The number of hours a man talks on a cellular phone each day may affect his fertility, with sperm count and quality deteriorating as the duration of calls increases, according to researchers in reproductive medicine.

Scientists in Cleveland, Mumbai, and New Orleans tracked 364 men who were being evaluated for infertility and split them into three groups based on sperm count. In the group whose sperm counts were within the normal range, those who used a cell phone more than four hours a day produced on average 66 million sperm a milliliter, 23% less than those in the group who didn’t use the phones at all.

The proportion of the cell-phone users’ sperm that possessed “normal forms” was 21%, almost half the 40% of normal sperm produced by men who didn’t use the phones, said the researchers, who presented their conclusions this week in New Orleans at the annual convention of the American Society for Reproductive Medicine.

“The effect of cell phones on sperm parameters may be due to the electromagnetic radiation the devices emit or to the heat they generate,” the group said in a summary of the research. “Further studies will be necessary to identify the mechanism involved in the reduction of sperm quality due to cell phones.”

The researchers involved in the study are from Ohio’s Cleveland Clinic, the Karthekeya Medical Research and Diagnostic Center in Mumbai, and the Tulane University Health Science Center in New Orleans.