Damage from photochemical reactions takes time to present itself: an individual may be exposed for the entire ‘work day’ before symptoms start to arise without stimulating an aversion response. Exposure limits for UV radiation are set as a total dosage (J m-2), similar to dosage limits with radiation exposure (X-Ray; alpha, beta, and gamma particles). UV exposure limits are set in an 8 hour time frame, though the onset time of symptoms can change depending on how much the exposure exceeds the threshold [1, p.106]. These thresholds, called Threshold Limit Values (TLVs), are set by the American Conference of Governmental Industrial Hygienists .
The exposure limits for UV radiation vary with wavelength (blue line in figure), since our tissue responds differently with wavelength (red line) . The UV-B and UV-C bands have the highest activity in our tissue, while the UV-A has the least. Exposure to about 10 mJ/cm2 of UV-A radiation (ex: 355 nm) on the skin would not show any effect, while significant effects would be observed from the same dosage (radiant exposure) in the UV-C band (ex: 266 nm).
The exposure dose limit during a work day is 3 mJ/cm2 for actinic UV radiation (200 nm - 315 nm) based on the ACGIH TLV booklet . The actinic region of the UV spectrum is the region of highest spectral sensitivity as shown in the red curve above. The actinic region contains some of the UV-C and most of the UV-B band. The total dosage in this spectral band translates into different effective irradiances of sources depending on the exposure duration, as shown in the following table.
|Exposure duration||Effective Irradiance (µW/cm2)|
Note that many drugs can modify an individual's sensitivity to UV and lead to over-exposure at dosage levels lower than that reported here.
 R. Henderson, K. Schulmeister, Laser Safety. Bristol, UK: IoP Publishing, 2004
 American Conference of Governmental Industrial Hygienists. (Accessed 2013, May 23). Ultraviolet Radiation: TLV® Physical Agents 7th Edition Documentation.
 International Commission on Illumination. (Accessed 2013, May 23). Erythema Reference Action Spectrum and Standard Erythema Dose.