors?

Swan: Banning these chemicals would have a huge economic impact, so undoubtedly much of the pushback came from industries that use them. Animal testing in the laboratory often showed results consistent with human studies, but the dosing in the laboratory at that time was high, whereas humans are exposed to very low doses. The industries argued that our doses were so small they couldn’t possibly cause harm.

Frisch: How long do scientists think that sperm counts have been in decline?

Swan: Our paper, published in 2017, included data from men who gave samples from 1973 to 2011. The original Denmark paper from 1992 went back to 1938.

The two curves from the Denmark study and those I produced under subsequent analyses all have approximately the same slope. So it’s reasonable to hypothesize that the decline goes back as far as 1938. There’s a caveat, however: those early years had very few studies, and the ones that were done were not stable.

I trust my own data the most, of course, but the declines in sperm count certainly did not start in 1973. Production of the chemicals that I think contributed to this decline — there are other factors as well — began to take off after the Second World War. So it’s plausible that this has been going on for sixty years, or two generations. Not a long time evolutionarily.

Frisch: You found about a 1 percent decline in sperm counts per year.

Swan: Yes, which would mean a 50 percent decline over fifty years. We’re actually seeing something a little steeper than that.

Production of the chemicals that I think contributed to this decline . . . began to take off after the Second World War. So it’s plausible that this has been going on for sixty years, or two generations.

Frisch: Since men produce a tremendous amount of sperm, why does it matter?

Swan: If you look at the relationship between how long it takes a couple to conceive and the amount of sperm the man produces, as long as he gets above 50 million per milliliter, it doesn’t matter how many he produces. It could be 70 million. It could be 90 million. It could be 100 million. The probability of a pregnancy is the same. When the sperm count falls below 40 million, though, the probability of conceiving drops off sharply. Men with sperm counts below 40 million per milliliter are considered subfertile. In other words, it takes them longer to conceive.

And when sperm count goes down, the motility — the motion, the swimming — becomes slower and inappropriate. They swim in circles. And morphology — the shape of the sperm — is not optimal. There could be two-headed sperm and two-tailed sperm and all kinds of abnormalities. The chromosomal integrity of the sperm goes down as well.

So once you get below that 40 million cutoff — and maybe it’s 45 million, or maybe it’s 38 million; we don’t know exactly — all aspects of sperm quality drop rapidly.

Frisch: So when sperm counts are high, the sperm tend to be a lot healthier?

Swan: Yes. The measures of sperm quality and quantity are highly correlated. When one measure is good, chances are the others are going to be good. When I was living in Missouri, we found that men in the central part of the state, which has a lot of agriculture, had only half as many moving sperm as men in Minnesota. That’s pretty dramatic. And when we looked within Missouri at men for whom every measure was subnormal — count, motility, and morphology — that group had significantly higher levels of four pesticides in their blood.

Frisch: You’re also looking at a decrease in testosterone in men.

Swan: Yes, the consequences of that are really devastating. Erectile dysfunction is increasing — and, by the way, it has been tied by one study to men working with BPA. The problem of lower testosterone is in some ways more profound than the problem of lowered sperm count because its consequences are broader: for fertility, for sexual function, for libido, for muscle mass. Lowering testosterone, in utero or adulthood, has effects on the entire reproductive system.

What differentiates the male and female reproductive systems is the amount of testosterone that’s present during fetal development. Girls whose mothers have polycystic ovary syndrome when they are in utero will get too much testosterone.

For women phthalates are not predominantly the problem during development. But women who had higher levels of phthalates in their blood did report less sexual satisfaction, consistent with what men reported. Though testosterone is not needed for the development of the genital tract in female fetuses, adult women do need testosterone for sexual function.

The other thing about females is that they are much harder to study because their genitalia are hidden, so it’s very hard to see when there are abnormalities. Most women won’t be aware of abnormalities in their reproductive organs unless the condition is painful, like endometriosis or fibroids.

If you want to know how many eggs a woman has, you have to do an ultrasound. That’s difficult and doesn’t reveal anything about the quality of the eggs. It’s much easier to get a sperm sample from a man.

That said, we do know quite a bit about women. We know about their menstrual function. We know their age of puberty has gone down. We know about the length of their ovarian reserves — how long their eggs last. When a woman stops menstruating early, it’s called “premature ovarian reserve” or “ovarian insufficiency,” and the rate of this condition is increasing.

When I was living in Missouri, we found that men in the central part of the state, which has a lot of agriculture, had only half as many moving sperm as men in Minnesota.

Frisch: So women are going into menopause early?

Swan: Yes, or maybe they’re not quite in menopause, but they’re producing fewer eggs and having a harder time getting pregnant. This happens naturally as a woman gets older, but it’s starting earlier.

Frisch: It’s like my garden in early fall. My green beans are so productive and beautiful, but they produce pods with nothing in them. [Laughter.]

