Numerous peer-reviewed studies have shown an association between cell phone use and the development of brain tumors. According to a Mar. 2008 meta-analysis of cell phone studies there is a "consistent pattern" connecting cell phone use and an increased risk of developing glioma, a type of brain tumor.  A Mar. 31, 2009 study found that long term cell phone use (10 years +) "approximately doubles the risk" of being diagnosed with glioma on the same side of the head where the cell phone is held.  In Apr. 2013 another study of Swedish cell phone users also found an association between cell phone use and the development of glioma and acoustic neuroma - a benign tumor formation on the nerve near the ear.  That study’s conclusions were confirmed by a different study in Apr. 2014.  Other studies published from 2005-2013 have similarly concluded that there is an association between cell phone use and increased risk of developing brain and head tumors.    
This is a powerful tool when troubleshooting an Android as it allows you to rule out poorly written, malfunctioning, or troublesome applications as possible causes. If you have a bad app or have installed a bad update for an app and your phone stops responding properly or starts having issues then you could put your phone into Safe Mode, uninstall that bad app, take your phone out of Safe Mode and be good to go, which actually brings us to our next objective, how to turn on Safe Mode and how to take your cell phone out of Safe Mode.
A 2012 study by NCI researchers (25) compared observed glioma incidence rates in U.S. SEER data with rates simulated from the small risks reported in the Interphone study (6) and the greatly increased risk of brain cancer among cell phone users reported in the Swedish pooled analysis (19). The authors concluded that overall, the incidence rates of glioma in the United States did not increase over the study period. They noted that the US rates could be consistent with the small increased risk seen among the subset of heaviest users in the Interphone study. The observed incidence trends were inconsistent with the high risks reported in the Swedish pooled study. These findings suggest that the increased risks observed in the Swedish study are not reflected in U.S. incidence trends.
Nevertheless, a group of scientists got together in the mid-2000s, calling themselves the BioInitiative Working Group. This group, which largely consisted of wireless radiation researchers, has written a harsh reply as feedback to the reports claiming that posed no health risks. The reply lists a wide range of health effects scientists at the European Commission have unfortunately either ignored or dismissed.