I would like to know the amount or threshold quantity of 254 nanometer
UV-C light to be harmful to the human eye or skin, in units of
Watt*seconds per square centimeter.
The 254 namometer wavelength of UV-C light is the "germicidal range"
that is usually used by UV sterilizers.  They all say that it's
dangerous, but, I cannot finds a threshhold quantity at which human
damage occurs.

For the eye, the peak effect of UV-B and UV-C radiation occurs at
270nm (in the UV-C).  Damage at this wavelength is entirely to the
cornea, and the threshold for damage is about 0.004 Joule/cm^2 (1
Joule = 1 Watt*sec).  The threshold for permanent corneal damage is
about twice this value.  See
<http://www.optometry.co.uk/files/48b2682cb5f43cbffaa4a831b75332ba_Voke1990730.pdf>

The UV-C wavelengths cannot penetrate the stratum corneum, the layer
of dead skin cells that makes up the outermost layer of the skin.  It
has little effect on normal skin tissue.

American Conference of Governmental Industrial Hygienists (ACGIH)
publishes Threshold Limit Values (TLVs), which are recommended
exposure limits. UV radiation exposure limits are wavelength
dependent.   For UV-C, the ACGIH TLV values are 0.250 Joules/cm^2 at
180 nm and 0.0031 Joules/cm^2 at 275 nm.  See
<http://www.ehs.unr.edu/Portals/0/Ultraviolet%20radiation%20safety%20050707.pdf>