Men who are heavy users of mobile phones have significantly lower sperm counts than usual, according to new research that suggests radiation from handsets could be damaging male fertility.

Both the quantity and quality of a man’s sperm declines as his daily mobile use increases, a study of 361 infertility patients in the United States has found.

The greatest effects were seen among very heavy users who talk on a mobile for more than four hours a day, these produce about 40 per cent less sperm than men who never use a mobile at all. Smaller falls in sperm count were also seen among those who use mobiles less frequently.

The findings, from a team led by Ashok Agarwal of the Cleveland Clinic in Ohio, could indicate that the electromagnetic fields generated by mobile phone handsets are interfering with sperm production. Previous studies have shown that close and heavy exposure to this form of radiation damages sperm in the laboratory, though an effect has never been convincingly demonstrated in the real people.

Other researchers cautioned that the study shows only an association between mobile phone use and sperm counts and not a causal link. It is more likely that heavy use is a proxy for another factor, such as stress or obesity, that is actually responsible for the effect, they said.

"On the face of it, the findings seem pretty robust, but I can only assume that mobile phone use is some kind of surrogate for something else," said Allan Pacey, a senior lecturer in andrology at the University of Sheffield. "If you are holding it up to your head to speak a lot, it makes no sense it is having a direct effect on your testes.

"Maybe people who use a phone for four hours a day spend more time sitting in cars, which could mean there’s a heat issue. It could be they are more stressed, or more sedentary and sit about eating junk food getting fat. Those seem to be better explanations than a phone causing the damage at such a great distance."

Dr Agarwal, who presented the results today at the American Society for Reproductive Medicine conference in New Orleans, said they were worrying because of the huge extent of mobile phone use.

"Almost a billion people are using cell phones around the world and the number is growing in many countries at 20 to 30 per cent a year," he said. "In another five years the number is going to double.

"People use mobile phones without thinking twice what the consequences may be. It is just like using a toothbrush but mobiles could be having a devastating effect on fertility. It still has to be proved but it could have a huge impact because mobiles are so much part of our lives."

In the study, 361 men who were having their sperm analysed prior to fertility treatment were asked about their mobile use and split into four groups: those who never used a phone and those who used one for less than two hours, two to four hours, and more than four hours a day.

Median sperm counts were measured at 85.89 million per millilitre for non-users, 69.03 for the second group, 58.87 for the third and 50.30 for the fourth. Sperm motility, or swimming ability, also fell with increasing phone use, as did other measures of quality.

"The main finding was that on all four parameters - sperm count, motility, viability and morphology - there were significant differences between the groups," Dr Agarwal said. "The greater the use of cell phones the greater the decrease in these four parameters. That was very clear and very significant."

The results are similar to those of a previous study from the University of Szeged in Hungary, which found a 30 per cent reduction in sperm count among men who kept a mobile on standby in their trouser pockets. That research also failed to control for lifestyle. Such controls are important because sperm production is very sensitive to a number of factors, including obesity and heat: lorry drivers and travelling salesmen, for example, tend to have low sperm counts because the long hours they spend sitting warms their testes.

Dr Agarwal said that if the effect is genuinely caused by mobiles, several explanations are possible. Animal work has shown that electromagnetic fields can damage Leydig cells in the testes and mobiles are also known to cause a heating effect on tissue that might be hazardous to sperm. Both phenomena occur over short distances, so holding a phone at a distance from the crotch while speaking should not be dangerous.

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