The most common effect is heat generation (though non-thermal biological harm has also been demonstrated), which can alter the characteristics of various bodily tissues depending on the amount of radiation present and its ability to penetrate the body. Tissue damage can promote the cellular mutations and increase your long-term risk of developing cancer.
To find out about the state of research on the link between phones and cancer, we spoke with Jonathan Samet, dean of the Colorado School of Public Health and an expert in phone radiation who led a World Health Organization working group on the subject. In 2011, the WHO group deemed phone radiation “possibly carcinogenic,” which is less certain than other classifications, but isn’t an outright “no” either. Six years later, Samet said the evidence in either direction is still mixed and that for the time being, there remains “some indication” of risk.

When you talk, your voice is transmitted from the antenna as radio frequency radiation (RFR) between 800 MHz and 2,200 MHz. A range equal to the middle of microwave frequency and 20% to 80% of the radiation emitted is deposited in the user's head. The microwave radiation is absorbed and penetrates the area around the head, some reaching an inch, to an inch and a half into the brain. Exposure to this microwave RFR has shown to have serious health consequences. Laboratory studies have shown that radiation from cell phones expose the user to a wide range of health problems including: 
EWG is calling on the FCC to update its testing guidelines to take account of the widespread use of smartphone cases. Such action is critical because mounting scientific studies have raised serious questions about the safety of cell phone radiation exposure over the short and long term. In the absence of meaningful action by the Commission, EWG offers consumers tips on how to reduce their exposure to cell phone radiation.
If not, then how can the seller claim that the reported effects on cell cultures, muscle strength testing, plants, skin heating, or other biological effects are due to electromagnetic changes (see http://www.cellphone-health.com/mret-howitworks.htm for example)? Electromagnetic field characteristics are well known and easy to measure. If there is truly an electromagnetic change, that change will be easy to detect and measure.

Among the hundreds of smartphone cases available for iPhone and a bevy of popular Android phones, there are some that claim to reduce the amount of radiation your body absorbs when you have the handset close to your body. Pong Research is a US brand that offers a range of products fitting this description, as is Cellsafe, a company based in Victoria, Australia.
SAR LEVELS ARE very deceptive when it comes to health hazards!  The Pong case is taking advantage of the fact that current safety regulatory guidelines provide no non-thermal assurances of safety what so ever! Other-words Pong is using inadequate safety standards to prove the legitimacy of the Pong cell phone case – Citing official documents being reviewed by the FCC’s “Reassessment of Federal Communications Commission Radio-frequency Exposure Limits and Policies” (Proceeding Number 13-84), as it turns out, the level SAR rating of your phone is almost irrelevant, as it doesn’t consider health effects from non-thermal levels of radiation exposure at all.

What the study showed: Most published analyses from this study have shown no statistically significant increases in brain or central nervous system cancers related to higher amounts of cell phone use. One analysis showed a statistically significant, although modest, increase in the risk of glioma among the small proportion of study participants who spent the most total time on cell phone calls. However, the researchers considered this finding inconclusive because they felt that the amount of use reported by some respondents was unlikely and because the participants who reported lower levels of use appeared to have a slightly reduced risk of brain cancer compared with people who did not use cell phones regularly (4–6).
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