Humans May Not Be Able to Reproduce Naturally Much Longer, Scientist Warns

Everyday chemicals are threatening the future of human fertility

Photo: Peter Dazeley/Getty Images

Many men today have just half the number of sperm their grandfathers had. The shocking discovery was published in 2017 by Shanna Swan, PhD, an environmental and reproductive epidemiologist at the Icahn School of Medicine at Mount Sinai in New York.

After analyzing 185 studies involving nearly 45,000 healthy men, Swan and her team found that over the past four decades, sperm counts among men in Western countries had dropped by more than 50%.

Why the huge decline? Swan says many factors are at play — alcohol use, smoking, body weight, and a lack of exercise are a few. But she has zeroed in on another, more insidious cause: exposure to common chemicals that interfere with the body’s production of hormones.

Swan has been studying these so-called endocrine disruptors for the past 30 years. And it turns out men’s sperm isn’t the only thing they affect: They may be changing human sexual development and reproduction in broader ways, too. In girls, exposure to such chemicals has been linked to earlier onset of puberty. Women, meanwhile, are experiencing a decline in egg quality and more miscarriages.

In a new book, 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 outlines the hidden damage these chemicals are having on our fertility and why we must act now to prevent them from causing further damage.

In the book, she makes a provocative claim: Humans may not have the ability to reproduce naturally for much longer. She estimates that by 2050, a large portion of the global population will need assisted reproductive technology to procreate. I talked to Swan to learn more about these chemical exposures and what can be done to avert an infertile future.

This interview has been edited for length and clarity.

Future Human: What are these endocrine-disrupting chemicals your book focuses on?

Shanna Swan: We’re talking about hundreds, if not thousands — because most of them haven’t been tested — of chemicals that are in our daily lives. The endocrine disruptors that I have studied the most are the phthalates. It’s a big class of chemicals. Several of them make plastics soft and flexible. Others have the ability to promote absorption and retention. They’re put in personal care products. For instance, they help hand lotion go through your skin, and they help the smell get into your nose. They also help the pesticides get to the plant. They’re added to many, many products.

The most common source of exposure to phthalates is food. That’s kind of surprising, but that’s because phthalates are in all soft tubing and soft coverings used to process and package food. Heat combined with food is a bad combination in the presence of phthalates. They have many impacts on humans, and the one that I am most concerned about is their ability to lower testosterone.

Then there’s the bisphenols, which are compounds that act on estrogen, as opposed to the phthalates, which act on testosterone. They make plastics hard. They’re found in baby bottles, drinking water bottles, and other products. The most familiar is BPA but there are substitutes like BPS, BPF, and so on.

Another class of chemicals that are very concerning are pesticides going all the way back to DDT and currently all the way up to Roundup. These are hormonally active in many, many ways, depending on the particular formulation. They get into our foods. They’ve also been found in people who are handling these products occupationally. Pesticides are a major source of exposure and a real risk to our endocrine system.

Then we have the flame retardants, which is a class of chemicals that has gotten a lot of attention over the years. They’ve changed, they’ve become “apparently” safer but never really safe. These are chemicals that are in everything from kids’ pajamas to firefighting foam to cushions on our sofas. They’re really hard to get rid of in our homes.

Then there’s the class of chemicals known as PFAS. Those are chemicals that are used, for example, in Teflon to make things stick-resistant. They also make things water-resistant, like our raincoats. These are chemicals that have strong hormonal effects, particularly on the thyroid and the immune system.

You mentioned the effect of these chemicals on male fertility, but what about women’s fertility?

That’s harder to study. Most of my work on specific hormonally active agents has been on men. But there are more and more studies coming out showing that they can also change the levels of hormone in a woman’s body. They can interfere with menstrual function, age at the first menstrual cycle, age at menopause, premature ovarian failure, and so on.

Men are much better studied because, traditionally, men have always been better studied. Until recently, the National Institutes of Health didn’t even require you to study both sexes. Secondly, in men, it’s very easy to look at the organs and sperm. It’s harder to examine a woman’s eggs and her ovaries. You have to have an ultrasound to do that. So, women are harder to study for many reasons.

Lifestyle factors, like diet and exercise, and sociological ones, like people waiting longer to have children, are linked to a decline in fertility too. How much can we blame on these chemicals?

That’s a great question, and I don’t know the answer to that. I believe, based on my work, that it is substantial. But even the lifestyle factors that appear to be quite different from these chemicals are not really that different. For example, one lifestyle factor that affects fertility is obesity. A number of these chemicals are obesogens. They cause obesity, they interfere with the hormones that are related to satiation and metabolism. They make us more likely to get fat, and so obesity is on the pathway between exposure and infertility.

Let’s take another example. In order to create a baby, you obviously have to have robust sexual function. We have quite a bit of evidence now that these endocrine-disrupting chemicals can interfere with sexual function. In a study in China, men who manufactured BPA had more sexual problems and more erectile dysfunction. In our study, we asked women how satisfied they were sexually, and women who had higher levels of some phthalates said they were less satisfied. So, you can see that there’s a lot of steps to making a baby and there are a lot of opportunities for chemicals to interfere with those all along the way. It may not be a direct effect, and that’s one of the reasons it’s hard to quantify.

We know that the quality of women’s eggs declines as they get older, which affects fertility. Are chemicals like phthalates exacerbating this?

