The most fundamental concept in photobiology is the action spectrum (or relative response) for a given effect. Although there is a standardized germicidal action spectrum in the IES Handbook, it was based on inactivation of E. coli bacteria, and action spectra for spores, other bacteria, and different viruses can vary. This standardized action spectrum extends from 235 nm to 313 nm and peaks at approximately 265 nm. A wavelength of 254 nm has a relative efficacy of 0.85; by contrast, 313 nm in the UV-B has a relative efficacy of only 0.01.
Germicidal effectiveness is proportional to the exposure dose (radiant exposure, typically in millijoules* per square centimeter, mJ/cm2, or joules per square meter, J/m2), which is the product of the dose-rate (irradiance, typically in mW/cm2 or W/m2) and time (from 1 μs to several hours). A nonlinear relationship exists between UV exposure and germicidal efficacy. For example, if a certain UV exposure kills 90% of a bacterial population (frequently referred to as "one-log kill"), doubling the exposure time or intensity can kill only 90% of the residual 10%, for an overall germicidal efficacy of 99% ("two-log kill"). Likewise, a 50% decrease in dose or exposure time decreases germicidal efficacy only from 99% to 90%.
Humidity can reduce the effectiveness of germicidal UV radiation. There is a reference dose to attain a survival of 37%; however, in practice a GUV dose of interest is 3 or 4 log-kills, corresponding to 99.9% or 99.99% inactivation, respectively. To be effective in practice, achieving two log-kills (99% inactivation) is frequently accepted.
 DiLaura D, Houser K, Mistrick R, Steffy S (editors). The Lighting Handbook, 10th ed. New York: Illuminating Engineering Society; 2011.
* One joule is equal to one watt times one second. (Or, 1 W = 1 J/s) A millijoule is one-thousandth of a joule.