A peer-reviewed paper, "Human Electromagnetic Field Exposure in 5G at 28 GHz," questions the safety of exposure to 5G millimeter waves.
The authors found in a simulation study that use of a 5G cell phone at 28 GHz could exceed ICNIRP (i.e. international) radio frequency exposure limits when held at 8 centimeters (i.e., 3 inches) or closer to the head or body.
Read Dr. Joel Moskowitz's commentary on the paper, as well as key excerpts from the original paper, here. Dr. Moskowitz is the Director of the Center for Family and Community Health at UC Berkeley's School of Public Health.