Perhaps when switched off :-)
Check the IPhone specs. When talking in low reception area it's 3W microwave blasting one inch from the brain. If there is one, that is.
I'm not seeing that spec, GSM peaks at 2W, and the others use lower power. Three watts is blasting? In any case, it's still microwave, which is non-ionizing radiation. The most this type of radiation can do is burn you. Serious burns can lead to cell damage, but these power levels will at most very slightly warm you. Non-ionizing radiation can't strip electrons off of atoms, causing the DNA damage that can lead to cancer.
Studies of literally hundreds of thousands of people have not been able to show a decent link between cell phones and cancer.