The review pooled data from 185 different studies, and found a 59.3 per cent drop between 1973 and 2011 in the average amount of sperm produced by men from North America, Europe, Australia and New Zealand. No similar pattern was seen in South America, Asia and Africa, although fewer studies had been conducted in these countries.
Dietary and environmental exposures as well as pharmaceuticals are all linked to the quality of male sperm, revealing that toxins in many substances we interact with affect sperm maturation and membrane function in men. This means that men who are at increased risk of sperm DNA damage because of advancing age can do something about it.
"Given the importance of sperm counts for male fertility and human health, this study is an urgent wake-up call for researchers and health authorities around the world to investigate the causes of the sharp, ongoing drop in sperm count," says Hagai Levine, of the Hebrew University of Jerusalem, who worked on the analysis.
"The fact that the decline is seen in Western countries strongly suggests that chemicals in commerce are playing a causal role in this trend," says team-member Shanna Swan, of the Icahn School of Medicine at Mount Sinai, New York.
Exposure to chemicals in the womb, adult exposure to pesticides, smoking, stress and obesity have all previously been linked to lower sperm counts. But previous studies reporting falling sperm counts have been challenged by some for being unreliable.
"Previous smaller studies have suffered from confounding factors, including the fact that methods of counting sperm in the laboratory might have changed over the years, or that the populations of men being studied might have changed," says Daniel Brison, at the University of Manchester, UK.
"This new analysis overcomes those problems by including a large number of studies of varying design and location around the world, to confirm that the decline in sperm counts is likely to be real," says Brison.
Allan Pacey, of the University of Sheffield, UK, says that, despite the decline found in the study, average sperm counts still remain in the normal range.