Sperm quality appears to be declining around the world but is a little discussed cause of infertility. Now scientists are narrowing in on what might be behind the problem.
"We can sort you out. No problem. We can help you," the doctor told Jennifer Hannington. Then he turned to her husband, Ciaran, and said: "But there's not much we can do for you."
The couple, who live in Yorkshire, England, had been trying for a baby for two years. They knew it could be difficult for them to conceive as Jennifer has polycystic ovarian syndrome , a condition that can affect fertility. What they had not expected was that there were problems on Ciaran's side, too. Tests revealed issues including a low sperm count and low motility (movement) of sperm. Worse, these issues were thought to be harder to treat than Jennifer's – perhaps even impossible.
Hannington still remembers his reaction: "Shock. Grief. I was in complete denial. I thought the doctors had got it wrong." He had always known he wanted to be a dad. "I felt like I'd let my wife down."
Over the years, his mental health deteriorated. He began to spend more time alone, staying in bed and turning to alcohol for comfort. Then the panic attacks set in.
"I hit crisis point," he says. "It was a deep, dark place."
Male infertility contributes to approximately half of all cases of infertility and affects 7% of the male population . However, it is much less discussed than female infertility, partly due to the social and cultural taboos surrounding it. For the majority of men with fertility problems, the cause remains unexplained – and stigma means many are suffering in silence.
Research suggests the problem may be growing. Factors including pollution have been shown to affect men's fertility, and specifically, sperm quality – with potentially huge consequences for individuals, and entire societies.
A hidden fertility crisis?
The global population has risen dramatically over the past century. Just 70 years ago – within a human lifetime – there were only 2.5 billion people on Earth. In 2022, the global population will hit eight billion . However, the rate of population growth has slowed, mainly due to social and economic factors.
Birth rates worldwide are hitting record low levels . Over 50% of the world's population live in countries with a fertility rate below two children per woman – resulting in populations that without migration will gradually contract. The reasons for this decline in birth rates include positive developments, such as women's greater financial independence and control over their reproductive health . On the other hand, in countries with low fertility rates, many couples would like to have more children than they do, research shows, but they may hold off due to social and economic reasons, such as a lack of support for families.
At the same time, there may also be a decline in a different kind of fertility, known as fecundity – meaning, a person's physical ability to produce offspring. In particular, research suggests that the whole spectrum of reproductive problems in men is increasing, including declining sperm counts , decreasing testosterone levels , and increasing rates of erectile dysfunction and testicular cancer .
Swimming cells
"Sperm are exquisite cells," says Sarah Martins Da Silva, a clinical reader in reproductive medicine at the University of Dundee and a practicing gynaecologist. "They are tiny, they swim, they can survive outside the body. No other cells can do that. They are extraordinarily specialized."
Seemingly small changes can have a powerful effect on these highly specialized cells, and especially, their ability to fertilize an egg. The crucial aspects for fertility are their ability to move efficiently (motility), their shape and size (morphology), and how many there are in a given quantity of semen (known as sperm count). They are the aspects that are examined when a man goes for a fertility check.
"In general, when you get below 40 million sperm per milliliter of semen, you start to see fertility problems," says Hagai Levine, professor of epidemiology at the Hebrew University of Jerusalem.
Sperm count, explains Levine, is closely linked to fertility chances. While a higher sperm count does not necessarily mean a higher probability of conception, below the 40 million/ml threshold the probability of conception drops off rapidly.
In 2022, Levine and his collaborators published a review of global trends in sperm count . It showed that sperm counts fell on average by 1.2% per year between 1973 to 2018, from 104 to 49 million/ml. From the year 2000, this rate of decline accelerated to more than 2.6% per year.
We are facing a public health crisis and we don't know if it's reversible – Hagai Levine
Levine argues this acceleration could be down to epigenetic changes, meaning, alterations to the way genes work, caused by environmental or lifestyle factors. A separate review also suggests epigenetics may play a part in changes in sperm, and male infertility.
