associated with brain tumors. but if you look just one layer deeper into the appendix of that same study, and you'll see something unsettling. turns out participants in the study who used a cell phone for ten years or more had doubled the rate of brain glioma, a type of tumor. and keep in mind 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, according to industry figures. >> environmental factors take decades to see their effect, not a few years. >> reporter: so if it may take decades to get a clear answer, what can we say about cell phone safety now? scientists here in san jose, california are trying to answer that very question. >> so one of the things we have to do first is literally put the brain inside the head. >> exactly. it's very light now.