i think that the public has a right to know that there could be a potential risk. the public generally assumes if one is selling something on the market, that we have had assurances that that device is safe. >> reporter: their conclusion, little or no evidence cell phones are associated with brain tumors. but if you look just one layer deeper into the appendix, it turns out participants in the study who use a cell phone for ten years or more, had double the rate of a type of tumor. and cell phone use in the united states has only been popular for around 15 years. back in 1996, there were 34 million cell phone users. today, nearly 300 million in use.