A 10-year study of cell phone users found no conclusive link between phone use and cancer, though it found a possible link between heavy use and a deadly brain tumor called glioma.

The $24 million study by the United Nations tracked nearly 13,000 people in 13 countries between 2000 and 2010. Among the 10 percent who used their phones for more than 30 minutes a day, there was a 40 percent higher chance of developing glioma. There was also a 15 percent spike in a more common but less deadly form of cancer called meningioma. Researchers did not attribute the increase in cancer to cell phones, citing flaws in how the research was conducted, the Associated Press reports.

Cell phones emit low levels of radiation, and the possible risk is greatest when the phone is right next to a person’s ear. Researchers disregarded the time a phone spends in a person’s pocket or on the night stand because once the phone is four inches away from the head, the amount of radiation reaching the brain is almost zero.

Critics of the study said it was far too soon to know whether cell phones cause brain cancer. Tumors can develop for 25 years before they are detected.