The quality of a woman’s eggs can be affected by environmental chemicals. There’s a program, a research program at Harvard called the Environment and Reproductive Health (EARTH) Study. In that study, what they do is they take couples that are coming in for assisted reproduction, like IVF, and they get urine and blood samples from the man and the woman. They’ve shown in multiple papers that the quality of the egg and embryo is worse and there are fewer successful births when the mother and the father are exposed to a wide number of environmental chemicals. That speaks to the quality of the egg and sperm. They’re both important. They both have to be of high quality, and those both can be impaired through environmental chemicals.

The explanation I hear a lot, and it’s very frustrating, is that people are choosing to have fewer children and the decline in fertility is because they’re choosing to delay childbearing. If it wasn’t for that, fertility would be just fine. Well, that’s actually not true because a lot of the fertility problems we’re seeing are in young women. We actually wrote a paper about that using the National Survey of Family Growth data. We showed that the problems in young women were actually higher than those in older ones. So, I don’t buy that the decline in fertility is solely due to delayed childbearing.

Is avoiding endocrine-disrupting chemicals as simple as not buying BPA plastic or eating organic foods that weren’t grown with pesticides?

It’s not that simple, although those things help. There have been studies where children and adults have eaten organic food, and they’ve shown a lot of positives. In one study we did in young men, we found that when the men ate organic food, their semen quality was better. So, I think it definitely helps, but the hard thing is, most of our exposure is hidden. You can’t always see what chemicals are in or on products, you don’t know when you’re getting exposed. We can’t buy our way out of this problem. We also can’t recycle our way out of this problem. We need much more fundamental changes.

Are these chemicals too entrenched in our environment at this point to be able to turn the clock back?

They certainly are entrenched, but they’re not too entrenched. For example, I conducted two pregnancy cohort studies. In one of them, the women were recruited between 2000 and 2002. In the second one, they were recruited 10 years later in 2010 to 2012. We measured the levels of phthalates in their urine in both populations, including the most harmful phthalates. After the Consumer Protection Act of 2008, there was a lot of change in behavior about phthalates and people were avoiding them. What we saw in the study was that there was a 50% reduction in certain phthalates in the women in our second population. Fabulous, right? The bad news is that other phthalates came in to take their place.

That happens over and over and over again with chemicals. It happened with the flame retardants, it happened with the bisphenols, it happened with the phthalates. It happens because we don’t require proof of safety or proof of lack of hormonal activity before we put a product into the market. It’s called regrettable substitution. It doesn’t happen to nearly the same extent in Europe, by the way. In the European Union, they have a legislation called Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). Under REACH, the manufacturer is obligated to prove the safety of the chemical before it is put into the marketplace and into our bodies. Here in the United States, we don’t have that legislation and we are the guinea pigs. Until we change the regulations, we can’t fix this problem.

How can chemical manufacturers be held responsible?

We have to make sure that the manufacturers understand what properties chemicals have before they’re going to be safe for our bodies. They shouldn’t be hormonally active, and especially not at very low doses. That’s important because we saw with BPA, the regulations could totally miss the risks because they test at high doses; the risks from low doses got right through. So, we have to have chemicals that are not harmful at low doses. Also, they can’t be stored in the environment. We have to make sure that they’re not going into our water, into our fish, and into our feed and coming back to us later through those various routes. We’re talking about a major overhaul of the chemical enterprise.

Are there ways we can use biotechnology to replace these chemicals with safer ones?

Absolutely. There’s a growing movement called “green chemistry.” Some companies have formed around this idea. They’re interested in making these safer chemicals and cleaning up our water and testing properly. It’s going to be expensive and it’s going to take time. But I think it’s possible. For example, bioplastics, which are made from renewable sources like potatoes and corn starch, were a first attempt at replacing our phthalates. I think it’s turning out to be costly in terms of the carbon footprint of these plastics. But I think the lesson is, we have alternatives. If the will is there, then it can happen.

What’s it going to take to get lawmakers and the public to care about the effect of these chemicals on our reproductive health?

People care about their own health, their immediate health. I think the more that we can show effects on people’s health right now — increased obesity, more erectile dysfunction, less sexual satisfaction, more menstrual problems — the more they’ll listen. I think that’s the kind of thing we’re going to have to show people. Showing the links between couples that are having trouble getting pregnant and what’s in their bodies is also persuasive. And, by the way, you’re not going to be able to stop passing them on to your children and later generations. We’re successively exposing ourselves and things get worse and worse. It’s cumulative. So, it’s really critical that we act quickly.

How big of a problem is this for the future of our species?

I think it’s a major problem. For example, the rate of surrogacy has gone up. The rate of assisted reproduction has gone up. More and more, we’re leaning on those technological solutions to have children, and I don’t think people are completely happy with reproducing that way. But here’s some good news. Patricia Hunt, PhD, at Washington State University has shown, first of all, that when male mice are exposed early in life to these chemicals, and then their offspring are exposed, and then their offspring is exposed, the chemicals build up over time. The chemicals are out there and they’re not going away. But she’s shown that if you take a male who’s been exposed and you stop the exposure, it will take three to four generations, but they will be back to reproductive health. So that’s really encouraging. The discouraging part of it is the length of time that it takes.

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.

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