Swan: Very good, yes. My work has been with humans, but, of course, endocrine-disrupting chemicals disrupt the reproductive health of wildlife as well. Frogs are damaged reproductively by the herbicide atrazine. Wildlife reproduction is diminished by estrogen exposure. And with wildlife we don’t have to rule out the many voluntary factors that can affect human fertility rates.

The expected number of children that a woman or a couple will have during their lifetimes has declined by a little more than 1 percent per year worldwide, from five children in 1960 to 2.4 children in 2020. The decline is very similar to the decline in sperm counts. Unlike sperm counts, however, the decline in fertility rate is attributable in part to factors that we have control over, such as the desire to have children, the use of contraception, the education of women, and urbanization. We know, however, that wildlife are not simply choosing to delay childbearing, using contraception, or getting more educated. For nonhuman species the main driver of declining fertility is going to be chemicals in the environment, as well as increasing temperatures and decreasing habitat.

I’m not saying that the decline in wildlife fertility is caused entirely by chemicals, but it’s happening too fast to be caused by genetic changes, and it’s not voluntary.

Frisch: Right, and when you see male fish laying eggs, you know something is wrong.

Swan: Yes, there are examples in nonhuman species of endocrine-disrupting chemicals causing disorders of sexual development — organisms that have male and female gonads in the same individual, for example.

You also have changes where female wildlife prefer to mate with other females, or males with other males. That’s been seen in bird species, in fish, and in frogs. Chemicals can cause this.

Mating behavior is a major aspect of reproduction in many species, particularly birds. If the behavior doesn’t happen, then the male and female birds won’t get together, and the females’ eggs won’t be fertilized, and that will be the end of the species.

Frisch: Might exposure to endocrine-disrupting chemicals be related to the rise in the number of trans or nonbinary or gender-fluid people?

Swan: We don’t even know if rates of gender fluidity and the rest are increasing. Since people didn’t talk about this in the past, we don’t have baseline data. We’re certainly more aware of it now. Whether there actually are more such people, we can’t say. So we cannot answer that question.

Frisch: Many people will be surprised to learn that genes or sex chromosomes alone do not determine whether a baby will have a penis or a vagina. What other factors are involved?

Swan: There are many, many sex traits. The genitals are just one. There are also sex traits in behavior, in body composition — fat distribution, width of the hips and shoulders, and so on — and in the brain.

Frisch: Have you gotten any pushback from the LGBTQ community on your work?

Swan: I have not gotten pushback. I’ve gotten mostly acceptance from the LGBTQ community. I’ve actually talked to a couple of groups in webinars. A trans woman organized one of these webinars, and it was a very positive event.

In Count Down we present a pretty extensive interview I had with a trans man I know. He was very comfortable with the way we described his experience. So I think we’re doing something right. But we cannot answer the question everyone asks: Are chemicals in the environment affecting the number of people on the LGBTQ spectrum?

Frisch: Are all endocrine disruptors products of modern industry?

Swan: No, there are natural endocrine disruptors. It has long been known that sheep and cows can become infertile when they graze on certain clovers. Some fungi are endocrine disruptors. Whether something is an endocrine disruptor is hard to say, because our bodies have close to a hundred different hormones, and any of them could be targets.

Frisch: I’m thinking also about herbs that women traditionally use to bring on a miscarriage or prevent pregnancy. You certainly see warnings on some herbal products: “Do not use during pregnancy.”

Swan: There are many things that impact fertility, like stress, which has strong endocrine-disrupting properties. Stress is not a chemical, but it causes chemical changes in the body. Men who report high levels of stress tend to have lower sperm counts, and stress is related to the probability of conception. But I’m most concerned about the chemicals we’re introducing into the body, and that can be avoided.

Many medications are endocrine disruptors. Acetaminophen, or Tylenol, for example, shares many characteristics with phthalates. Of course, oral contraceptives are clearly endocrine disruptors. You take them for that purpose. And then they make their way into the water, and people and other animals get exposed to them.

I was on the advisory committee for a program in Israel that was looking at reusing wastewater for agriculture, and they demonstrated that certain plants irrigated with reused water were affected by medications in it. They could measure the amounts.

So there are nonindustrial endocrine disruptors, but when we talk about endocrine-disrupting chemicals, we’re usually talking about the big five: phthalates, bisphenols, PFOAs, pesticides, and flame retardants. Also tributyltin, which is an ingredient in some paints and can cause female mollusks to develop penises.

Frisch: One thing we haven’t touched on yet is the rising rate of miscarriages. We tend to ascribe the cause of miscarriages to the woman without thinking about the male partner. You cite a study where women with older male partners had higher rates of miscarriages because the man’s sperm tended to have chromosomal abnormalities.

Swan: It’s great that people are now recognizing the male role in successful conception, and also in miscarriage. Men are producing sperm all the time. It takes seventy days to produce a mature sperm. And if, during that period, the man is exposed to something that could increase chromosomal abnormalities or somehow decrease the success rate, that will impact fetal development.

