By Caroline Ryan
BBC News Online health staff in Berlin
Carrying a mobile phone could significantly affect a man's fertility, scientists have suggested.
Sperm numbers were cut by a third, researchers said
Radiation from the phones could cut the number of sperm a man has by a third, researchers from the University of Szeged, Hungary, said.
The research, presented to the European Society of Human Reproduction and Embryology (ESHRE) meeting in Berlin, studied over 200 men.
But other experts have criticised their research, saying it did not take any other aspects of the men's lives into account.
The Hungarian study is the first to look at how electromagnetic radiation from mobiles could affect sperm.
The men who had their phone on standby throughout the day had about a third less sperm than those who did not.
And, of the remaining sperm, high numbers were found to be swimming abnormally, reducing the chances of fertilisation.
The researchers say their findings suggest that mobiles do have a "negative effect" on male sperm and fertility.
But other experts at the ESHRE conference said the Hungarian study did not allow for other factors which could have affected the men's fertility.
Professor Hans Evers, the former chairman of the European Society of Human Reproduction, said it "raises more questions than it answers".
He said the study had not taken into account the "many potential confounding factors which could have skewed the results".
These could include whether the men who had affected sperm came from a different social class or age range than those who did not.
It was also unclear whether the men were carrying their phones in their trouser pockets, close to their bodies, or in briefcases away from them.
He said: "These factors would have a considerable effect on the outcome of the research."
Professor Evers added: "Mobile phones are related to a certain lifestyle and they may be related to stress, to a heavily occupied business man rushing around from one office to another, having a lot of concerns in the life; there's all kind of things.
"And it is known that this decreases sperm counts and if you compare that to farmers living in the open air, on the land and not carrying a mobile phone at all you can explain the difference by completely other ways than the use of a mobile phone."
He said the only way to get a clearer picture would be for scientists to carry out a study which was carefully designed only to look at the effect of mobile phones on sperm.