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Plummeting sperm counts “threaten mankind’s survival”, researchers have warned.
Counts have more than halved since the 1970s.
And the decline has only accelerated since the turn of the century, according to a global analysis.
Scientists tracking the data, taken from more than 50 countries, said “we have a serious problem on our hands”.
Men’s bulging waistlines are blamed for the worrying trend, as well as “everywhere chemicals” in the environment.
The warning comes on the day Earth’s population officially surpassed eight billion.
Experts predict the toll could even breach 10 billion by 2100.
International researchers tracked sperm counts and concentrations from semen samples across the world between 1973 to 2018 by looking at older studies.
Men involved came from 53 countries including Britain, the US and Australia.
The study, in Human Reproduction Update, built on existing findings, which only looked at a fraction of the nations.
The analysis, which gives the first insight on trends in South America, Asia and Africa, also includes an additional seven years of data from 2011 to 2018.
Data included figures for sperm count and concentration in semen samples.
Count refers to the overall number of sperm in a sample, while concentration accounts for how many of the reproductive cells there are per volume of semen.
Results showed the mean sperm count fell by 51.6 per cent between 1973 and 2018 across men from all continents.
And concentrations have been falling by 2.64 per cent per year since 2000, quicker than the previous drop of 1.16 per cent annually from 1972.
Trends seen previously in men in North America, Europe and Australia have gotten quicker, the team said.
And they were also seen in those from South America, Asia and Africa, suggesting any future baby bust may not be limited to the global North.
Lead author Professor Hagai Levine, an epidemiologist at Hebrew University of Jerusalem, said, “Our findings serve as a canary in a coal mine.
“We have a serious problem on our hands that, if not mitigated, could threaten mankind’s survival.
“We urgently call for global action to promoted healthier environments for all species and reduce exposures and behaviours that threaten our reproductive health
“Overall, we’re seeing a significant worldwide decline in sperm counts of over 50 per cent in the past 46 years, a decline that has accelerated in recent years.”
While the current study did not examine the causes of sperm count declines, recent research indicates disturbances in the development of the reproductive tract in foetuses are linked to lifetime impairment of fertility.
He said “lifestyle choices and chemicals in the environment are adversely affecting this foetal development”.
Plummeting sperm counts are not just a symptom of reduced fertility, but will also be accompanied by higher levels of chronic disease, testicular cancer and shorter lifespans in men, they warned.
Low testosterone, one of the physiological causes of a low sperm count, increases the risk of chronic disease, while testicular cancer can also cause declining sperm levels.
Co-author Professor Shanna Swan, an epidemiologist at the Icahn School of Medicine, Mount Sinai, New York, said the fall could seriously harm men’s health.
She said, “The troubling declines in men’s sperm concentration … are consistent with adverse trends in other men’s health outcomes, such as testicular cancer, hormonal disruption, and genital birth defects, as well as declines in female reproductive health.
“This clearly cannot continue unchecked.”
She previously warned that “everywhere chemicals”, such as phthalates found in toiletries, food packaging and children’s toys, are to blame.
The chemicals cause hormonal imbalance which can trigger “reproductive havoc”, she said.
Factors including smoking tobacco and marijuana and rising obesity rates may also play a role, Dr Swan said previously.
It comes after a UN report yesterday revealed it has taken the global population 12 years to grow from 7 to 8billion, fuelled by rapid growth in sub-Saharan Africa and parts of Asia.
The report said it will now take approximately 15 years — until 2037 — for it to reach nine billion, which shows the overall growth rate of the global population is slowing.
Previous research has suggested birth rates in developed nations have been falling for years, which has dragged down the overall rate.
The average woman had two-and-a-half children by 2020, compared to five 50 years ago.
Plummeting sperm rates were one potential factor mentioned for the decline, a trend often discussed by the world’s richest man Elon Musk.
But some experts are less convinced the quality of sperm is to blame.
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