Sperm counts are falling at a rate that threatens the long-term survival of the human race, a leading epidemiologist has warned.
In a new book, Shanna Swan, an environmental and reproductive epidemiologist at the Icahn School of Medicine in New York, warns there is a looming fertility crisis comparable in its impact to climate change.
She projects that if sperm counts continue to fall at current rates, they will reach zero by 2045.
To blame, she says, are lifestyle changes that alter hormone balances and exposure to "everywhere chemicals" that impair the endocrine system of chemical messaging in the body.
Additional factors, Professor Swan says, are smoking tobacco and cannabis and increased obesity.
In a research paper published in 2017 that she co-authored, Swan found that total sperm count in the Western world had fallen 59 per cent between 1973 and 2011.
"If you look at the curve on sperm count and project it forward — which is always risky — it reaches zero in 2045," Swan told Axios, a US news website.
Her new book, Count Down, expands on that research and argues the fertility crisis is as serious as climate change.
"The climate crisis has been accepted — at least by most people — as a real threat. My hope is that the same will happen with the reproductive turmoil that's upon us," Swan writes.
Fertility rates across the world have been falling for decades. The average number of children born per woman has fallen from 5.06 in 1964 to 2.4 in 2018.
Much of that fall is associated with the positive results of economic development. Nevertheless, Swan writes in the book there is clear evidence of biological factors at play and not just socio-economic ones.
As well as falling sperm counts, Swan identifies increased rates of miscarriages and impaired fecundity, a broad term for conception difficulties in couples, as evidence of a wider problem in the West.
She blames "everywhere chemicals" such as phthalates and bisphenol-A, which are found in cosmetics, pesticides and plastics, as well as lifestyle changes.
"Chemicals in our environment and unhealthy lifestyle practices in our modern world are disrupting our hormonal balance, causing various degrees of reproductive havoc," Swan writes.