Environmental pollution from industries such as coal mining and metal works may play a role in the increasing numbers of boys born with undescended testicles, new research suggests.
Researchers say their findings are hypothesis-generating, cannot show that these industries cause the condition called cryptorchidism and that further, targeted research is required.
However, the study of nearly 90,000 boys is the first to describe at a national level a recent increase in incidence of the condition over time.
It also identified clusters of cases in parts of France that are former mining or metal-working areas, such as the Pas de Calais in northern France.
All of the boys had operations to correct undescended testicles between 2002 and 2014 when they were younger than seven years old.
Cryptorchidism is the absence from the scrotum of one or both testes around the time of birth.
It is the most common male genital defect, occurring in between one and eight per cent of babies.
In most cases, the condition corrects itself within six months of birth, but approximately one in 100 boys have testicles that remain undescended, and this requires surgery to move them into the right position.
Those with untreated cryptorchidism may have fertility problems in later life and are at higher risk of testicular cancer.
Other research has shown that certain chemicals, such as phthalates and pesticides, are associated with the condition.
In the current study, researchers set out to find trends over time and in areas of metropolitan France to see if the local geographical environment might be an important trigger for the defect.
They identified 89,382 cases of operated cryptorchidism between 2002 and 2014 from public records, and found that the incidence of the condition increased by 36% during this time.
Dr Joelle Le Moal, a medical epidemiologist at the DATA Science Department, Public Health France, said: “Our main findings are the increase in the frequency of operated cryptorchidism in France during the study period, and the strong tendency for cases to cluster together in particular locations.
“This is the first time that such a finding has been documented at a country level for this birth defect.
“Our results suggest that the geographical environment could contribute to the clustering of cryptorchidism and interact with socio-economic factors.
“The industrial activities identified in the clusters are potentially the source of persistent environmental pollution by metals, dioxins and polychlorinated biphenyls, known as PCBs.
“PCBs, pesticides and dioxins are suspected to play a role in cryptorchidism and other testicular problems by disrupting hormones.”
Researchers used a disease-mapping model to describe the risk of cryptorchidism according to postcode, and identified 24 clusters scattered across France apart from in the south west.
The clusters were mainly in the north and central east of France, with the main cluster being around the city of Lens in the Pas de Calais, a former coal mining area.
According to the study published in Human Reproduction, the risk of having one undescended testis increased by more than a half and the risk of both testes being undescended increased more than five-fold when compared to the national level.
There were 1,244 cases and the researchers estimated this was an excess of 453 cases above the expected number for the area.
Professor Ieuan Hughes, emeritus professor of paediatrics, University of Cambridge, said: “This French study is a trail-blazer in research on male reproductive tract disorders by using a nationwide analysis of time and spatial trends in cryptorchidism identified by surgical records.”
He added: “The French study is a clarion call to other countries to replicate their work.
“Where better for that to occur than in the UK with its vast metropolitan regions, an industrial heritage, contrasting agricultural regions, rank child poverty and ‘levelling up’ on the political agenda.”
Rod Mitchell, professor of developmental endocrinology, MRC Centre for Reproductive Health, University of Edinburgh, said: “Cryptorchidism is associated with several other male reproductive disorders including testicular cancer and infertility, which may result from reduced testosterone in males during foetal life.
“Therefore, these findings may also have implications for the current decline in male reproductive health in general.
“We have a moral duty to identify and eliminate the factors that are behind the recent increase in the incidence of male reproductive disorders.”