There has been debate for years and years about whether or not excessive use of cellphones increase the chances of getting brain cancers. I guess that prolonged use of a high power VHF or UHF handheld may pose similar risks?
According to Southgate News a judge in California has asked for a 2014 report to be made public. A quick read seems to suggest some sort of increased risk. Basically, this is all relative, but prolonged RF exposure is probably not good and is best avoided. Another argument in favour of QRP?