Can far-UVC light be safely used as germicide? (help understanding a paper)
This is a great biological question! It asks a lot about how empirical science is done in the field of modern biology! I'm glad we encourage such questions from curious people who want to learn more.

There is another paper here that discusses using a similar frequency range of UVC radiation (207-222 nm) as a biocide:


  The biophysical reason is that, due to its strong absorbance in biological materials, far-UVC light does not have sufficient range to penetrate through even the outer layer (stratum corneum) on the surface of human skin, nor the outer tear layer on the outer surface of the eye, neither of which contain living cells; however, because bacteria and viruses are typically of micron or smaller dimensions, far-UVC light can still efficiently traverse and inactivate them.


Safety perhaps derives from the relatively exponential damage that far-UVC light can do to bacteria and viruses, while the layers and types of human cells that would get exposed are surface cells and mostly already dead or expendable. A 20% constant survival rate of vulnerable cells may be an acceptable trade-off, as those cells which are vulnerable would not typically be exposed (short of a wound or other exposed tissue, maybe), while the figure you cite suggests that you can raise the intensity of UV light linearly and kill exponentially more MRSA (or other disease-causing bacteria).