  Ultraviolet (UV) exposure significantly contributes to non-melanoma skin
cancer. In the context of health, UV exposure is the product of time and the UV
Index (UVI), a weighted sum of the irradiance I(lambda) over all wavelengths
from lambda = 250 to 400nm. In our analysis of the United States Environmental
Protection Agency's UV-Net database of over four-hundred thousand spectral
irradiance measurements taken over several years, we found that the UVI is well
estimated by UVI = 77 I(310nm). To better understand this result, we applied an
optical atmospheric model of the terrestrial irradiance spectra and found that
it applies across a wide range of conditions.
