Between 1989 and 2005 the sperm count of French men dropped by a third, according to research.
Compiling data from over 26,000 men, the study is thought to be the most extensive ever performed. It draws on data collected from French IVF clinics with researchers testing semen samples provided by men who were partners of women with diagnosed fertility problems.
In a paper published in the journal Human Reproduction, researchers note a continuous 32.2 percent decrease in total sperm count with an overall 33.4 percent decrease in healthy sperm.
The researchers say that their results 'constitute a serious public health warning'. The research will be added to other evidence that points toward declining male fertility in the industrialised world.
In the study, for men with average age of 35, semen concentrations declined from an average of 73.6 million per millilitre in 1989 to 49.9 million per millilitre in 2005.
Co-author Dr Joëlle Le Moal, an environmental health epidemiologist at the Institut de Veille Sanitaire, Saint Maurice, France, says that the values for 2005 'fall within the "fertile" range for men according the definition of the World Health Organisation'.
However, Dr Le Moal continues, 'the 2005 values are lower than the 55 million per millilitre threshold, below which sperm concentration is expected to influence the time it takes to conceive'.
The findings have been greeted with scepticism by some scientists who question the study's methodology. The researchers controlled their results for the effect that age has on semen concentration but not other variables.
Dr Allan Pacey, a senior lecturer in andrology at Sheffield University and chairman of the British Fertility Society, told the Guardian: 'I would urge much caution in its interpretation as there remain too many unknowns. In my view, the paper certainly does not resolve the issue of whether or not sperm counts have declined or not'.
In particular, Dr Pacey said that the paper claimed 'that the methods for measurement of sperm concentration and motility "have not changed noticeably during the study period", yet to me this is an odd thing to say as in my experience they have changed remarkably'.
Accordingly, Dr Pacey suggested, the decrease noted in the paper might just be 'a function of alterations to laboratory method'.
Contrastingly, Professor Richard Sharpe, at the Medical Research Council Centre for Reproductive Health at Edinburgh University, who was not involved in the study, said he did 'not accept that the basic methods for counting sperm have changed down the years, so there's no reason why sperm counts should go down unless it is real'.
Professor Sharpe added that the study meant it was 'time for action' and that research should be undertaken to determine the reasons for the supposed decline in male fertility.
'We still do not know which are the most important factors', he said, 'but perhaps the most likely is that it is a combination, a "double whammy", of changes such as a high-fat diet combined with increased environmental chemical exposures'.