For now, it’s probably better not to spend too much of your time worrying: you’re surrounded by cellphone signals, Wi-Fi signals, and all other kinds of radio frequency radiation day in and out — not just when you put your phone up to your face. And until the evidence suggests otherwise, all of this is still considered less of a cancer risk than eating red meat (which you shouldn't freak out about that much either).
In 2007, Dr Devra Davis founded non­profit Environmental Health Trust to provide basic research and education about environmental health hazards. Dr Davis served as the President Clinton appointee to the Chemical Safety and Hazard Investigation Board in the U.S.A. from 1994–­1999, an independent executive branch agency that investigates, prevents and mitigates chemical accidents.
Bonus application! In addition to shielding magnetic fields, PaperSHIELD is also quite good at shielding radiowaves (cellphone, wifi, etc). And because of the adhesive backing, it can be adhered to almost any surface you need such as the inside or back of your cellphone case. (Use a near field meter to test RF shielding performance.) Note that it is not transparent, so it can't be used on the touch screen side. Cover the cut edges with sturdy tape as they can be sharp. 36 inches wide. Made in USA.

Rats were exposed to radiation with a frequency of 900 megahertz, typical of the cellphones in use when the study was conceived in the 90s, for about nine hours per day for two years, The New York Times reports. The lowest levels of radiation used in the study were equivalent to the maximum exposure a phone can cause and still receive federal regulatory approval; the highest levels to which the animals were exposed were four times that. 
Cell-phone designs have changed a lot since the studies described above were completed. For example, the antennas—where most of the radiation from cell phones is emitted—are no longer located outside of phones near the top, closest to your brain when you talk, but are inside the phone, and they can be toward the bottom. As a result, the antenna may not be held against your head when you’re on the phone. That’s important because when it comes to cell-phone radiation, every milli­meter counts: The strength of exposure drops dramatically as the distance from your body increases.
There are theoretical considerations as to why the possible risk should be investigated separately in children. Their nervous systems are still developing and, therefore, more vulnerable to factors that may cause cancer. Their heads are smaller than those of adults and consequently have a greater proportional exposure to the field of radiofrequency radiation that is emitted by cell phones. And, children have the potential of accumulating more years of cell phone exposure than adults do.
In one type of study, called a case–control study, cell phone use is compared between people with these types of tumors and people without them. In another type of study, called a cohort study, a large group of people who do not have cancer at study entry is followed over time and the rate of these tumors in people who did and didn’t use cell phones is compared. Cancer incidence data can also be analyzed over time to see if the rates of brain tumors changed in large populations during the time that cell phone use increased dramatically. These studies have not shown clear evidence of a relationship between cell phone use and cancer. However, researchers have reported some statistically significant associations for certain subgroups of people.