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Phones are almost always on our person or right beside us, emitting heat and radio-frequency radiation. But, by and large, research has shown these factors don’t effect our health.

Last year, the Food and Drug Administration (FDA) issued a statement on the topic, citing nearly 30 years of evidence that found no link between cell phone use and health problems, including cancer. In fact, the FDA found that cell phones have no health effects whatsoever besides generating heat and warming the body’s tissues.

However, a new study raises questions about the extent to which our phones impacting our bodies by finding a link between cell phone use and one important men’s health outcome: sperm concentration.

For the study, published in the journal Fertility and Sterility, researchers analyzed data about health and lifestyle factors including cell phone use and sperm quality from close to 3,000 college-aged men who were recruits in Switzerland’s military between 2005 and 2018.

The researchers found that men who frequently used their cell phones had lower sperm concentrations than men who didn’t use them much. Specifically, the median concentration of sperm was 21% lower for men who used their phones more than 20 times a day compared to men who used them about once a week. According to the study, that means men who used their phones more had a 30% and 21% increased risk of their sperm concentration and total sperm count, respectively, being below what’s considered the fertile range.

Semen quality is vulnerable, and it is really sensitive to outside factors.

The researchers didn’t specifically measure whether this decrease in sperm concentration impacted fertility. However, because the difference in sperm concentration was relatively low, it likely did not for most of the men, says Rita Rahban, Ph.D., a senior scientist at the University of Geneva in Switzerland and first author of the study.

According to thresholds established by the World Health Organization, the chances of getting pregnant can decrease if a man’s sperm concentration is below about 40 million per milliliter, and men in the study who used their phones more than 20 times a day still had an average concentration of 44.5 million per milliliter.

Fortunately, the relationship between cell phone use and sperm likely isn’t as strong now as it was at the start of the study. The researchers also found that, as time progressed from the mid-2000s to the late 2010s, that link got weaker, potentially because cell phone technology got more efficient.

At the beginning, phones used 2G technology, which took more time to load and produced more heat in the process. Studies show that heat can reduce sperm quality, with bakers or other people who are regularly exposed to heat experiencing reductions in sperm concentrations compared to other groups. Because earlier cell phone technology may have increased heat exposure for users, it could have had more of an effect on sperm than later 4G technology, says Kevin Y. Chu, M.D., a fertility specialist at Torrance Memorial in California, who was not involved in the study.

However, the study found no association between reduced sperm concentrations and whether men kept their phones in their pockets or not. This implies that something else may be at play other than heat, Rahban says, such as radiation.

When men are using phones close to their heads, it could be that radiation from their phone somehow affects hormones in the brain that regulate testicular function and therefore sperm count, Rahban says. “But,” she added, “we still need a lot of further investigation to be able to say anything about how and which mechanism of action is actually at play.”

One important limitation of the study, per Chu, is that the researchers measured cell phone use through questionnaires, so people’s responses could be clouded or limited to what they remember their cell phone use to be.

Overall, the associations found in this study don’t suggest cell phone use will make men infertile.

Regardless, some evidence suggests that semen quality has been declining since the 1940s, and many have blamed environmental factors like exposure to pesticides or radiation, along with risky behaviors such as smoking tobacco or drinking alcohol.

“Semen quality is vulnerable, and it is really sensitive to outside factors,” says Rahban. Her research team is currently conducting another study to further explore the effect electromagnetic waves produced by cell phones have on the body and male reproductive health.

If you’re concerned about your sperm concentration, experts say to avoid heat exposure, such as by not using hot tubs or saunas. Chu also recommends not using your hot laptop on your lap, especially if you work from home and are frequently on your computer. The standard health lifestyle tips — getting enough exercise, eating a balanced diet, maintaining a healthy weight, etc. — can also help keep sperm concentration from dipping.

Overall, the associations found in this study don’t suggest cell phone use will make men infertile, and “for the majority of the population, this is probably not going to be an issue,” Chu says. “None of this is definitive, and none of it should be alarmist either.”

However, the results do suggest that assuming cell phones are neutral for our health isn’t the correct approach.

“We shouldn’t just take as foregone conclusions that things are not harmful,” Chu says. “We need to look at this more and be more cognizant of it.”