Since 2001, the FCC has allowed manufacturers to test phones at a distance of up to one inch from the body to account for the use of a holster. In a 2012 report, however, the Government Accountability Office, the Congressional watchdog agency, noted that many cell phone owners actually keep and use their phones right next to the body, so these outdated testing policies could result in radiofrequency (RF) radiation exposure greater than the FCC’s legal limit (GAO 2012). The GAO report concluded:

Note: Although it is true that cell phones emit low frequency magnetic fields that can be measured in milliGauss, they also emit high frequency microwave radiation which is what all the concern (and publicity) is about, and which the products below are designed to shield. Many clients have contacted us seeking a shield for BOTH magnetic fields and microwaves for their phone. Such a shield does not currently exist. To reduce your exposure to BOTH types of radiation, use an airtube headset and keep the phone itself at a distance.
Think of it as a luxurious pillow case for your phone. Soft and attractive, it protects your phone like an ordinary phone case, PLUS innovative near field shielding material built-in to one side shields your body while carrying the phone and shields your head while making calls. BlocSock™ has two compartments, the main compartment covers the whole phone for transport. During calls, put the phone in the smaller “kangaroo style” pouch.

An analysis of data from NCI's Surveillance, Epidemiology, and End Results (SEER) Program evaluated trends in cancer incidence in the United States. This analysis found no increase in the incidence of brain or other central nervous system cancers between 1992 and 2006, despite the dramatic increase in cell phone use in this country during that time (22).
Radiofrequency radiation is a form of electromagnetic radiation. Electromagnetic radiation can be categorized into two types: ionizing (e.g., x-rays, radon, and cosmic rays) and non-ionizing (e.g., radiofrequency and extremely low frequency, or power frequency). Electromagnetic radiation is defined according to its wavelength and frequency, which is the number of cycles of a wave that pass a reference point per second. Electromagnetic frequencies are described in units called hertz (Hz).