People who had a used their cell phones for 896 hours or more for lifetime use had 2.6 to 2.9 times greater risk of brain tumors compared to people who were not regular cell phone users according to a 2014 study from France called the CERENAT which was a multicenter case-control study carried out in four areas in France in 2004–2006.

People who used their cell phones to make calls for an average of 15 hours or more per month—30 minutes per day—were 4.2 times more likely to have a brain tumor than those who were not regular cell phone users.

Practical advice: Use earbuds with a microphone when making talking on the phone so that the cell phone is not next to your head / brain.