Any kind of wireless device, even wired ones for that matter emit certain types of electromagnetic radiation which are potentially dangerous over a long term exposure!! Researches are on to recognize the exact nature of these emissions and the way they affect our body.
Gulshan is right to point out that the mobile tower emissions are already playing a havoc on the way the bees, butterflies and even birds work. Due to these towers, the numbers of sparrows in the city are rapidly declining.
@Abid, why should a phone be switched off during journey? Please do let us know how a journey affects the phone usage!
"I am free of all prejudice. I hate everyone equally." - W. C. Fields :)
