Cancer is the obvious start. An early concern with mobile technology was clusters of the disease around those living near phone masts. One study in Israel found a 4.5-fold increase in cancers of all kinds in the immediate vicinity of a mast (Int. J. Cancer Prev., 2004). In 2009, a Korean team of researchers carried out a pool analysis of the results of 23 studies, which involved almost 38,000 subjects.  
In this study, the effect of Wi-Fi radiation exposure as a threat to brain health was studied using genomic analysis and histopathological study which showed the high risk of its genotoxicity especially in prolonged exposure spectrum through the findings from this study. The genomic analysis confirmed DNA damage due to Wi-Fi radiation toxicity and DNA damage effect which was seen through the RAPD profiles of animals from the exposed groups. The histopathological analyses also confirmed significant deleterious alterations in the brain tissues of Wi-Fi-exposed animals. Hence, the need to exhibit caution in handling smart devices that are used from day to day is fast becoming a threat to human health and wellness.
You know what’s equally as concerning? Even though we don’t fully understand the impacts of long-term wireless radiation, we’re forging ahead with 5G wireless technology. Within a few years, this technology could mean millions of mini cell towers could pop up on street corners, according to the Federal Communications Commission chairman. (3) Developing a 5G network is moving forward despite the fact that we don’t even remotely understand 5G health effects.
Since children’s skulls are thinner than adults’ and their nervous systems are still developing, researchers believe they may be at a greater risk for cell phone-related cancers. They’ll also be exposed to the potentially harmful radiation significantly longer than their parents’ generation. Until more is known about the possible carcinogenic effects of cell phone use, curb your kids’ cell phone habits by encouraging texting or using a landline instead—if they can actually recognize a landline anymore, that is.
A 2005 study in the International Journal of Cardiology found that mobile phones may have "adverse effects" on pacemaker functions under certain conditions. [59] According to the US Food and Drug Administration (FDA), radiofrequency energy from cell phones can create electromagnetic interference (EMI) that may disrupt the functioning of pacemakers, especially if the cell phone is placed close to the heart. [21] The American Heart Association includes cell phones on its list of "devices that may interfere with pacemakers." [60]
Cell phone storage in front pockets has been linked to poor fertility and higher chances of miscarriage and childhood cancer. [18] According to the Cleveland Clinic Center for Reproductive Medicine, semen quality "tended to decline as daily cell phone use increased." [19] According to a May-June 2012 meta-study in the Journal of Andrology, "men using mobile phones have decreased sperm concentration" in addition to "decreased viability" of their sperm. [64]

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I am 60 years old, single. I got a tumor on my left breast when I took a bath on Sunday, Sep. 4th. I came to HCM City Medic Hoa Hao Clinic on Sep. 5th for an X-ray then a biopsy test. In the afternoon I came back to have the result, my case fell as ‘invasive ductal carcinoma. Then I came to Ho Chi Minh City Oncology Hospital on September 7th for some more tests: blood, urine, X-ray, mammogram, ECG. Can I drink ‘Ensure Original nutrition shake’ as my students offer me 12 bottles (8 FL oz /237 mL), I am looking forward to your soonest consideration.
We investigated the long-term effects of radiofrequency radiation (RFR) emitted from Wi-Fi systems on hearing. Sixteen Wistar albino rats were divided equally into two groups: sham control and exposure groups. The rats in the experimental group were exposed to 2.4 GHz RFR emitted from a Wi-Fi generator for 24 h/day for one year. The same procedure was applied to the rats in the sham group, except that the Wi-Fi generator was turned off. All groups were kept in Faraday cages during the 12 months to eliminate external electromagnetic fields. The distance between the Wi-Fi generator antenna and the exposure cages was 50 cm. Pre-exposure distortion product otoacoustic emissions (DPOAE) of all rats were measured at the beginning, 6th and 12th months of the study. The DPOAE values of the sham, baseline and exposure groups were compared statistically. For the 6000 Hz hearing frequency, the DPOAE values in the exposure group were lower than those in the sham group (p < 0.05). Similarly, the 6000 Hz hearing frequency values obtained at the end of the 12th month were also lower than the baseline and 6-month values in the exposure group (p < 0.05). In contrast, the DPOAE values at the 6th and 12th months of exposure for the 2000 Hz hearing frequency were higher than the baseline value (p < 0.05). These results indicated that 12 months of RFR (24 h/day) at 50 cm from a 2.4 GHz Wi-Fi source can affect hearing. However, further studies are necessary.

SafeSleeve Anti Radiation Case: Safe Sleeve website says they integrate FCC accredited independent lab tested "shielding technology" in their products. There are many links to "SafeSleeve, Is it a Scam?" and "Does SafeSleeve work?" online, but if you follow those links you'll find they go to the actual SafeSleeve site where the question of SafeSleeve protection is addressed.  You'll see in this demonstration that the radiation level was higher with the SafeSleeve Case on.

I really want to caution you to be careful of any "expert" or EMF blogger recommending anti-radiation filtering cases, stickers that attach to the phone or necklaces to "ward off" EMF.  I get it, when I first realized 25 years ago that I was sensitive to electromagnetic energy and then later, when I understood that without a doubt, man made electromagnetic radiation has a biological effect on my body, I wanted an easy fix.  I tried the shielding materials and cases and I just did not feel any better.  Then I bought meters to see if the energy changed and I actually noticed what the California Department of Health now says and it's what the FTC and FCC post warnings about:  "Some shielding materials can make exposures worse". So please, be careful and read on for what you can do to avoid electromagnetic radiation exposure.
Joel Moskowitz (@berkeleyprc) of the University of California, Berkeley School of Public Health, US, says: “This is the largest technological experiment in the history of our species, with potential health risks we still know next to nothing about.” This view is shared by Denis Henshaw, professor of human radiation effects at Bristol University, UK, who said: “Vast numbers of people are using cell phones and this could be a time bomb of health problems.”