"There are signs that it could be cumulative across generations," he says.
The idea that epigenetic changes can be inherited across generations has not been without controversy , but there is evidence suggesting it may be possible.
"This [declining sperm count] is a marker of poor health of men, maybe even of mankind," says Levine. "We are facing a public health crisis – and we don't know if it's reversible."
Research suggests that male infertility may predict future health problems, though the exact link is not fully understood. One possibility is that certain lifestyle factors could contribute to both infertility, and other health problems.
"While the experience of wanting a child and not being able to get pregnant is extraordinarily devastating, this is a much bigger problem," says Da Silva.
Individual lifestyle changes may not be enough to halt the decline in sperm quality. Mounting evidence suggests there is a wider, environmental threat: toxic pollutants.
A toxic world
Rebecca Blanchard, a veterinary teaching associate and researcher at the University of Nottingham, UK, is investigating the effect of environmental chemicals found within the home on male reproductive health. She is using dogs as a sentinel model – a kind of early-warning alarm system for human health.
"The dog shares our environment," she says. "It lives in the same household and is exposed to the same chemical contaminants as us. If we look at the dog, we could see what's going on in the human."
Her research concentrated on chemicals found in plastics, fire retardants and common household items. Some of these chemicals have been banned, but still linger in the environment or older items ( read more about this in BBC Future's story on "forever chemicals" ). Her studies have revealed that these chemicals can disrupt our hormonal systems, and harm the fertility of both dogs and men.
"We found a reduction in sperm motility in both the human and the dog," says Blanchard. "There was also an increase in the amount of DNA fragmentation."
Sperm DNA fragmentation refers to damage or breaks in the genetic material of the sperm. This can have an impact beyond conception: as levels of DNA fragmentation increase, explains Blanchard, so do instances of early-term miscarriages.
The findings chime with other research showing the damage to fertility caused by chemicals found in plastics, household medications, in the food chain and in the air . It affects men as well as women and even babies. Black carbon , forever chemicals and phthalates have all been found to reach babies in utero.
Climate change may also negatively impact male fertility, with several animal studies suggesting that sperm are especially vulnerable to the effects of increasing temperatures. Heatwaves have been shown to damage sperm in insects, and a similar impact has been observed in humans. A 2022 study found that high ambient temperature – due to global warming, or working in a hot environment – negatively affects sperm quality.
Poor diet, stress and alcohol
Alongside these environmental factors, individual problems can also harm male fertility, such as a poor diet , sedentary lifestyles , stress , and alcohol and drug use .
In recent decades there has been a shift towards people becoming parents later in life – and while women are often reminded about their biological clock, age was thought not to be an issue for male fertility. Now, that idea is changing . An advanced paternal age has been associated with lower sperm quality and reduced fertility .
There is a growing call for greater understanding of male infertility and new approaches for its prevention, diagnosis and treatment – as well as an increased awareness of the urgent need to tackle pollution. Meanwhile, is there anything an individual can do to protect or boost their sperm quality?
Exercise and a healthier diet may be a good start, since they have been linked to improved sperm quality. Blanchard recommends choosing organic food and plastic products free of BPA (Bisphenol A), a chemical associated with male and female fertility problems . "There are small things that you can do," she says.
And, says Hannington, don't suffer in silence.
After five years of treatment and three rounds of ICSI (Intracytoplasmic Sperm Injection), an IVF technique in which a single sperm is injected into the center of an egg, he and his wife had two children. For people who have to pay for fertility treatments themselves, such a procedure may however not be affordable. In the US, a single round of IVF can cost upwards of $30,000 (£24,442) and insurance coverage for IVF can depend on the state you live in and who your employer is. And Hannington says he still feels the mental toll of his order.
"I'm grateful for my children every day, but you just don't forget," he says. "It will always be part of me."
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