There is no conclusive evidence that cellphone masts are not harmful. Many studies have been conducted which have shown that the radiations from cellphone masts may cause damage in the long term. The problem is that the studies are only for a short term where in order to see whether they effect or not the studies should be conducted over the life time of that subject.
Some raidation can penetrate whereas others are not able to penetrate.
I remember one study that was conducted at the University of Toronto on the effects of radiations emitted from the TV. They mesaured the radiations and said that the radiations from the TV have no effect on kids eyes. Whereas we are seeing kids as old as four with glasses. Maybe the evolution wants us with bad eye sight or maybe it is the TV?