Two reviews, by the National Academy of Sciences and the National Institutes of Health, have confirmed earlier findings showing that average sperm counts have almost halved over the past 50 years.
Shanna Swan, chief of the reproductive epidemiology section at the California Department of Health Services, who carried out the review for the National Academy of Sciences said: "I think this study will change the debate about sperm decline from `if' to 'why'."
The academy asked Dr Swan to write the definitive report on the issue, which has been bubbling since 1992 when Niels Skakkebaek and colleagues at Copenhagen University reported that sperm counts were falling around the world.
Their announcement caused a flurry of debate, and studies published since have shown conflicting results. British research found that men born in the 1970s had 25 per cent fewer sperm than those born in the 1950s, while a US study found men in New York had high sperm counts, with no evidence of a decline.
Some experts have questioned the accuracy of sperm counts. British researchers who sent the same samples to different hospital laboratories found counts varied widely, with some rated infertile. They said that counts made 30 or 40 years ago are likley to have been still less accurate.
Dr Swan's group re-analyzed the 61 published studies on sperm count that the Copenhagen team originally used. She said she had expected to contradict their finding which she had found "frankly suspicious because of its simplicity". But, after careful analysis, she changed her mind.
"Overall, in Europe and the United States there is a strong and significant decline," she said. She has since started her own analysis of sperm counts from 1938 to 1996 and the early results show the same downward trend. "I have done enough work to be sure of that," she said.
The National Institutes of Health review found sperm counts in the United States declined annually by an average of 1.5 million sperm per millilitre, or about 1.5 per cent per year between 1938 and 1990. Those in European countries declined at about twice that rate (3.1 per cent per year).
Environmental pollution is the most likely cause of the decline. Dr Swan said: "Once we rule out differences such as smoking, temperature, age and ethnicity, what we will have left are environmental factors."
Most experts blame industrial chemicals, including the pesticide DDT and those used in making plastics, which mimic the hormone oestrogen in their effect on the body, either bringing out feminine characteristics or counteracting male hormones. They are found in soil, water and food, and are long lasting.
Dr Swan said fertility was not the big issue, as babies were still being born. "However, sperm count is a marker, a red flag, for testicular cancer. We would expect wide-ranging effects. You cannot affect something like the reproductive system without affecting other systems in the body."Reuse content