ON the same day that the global population officially reached a record eight billion on Tuesday, scientists were sounding the alarm over something that might just put the brakes on future growth: the mysterious, and apparently accelerating, decline in sperm count worldwide.

The ominous findings from the largest analysis of its kind to date - tracking measurements from 1973 to 2018 - were described as “desperately bad news” for fertility.

“The human race is not at immediate risk of extinction but we really need research to understand why sperm counts are falling,” said Dr Sarah Martins da Silva, a reader in reproductive medicine at Dundee University.

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The study, published in Human Reproduction Update and led by researchers in Israel, found that sperm counts are falling by an average of 1.2 per cent annually with an overall decline of more than 50% in the 45 years to 2018.

Notably, the rate of decline appears to have more than doubled, to 2.6%, since the year 2000.

“We genuinely don’t know why,” said Dr da Silva, who said the results were “of concern”.

She added: “Exposure to pollution, plastics, smoking, drugs and prescribed medication, as well as lifestyle, such as obesity and poor diet, have all been suggested to be contributory factors although effects are poorly understood and ill-defined.”

HeraldScotland: Graph showing decline in sperm count based on 2018 estimates, compared to 1973Graph showing decline in sperm count based on 2018 estimates, compared to 1973 (Image: Human Reproduction Update)

Certain chemicals such as Bisphenol A (BPA) - which can leach from packaging into canned goods and milks - are thought to disrupt hormones and damage sperm quality, leading some scientists to call for tighter regulation around their use.

A lack of exercise and fatty junk foods have also been implicated in reduced sperm counts

Professor Richard Sharpe, an expert in male reproductive disorders at Edinburgh University’s MRC Centre for Reproductive Health, said the big question raised by the study “is whether this is a continuing trend and where might it end up”.

Although he cautions that we are unlikely to reach some sort of “rock bottom” scenario, the outlook points to a steady rise in problems conceiving.

Prof Sharpe said: “The key point that needs to be made (once again!) is that this is desperately bad news for couple fertility, because in our modern world (across the globe) couples are delaying putting their fertility to the test until the female partner is in her 30’s-40’s, when her fertility (and therefore chances of conceiving) is already reduced by 30-60% compared with in her 20’s and will continue to decline with her age.

“If her male partner has a low sperm count (and the present data shows that this is increasingly likely), then we know from prospective couple studies that the chances of him impregnating his partner are reduced – he may be able to get her pregnant but it will take longer and time is not on their side (because of the progressive decline in the female partners fertility as she ages); and the lower his sperm count the longer it will take. As I term it, a perfect recipe for increased couple infertility."

Relying on assisted reproduction is "unlikely to be of much use", stressed Prof Sharpe, given that "its effectiveness also reduces progressively with age (including of the male)".

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Controversy over declining sperm counts is not new.

The study published this week follows on from a prior analysis in 2017 which reported a “very strong decline” in both sperm concentration and total sperm count across North America, Europe, Australia and New Zealand.

The latest analysis simply builds on those findings by adding the results from an additional 38 studies.

Previously there had been too few studies from men in South and Central America, Asia and Africa to reliably estimate wider trends.

The updated study - which is now based on an analysis of more than 57,000 men in 53 countries - provides “strong evidence”that the decline in sperm counts is occurring worldwide, not just in the West.

Its lead author, public health physician and epidemiologist Professor Hagai Levine, situation was “like a pandemic” - even going so far as to say it could “threaten mankind’s survival” unless more was done to mitigate the problem.

“As in climate change, the impact could be different in different places, but generally the phenomenon is global and should be treated as such,” he added.

HeraldScotland: The decline in average sperm count has steadily increased since the 1970s The decline in average sperm count has steadily increased since the 1970s (Image: Human Reproduction Update)

While the study’s authors have cautioned that sperm count is an “imperfect proxy for fertility” (in the sense that past a certain upper threshold the chances of conception are not necessarily any higher), it is closely linked.

The World Health Organisation considers a “healthy” sperm count to be at least 15million per millilitre of semen; anything below that might be expected to lead to fertility issues.

At a global population level, average sperm count looks to have fallen from 101 million per millilitre in 1973 to 49 per million/ml by 2018.

Such a drop, note the authors, “implies a substantial increase in the proportion of men with delayed time to conception”.

Not everyone agrees that we are heading for a fertility catastrophe, however; some are sceptical that sperm counts genuinely are falling, in fact.

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Allan Pacey, a professor of andrology [male reproductive health] at Sheffield University, said the question of whether or not sperm counts are falling is a “really important question that we haven’t got to grips with in a serious way”.

In particular, he said the gold standard technique used to count sperm, using a haemocytometer (originally created to count blood cells), is “really difficult”.

He added: “I believe that over time we have simply got better at it because of the development of training and quality control programmes around the world.

"I still think this is much of what we are seeing in the data.”

He is not alone in this view, but as with so many public health crises, the longer science takes to reach a consensus, the harder it may become to tackle the causes and reverse the problem in due course.

As Dr da Silva put it: “Critics of this study might cite that we count sperm differently to 40 years ago, that microscopes are better and that they don’t believe the findings.

"But the numbers and consistent findings are difficult to ignore."