I know several people who use a mobile only when needed --mostly with a headset-- and just for a minute or so, because they believe (not without a reason) that it might harm their heads.
Seemingly, using it for some 30 minutes may frighten them as hell.
Interesting; the thing about mobiles being harmful to heads (brains) is very conclusive, as far as I know... Though if there's any truth in it, most people in Finland will soon have very damaged heads, as like I said probably 99% use mobiles only, and wherever you go people have mobiles stuck in their ears, all the time, be it public transport or anywhere else in public (Yes, it can get very annoying... I certainly prefer to talk about my private or not-so-private issues when *not* in public!)