Action Spectra

What is an action spectrum?

• An action spectrum is a parameter that describes the relative effectiveness of energy at different wavelengths in producing a particular biological response. "Biological response" may refer to effects at a molecular level, such as DNA damage, or at a whole organism level, such as plant growth. An action spectrum is used as a "weighting factor" for the UV spectrum to find the actual biologically effective dose (BED) for a given effect. This relation is described mathematically below,
  

  where UV and A are the ultraviolet irradiance and action spectrum values at a given wavelength (lambda), respectively. The total dose is then found by integrating over the entire wavelength range. Several action spectra are shown in the figure below: the erythemal action spectrum given by McKinlay and Diffey (1987), the mammalian non-melanoma skin cancer action spectrum of de Gruijl and van der Leun (1994); the spectrum for melanoma induction in platyfish-swordtail hybrids derived by Setlow et al., (1993); and the DNA damage action spectrum of Setlow, (1974).

Three of the four action spectra displayed here show a rapid decline in relative response as the wavelength increases; this indicates that the UV-B portion of the spectrum is much more effective at eliciting a response than the UV-A wavelengths (note the logarithmic scale). These biological responses would then theoretically be more sensitive to ozone depletion since irradiance levels in the UV-B region increase with lower ozone levels. Conversely, the melanoma action spectrum for platyfish-swordtail hybrids shows a significant UV-A dependence, which indicates that ozone depletion would not have as great an impact on melanoma induction in this species of fish as would the responses described by the other action spectra.

Note, however, that a direct comparison of dose amounts for any induced effect cannot be made on the basis of action spectrum alone; these spectra only give an indication as to the relative effectiveness of particular wavelengths, not the actual dose amount required to produce a biological response.

References

de Gruijl, F.R. and J.C. van der Leun. 1994. Estimate of the wavelength dependency of ultraviolet cacinogenesis in humans and its relevance to the risk assessment of stratospheric ozone depletion. Health Physics, 67: 319-325.

McKinlay, A.F. and B.L. Diffey. 1987. A reference action spectrum for ultra-violet induced erythema in human skin. In Human Exposure to Ultraviolet Radiation: Risks and Regulations. W.F. Passchier and B.F.M. Bosnjakovich, eds. International Congress Series. pp. 83-87.

Setlow, R.B. 1974. The wavelengths in sunlight effective in producing cancer: a theoretical analysis. Proceedings of the National Academy of Sciences U.S.A., 71: 3363-3366.

Setlow, R.B., E. Grist, K. Thompson, and A.D. Woodhead. 1993. Wavelengths effective in induction of malignant melanoma. Proceedings of the National Academy of Sciences U.S.A., 90: 6666-6670.

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