Cited from Mercola.com

Exposure to cellphone radiation may or may not increase your risk of brain tumor formation, but this is a minor risk compared to damage done by free radicals from peroxynitrites that radically impair mitochondrial function.

Science has linked exposure to peroxynitrites from low-frequency microwave radiation emitted from cellphones and Wi-Fi networks with chronic diseases such as heart disease, obesity and inflammatory bowel disease.

You can reduce your exposure by shutting off your Wi-Fi at night, keeping cellphones in airplane mode unless using them, using the speaker phone and using a selfie stick when talking on it