Men who are heavy users of cellphones have significantly lower sperm counts than those who are not, according to research.
The research suggests that radiation from handsets might be damaging male fertility.
The study, which was conducted on 361 infertility patients in the US, indicates that both the quantity and quality of a man's sperm decline as his daily use of his cellphone increases.
The greatest effects were seen among very heavy users - those who talked on a cellphone for more than four hours a day.
They produced about 40percent less sperm than men who never used a cellphone. Smaller falls in sperm count were found in those who used the phones less frequently.
The findings of the team led by Ashok Agarwal, of the Cleveland Clinic in Ohio, indicate that the electromagnetic fields generated by cellphones interfere with sperm production.
Previous studies have shown that close and heavy exposure to this form of radiation damages sperm in the laboratory, but an effect has never been demonstrated convincingly in daily life.
But other researchers cautioned that the study showed only an association between cellphone use and sperm counts, not a causal link.
It was more likely that heavy phone use was linked to another factor, such as stress or obesity, and that it was this factor that was responsible for the effect, they said.
Allan Pacey, a senior lecturer in andrology at the University of Sheffield, said: "The findings seem pretty robust, but I can only assume that cellphone use is a surrogate for something else.
"If you hold it up to your head, it makes no sense to say 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 that they are more stressed or more sedentary and sit around eating junk food, getting fat. These seem to be better explanations than a phone causing damage at such a great distance."
Agarwal, who presented his team's results on Monday at a conference of the American Society for Reproductive Medicine, said they were worried because of the extent of cellphone use.
"Almost a billion people use cellphones and the number is growing in many countries at 20percent to 30percent a year.
"People use cellphones without thinking of the consequences. It is just like using a toothbrush, but cellphones could be having a devastating effect on fertility. It still has to be proved, but it could have a huge effect."
In the study, 361 men whose sperm was being analysed before they underwent fertility treatment, were asked about their cellphone use. They were split into four groups.
One group was made up of those who never used a phone, the second of those who used a phone for less than two hours a day, the third of those who used a cellphone for two to four hours a day and those who used a phone for more than four hours a day.
The median sperm counts were 85,89 million sperm a millilitre for nonusers, 69,03 million for the second group, 58,87 million for the third and 50,30 million for the fourth.
Sperm motility, or swimming ability, also fell as phone use increased, as did other measures of sperm quality.
Agarwal said: "The main finding was that on all four parameters - sperm count, motility, viability and morphology - there were significant differences between the groups.
"The greater the use of cellphones, the greater the decrease in these parameters. That was very clear and very significant."
The results are similar to those of a previous study by researchers at the University of Szeged, in Hungary.
It suggested a 30percent reduction in the sperm count of men who kept a cellphone on standby in a trouser pocket. But the research failed to eliminate the possibility of other lifestyle causes.
Such controls are important because sperm production is sensitive to a number of factors, including obesity and heat.
For example, lorry drivers and travelling salesmen tend to have low sperm counts because the long hours they spend sitting increases the temperature of their testes.
Agarwal said that if the effect were caused by cellphones, several explanations were possible.
Studies have shown that electromagnetic fields can damage Leydig cells in the testes, and cellphones can heat tissue, which might harm sperm. - The Times News Service, London