UV Exposure Limits

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 [2].

The exposure limits for UV radiation vary with wavelength (blue line in figure), since our tissue responds differently with wavelength (red line) [3]. 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).

UV Wavelength-Dependent Exposure Limit and Weighting Curve

Figure: The Spectral Weighting Factor (red line) is a multiplicative factor that weights the effect of UV radiation for each wavelength. The Exposure Limit (blue) is smallest at the peak of the Weighting Factor (270 nm in the UV-C band) because our tissue responds the most at this wavelength [3]. Other than the mercury lamp (Hg lamp), all sources highlighted here are common laser wavelengths.

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 [2]. 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)
  8 hours   0.1
  4 hours   0.2
  2 hours   0.4
  1 hour   0.8
  30 minutes   1.7
  15 min.   3.3
  10 min.   5
  5 min.   10
  1 min.   50
  30 seconds   100
  10 s   300
  1 s   3000
  0.5 s   6000
  0.1 s   30,000

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.

References

[1] R. Henderson, K. Schulmeister, Laser Safety. Bristol, UK: IoP Publishing, 2004

[2] American Conference of Governmental Industrial Hygienists. (Accessed 2013, May 23). Ultraviolet Radiation: TLV® Physical Agents 7th Edition Documentation [Online]. Link to source at www.acgih.org/forms/store/ProductFormPublic/ultraviolet-radiation-tlv-r-physical-agents-7th-edition-documentation

[3] International Commission on Illumination. (Accessed 2013, May 23). Erythema Reference Action Spectrum and Standard Erythema Dose [Online]. Available: www.cie.co.at/publications/erythema-reference-action-spectrum-and-standard-erythema-dose

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