Current regulatory standards (SAR Test) only protect us from thermal or heating risks; yet, many hundreds of laboratory studies have found that low-intensity, non-thermal exposure to cell phone radiation can promote carcinogenic mechanisms. Moreover, research on humans has found that 25 years of mobile phone use is associated with a three-fold risk of brain cancer.  –Joel M. Moskowitz, Ph.D. School of Public Health. University of California, Berkeley

(Some common flaws in these studies: The summaries of the evidence weren’t comprehensive, the researchers often didn’t look at the quality of the studies they found, and they failed to do other simple things that would limit bias from creeping in. They also relied on case-control studies, a poor method to determine causality — more on that soon.) So we didn’t include these eight reviews in our analysis.
Many mobile phone protection stick on shields, chips or buttons make claims to manage the radiation problem in ways that are not verifiable by recognised scientific methods. We have evaluated a number of such products and have found in many cases that many of them simply do not work. On the other hand, our Anti-radiation phone covers, headsets and other solutions offer outstanding levels of protection (up to 100%) and are proven by verifiable SAR testing in accredited laboratories. In fact we are proud to say that our range includes some of the most effective radiation reducing solutions available anywhere in the world.
In a February 2 statement, Jeffrey Shuren, director of the FDA’s Center for Devices and Radiological Health, wrote that despite the NTP study’s results, the combined evidence on RF exposure and human cancer—which by now amounts to hundreds of studies—has “given us confidence that the current safety limits for cell phone radiation remain acceptable for protecting the public health.” Chonock says that for him, evidence from the Ramazzini study does not alter that conclusion. “We continue to agree with the FDA statement,” he says.
Until the FCC establishes testing procedures that fully correspond to real-world cell phone use and reviews its radiation standards to ensure that they are fully protective for all users, including young children, EWG advises phone users to adopt these simple measures to minimize radiation exposure: Use a headset, keep the phone away from the body and text rather than talk.  
Of course, scientific seesawing like that doesn’t provide a lot of clarity or confidence for the 90 percent of American adults and roughly 80 percent of teens who report having a cell phone. So how concerned should you be about cell-phone radiation? Consumer Reports’ health and safety experts conducted a thorough review of the research and offer some guidance.
Universal Cell Phone EMF Radiation Shield – Wallet Case (Small: 3.3"x5.6" Cell Phones, Universal Cell Phone EMF Radiation Shield – Wallet Case (Large: Up to 3.2"x6.2" Cell Phones, iPhone X EMF Radiation Shield & RFID Blocker- Wallet Case, iPhone 8 / 7 EMF Radiation Shield – Wallet Case, iPhone 8 / 7+ (Plus) EMF Radiation Shield – Wallet Case, iPhone 8 / 7 EMF Radiation Shield – SlimFlip Case, iPhone 8 / 7+ (Plus) EMF Radiation Shield – SlimFlip Case, iPhone 6/6S EMF Radiation Shield – Wallet Case, iPhone 6 / 6S+ (Plus) EMF Radiation Shield – Wallet Case, iPhone 6/6S EMF Radiation Shield – SlimFlip Case, iPhone 6 / 6S+ (Plus) EMF Radiation Shield – SlimFlip Case, iPhone 5/5s/SE EMF Radiation Shield – SlimFlip Case, Galaxy S7 EMF Radiation Shield – Wallet Case, Galaxy S7 EMF Radiation Shield – SlimFlip Case, Galaxy S6/S6 Edge EMF Radiation Shield – SlimFlip Case, iPhone 5/5S/SE Case -Wallet
So, what types of RF are these cases intended to block? If they block the frequency being used by the phone than if the blocking is complete the phone won’t work. If it’s partial than, as noted by the FCC and related in the article, the phone may attempt to compensate for the weakened signal by increasing it’s signal strength, thereby emitting higher levels of radiation itself, and reducing battery life in the process.
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.

Your phone sends radiofrequency, or RF, waves from its antenna to nearby cell towers, and receives RF waves to its antenna from cell towers when you make a call or text or use data. The frequency of a cell phone’s RF waves falls between those emitted by FM radios and those from microwave ovens, all of which are considered “non-ionizing” forms of radiation. That means that—unlike radiation from a nuclear explosion, a CT scan, or a standard X-ray—the radiation from your phone does not carry enough energy to directly break or alter your DNA, which is one way that cancer can occur. (FM radios and microwaves don’t raise alarms, in part because they aren’t held close to your head when in use and because microwave ovens have shielding that offers protection.)

What the study showed: Self-reported cell phone use was not associated with an increased risk of glioma, meningioma, or non-central nervous system tumors. Although the original published findings reported an association with an increased risk of acoustic neuroma (14), this association disappeared after additional years of follow-up of the cohort (15).