Conception failures and pregnancy failures of various kinds have been linked to endocrine-disrupting chemicals on both the male side and the female side. Russ Hauser and his group at Harvard have an ongoing study called the EARTH Study, which collects sperm from men and, I think, blood and urine from both women and men who come in for assisted reproduction. They’ve shown that the success of that procedure is affected by the levels of endocrine-disrupting chemicals in both the man and the woman. It can affect the quality of the embryo, the implantation rate, and the survival rate, for example. In general, if the man or the woman or both are contaminated with these chemicals, their chances of successful reproduction decrease.

Frisch: How does the environment experienced in the womb affect a person’s later reproductive potential?

Swan: The experience in the womb, particularly in the first trimester, is enormously important. While there are later sensitive periods — such as prepubescence and puberty — the major hormonal changes take place in utero. Depending on what the genetic sex is and what the hormonal environment is, the fetus will develop in a more or less typical male or female manner.

Consider, for example, the development of the testicles: How big are they? Are they descended? Also the size of the genitals and other organs. The female is born with all of the eggs she is going to have, so anything she’s exposed to in utero can affect her eggs.

Frisch: One of the most alarming scientific findings that you discuss is how endocrine-disrupting chemicals we were exposed to before we were born can affect our grandchildren.

Swan: Let’s say a mother is carrying a fetus and gets exposed to phthalates, maybe from her food or personal-care products. They enter her bloodstream and are transmitted to the fetus. So then the fetus is also exposed.

That fetus is carrying the germ cells for either the egg or sperm that will be passed on to their children. And so, when that child is born, they may already be carrying a damaged set of germ cells, which will then play out in the third generation.

It’s now widely believed that transmission to four generations or more is possible. Some scientists have gone as far as seven generations.

Frisch: What actions can we take to protect our own reproductive health, as well as other aspects of our health that are influenced by reproductive hormones?

Swan: There are lots of ways that you can improve your reproductive health and the reproductive health of your offspring, many of which are not necessarily tied to endocrine-disrupting chemicals.

We know that smoking by the mother leads to probably a 40 percent decrease in the sperm count of her son when he is an adult, because smoking damages those germ cells. You don’t see that effect at birth or before puberty, but when he finally starts to make sperm, you’ll see a decrease.

When the mother is obese, that’s going to have an effect on her child’s reproductive function. A woman with polycystic ovary syndrome will convey excess testosterone to her offspring. A daughter may be born with less-typical female genitals because she’s had too much testosterone exposure.

The medications the mother takes during pregnancy can influence the postnatal development of reproductive health. In my study of endocrine-disrupting chemicals we’re trying to find out the critical time for causing damage and then measure how this plays out in the fertility of the mature adult.

Frisch: Can banning endocrine-disrupting chemicals reverse the infertility trend in humans and other animals, or are there already too many chemicals in the environment for it to matter?

Swan: I do think that every little bit matters. Let me give you an example: There was a chemical pesticide called dibromochloropropane. It was used to kill nematodes in agriculture, and it caused men who worked with it to be azoospermic — they had no sperm at all. When this pesticide was banned, the men’s sperm recovered. So did that help them? Absolutely.

There’s no question that lessening exposures during childhood and adult life can reverse or minimize harmful effects. We know this from lead. When we removed lead from gasoline and lowered overall exposure to it, the population’s average IQ increased. So it wasn’t too late. In fact, it’s never too late to make changes that will improve health.

But we don’t have a lot of time, because we’re getting close to the point where a large number of people are going to need assisted reproduction such as in vitro fertilization. And I want to stress that this is not an equal-opportunity world. Some won’t be able to afford assisted reproduction. Some won’t have the education to know about these procedures. And people for whom both of those things are true are more likely to be exposed. There’s a social-justice issue here that we have to consider. We have to pay attention to everyone who’s exposed, not just those who can purchase more-expensive BPA-free products or do in vitro fertilization.

It’s never too late to make changes that will improve health. But we don’t have a lot of time, because we’re getting close to the point where a large number of people are going to need assisted reproduction such as in vitro fertilization.

Frisch: What studies are you currently involved in?

Swan: I’m currently working on a study of glyphosate, a weed killer often sold under the brand name Roundup. And I am starting to think about how we should study microplastics: what population to study, what outcomes to look at, and so on. I try to be informed by the animal studies. They have really helped me over the course of my career.

Frisch: Of course, many people want to ban animal studies. Would that make it difficult to investigate the effects of these chemicals?

Swan: I think we need the animal studies. The National Toxicology Program is set up under the assumption that studies in animals teach us about risks to human health. They do them carefully, losing as few animals as possible, working as humanely as possible. I think the information these studies produce is invaluable for our health. We can’t get this information from a computer model.

Frisch: Do you believe it’s possible to restore human fertility?

Swan: I hope so. We have some evidence it is. In some Scandinavian countries — Denmark in particular — the decline in sperm counts, which was very dramatic, has slowed and perhaps even stopped. We will need more data to know for sure.

Patricia Hunt at Washington State University has shown that in rodents you can restore fertility in three generations. If the first generation is exposed, but the second and third are not, by the third generation you see significant improvement in reproductive health. So we have to get on with it. Let’s start turning this around while we still can.