There are numbers out there, but the problem isn't just the amount of energy, it's where it's concentrated. The theory goes that because of its very short wavelength everything that gets absorbed is concentrated in the topmost layers of the skin, so the energy density where it might be damaging is much higher than the absolute amount of energy might suggest.
From memory part of the argument was that the studies hadn't yet been done which might show whether this could be a health issue, so claiming the scanners were safe was at best premature.
