(CNN) -- Keeping a cell phone on talk mode in a pocket can decrease sperm quality, according to new research from the Cleveland Clinic.

"We believe that these devices are used because we consider them very safe, but it could cause harmful effects due to the proximity of the phones and the exposure that they are causing to the gonads," says lead researcher Ashok Agarwal, the Director of the Center for Reproductive Medicine.

In the small study, Agarwal's team took semen samples from 32 men and brought them to the lab. Each man's sample was placed into small, conical tubes and divided into two parts: a test group and a control group. The control group was unexposed to cell phone emissions, but kept under the same conditions and temperature as the test group.

The semen in the test group was placed 2.5 centimeters from an 850 MHz cell phone in talk mode for 1 hour. Researchers say that 850 MHz is the most commonly used frequency.

They used the measurement of 2.5 centimeters to mimic the distance between the trouser pocket and the testes. Agarwal reasoned that many men keep their active cell phones in their pants pocket while talking on their headsets.

Don't Miss Read the entire study

Overall, researchers found an increase in oxidative stress such as a significant increase in free radicals and oxidants and a decrease in antioxidants. Agarwal says that equals a decrease in sperm's quality, including motility and viability. Evidence of oxidative stress can appear under other conditions, including exposure to certain environmental pollutants or infections in the urinary genital tract.

"On average, there was an 85 percent increase in the amount of free radicals for all the subjects in the study. Free radicals have been linked to a variety of diseases in humans including cancer," said Agarwal. Free radicals have been linked to decreased sperm quality in previous studies.

However, the study does have major limitations, he acknowledged, such as the small sample size. It also was conducted in a lab and so cannot account for the protection a human body might offer, such as layers of skin, bone and tissue. Agarwal is in the early stages of further research that can model the human body's role in protecting from radio-frequency electromagnetic waves emitted from cell phones.

Agarwal also admits that there is no clear explanation of this demonstrated effect, but he shared some of his theories. "Perhaps the cell phone radiation is able to affect the gonads through a thermal effect thereby increasing the temperature of the testes and causing damaging effects in the sperm cell."

In a previous study, Agarwal and his team found that men who used their cell phones more than four hours a day had significantly lower sperm quality than those who used their cell phones for less time. Those findings were based on self-reported data from 361 subjects.

While representatives from the cell phone industry had not yet reviewed the latest study, they were careful not to give this study much merit. "The weight of the published scientific evidence, in addition to the opinion of global health organizations, shows that there is no link between wireless usage and adverse health effects," said Joe Farren, a spokesman for the CTIA-the Wireless Association.

"We support good science and always have," he said. "It's important to look at studies that are peer-reviewed and published in leading journals and to listen to the experts."

Agarwal emphasized that it is far too early for men to start changing cell phone carrying habits, noting that his own cell phone was in his pocket as he talked to CNN.

"Our study has not provided proof that you should stop putting cell phones in your pocket. There are many things that need to be proven before we get to that stage," he said.

All About Sexual and Reproductive Health • Wireless and Mobile Networking