Due to the relatively recent adoption of cell phones, the long-term safety of the technology cannot be determined conclusively and caution is warranted. Research on glioma brain tumors shows the average latency period is 20-30 years.  Although cell phones were introduced in 1983, it was not until 2003 that over 50% of the US population had a wireless subscription, so the 20 year mark for mass cell phone use has not yet been reached.   The May 17, 2010 INTERPHONE study, the largest study ever to examine possible links between cell phones and brain tumors, concluded that overall there was "no increase in risk" for glioma or meningioma brain tumors,  but the average user in the study had less than eight years of cell phone exposure.  In his review of the INTERPHONE study results, Dr. Rodolfo Saracci stated that "none of today’s established carcinogens, including tobacco, could have been firmly identified as increasing risk in the first 10 years or so since first exposure." 
In 2011, the American Cancer Society (ACS) stated that the IARC classification means that there could be some cancer risk associated with radiofrequency radiation, but the evidence is not strong enough to be considered causal and needs to be investigated further. Individuals who are concerned about radiofrequency radiation exposure can limit their exposure, including using an ear piece and limiting cell phone use, particularly among children.
To be sure, the industry could not have been pleased with some of the Interphone study’s conclusions. The study found that the heaviest cell-phone users were 80 percent more likely to develop glioma. (The initial finding of 40 percent was increased to 80 to correct for selection bias.) The Interphone study also concluded that individuals who had owned a cell phone for 10 years or longer saw their risk of glioma increase by nearly 120 percent. However, the study did not find any increased risk for individuals who used their cell phones less frequently; nor was there evidence of any connection with meningioma.36
Carlo’s October 7, 1999, letters to wireless-industry CEOs are the smoking-gun equivalent of the November 12, 1982, memo that M.B. Glaser, Exxon’s manager of environmental-affairs programs, sent to company executives explaining that burning oil, gas, and coal could raise global temperatures by a destabilizing 3 degrees Celsius by 2100. For the tobacco industry, Carlo’s letters are akin to the 1969 proposal that a Brown & Williamson executive wrote for countering anti-tobacco advocates. “Doubt is our product,” the memo declared. “It is also the means of establishing a controversy…at the public level.”18
Outside critics soon came to suspect that Carlo would be the front man for an industry whitewash. They cited his dispute with Henry Lai, a professor of biochemistry at the University of Washington, over a study that Lai had conducted examining whether cell-phone radiation could damage DNA. In 1999, Carlo and the WTR’s general counsel sent a letter to the university’s president urging that Lai be fired for his alleged violation of research protocols. Lai accused the WTR of tampering with his experiment’s results. Both Carlo and Lai deny the other’s accusations.5
Long-term exposure to Radio Frequency waves or microwave radiation has long been studied and shown to produce all sorts of adverse health effects. See my post on EMF Radiation Exposure Symptoms for a full list of common symptoms. Even the American Cancer Society, who in large part will not fully say that EMF radiation causes long-term harm, and instead errs on the side of not enough research, admits on their website, that:
The wireless industry has sought to downplay concerns about cell phones’ safety, and the Federal Communications Commission has followed its example. In 1996, the FCC established cell-phone safety levels based on “specific absorption rate,” or SAR. Phones were required to have a SAR of 1.6 watts or less per kilogram of body weight. In 2013, the American Academy of Pediatrics advised the FCC that its guidelines “do not account for the unique vulnerability and use patterns specific to pregnant women and children.” Nevertheless, the FCC has declined to update its standards.30
The most compelling evidence though comes from a Swedish team of cancer experts whose research stretches back 15 years. The results clearly demonstrate “a consistent association between long-term use of cell phone and cordless phones and glioma and acoustic neuroma”. Overall, they found that using a cell phone for more than a decade significantly increases the risk of a malignant tumour by almost two times with an analogue cell phone and by nearly four times with a digital phone. Interestingly, the risks were even higher for people who had started using mobiles as teenagers.