Positive Impacts

It has long been recognized that some diseases are alleviated by moderate UV-B exposure (van der Leun and de Gruijl, 1993), though the benefits of vitamin D from sun exposure were generally not emphasized.

UV Radiation in Global Climate Change: Measurements, Modeling and Effects on Ecosystems 12.2.7.1 Vitamin D

Although more literature is now available regarding the benefits of UV in the production of vitamin D and the consequent reductions in non-cutaneous cancers, sorting out the risks and benefits of UV exposure is not a simple task. Humans benefit from UV radiation by the production of vitamin-D (e.g., Diffey, 1991; Webb, 1993; Harvard Women's Health Watch, 2004; Webb, 2006). Exposure to UV-B is involved in both synthesis and the breakdown of vitamin D by a complex series of photochemical reactions. These reactions regulate the production of vitamin D so that toxic levels are not reached (Webb, 1993; Webb, 2006).

Balancing the negative influences of exposure to UV radiation against possible benefits from vitamin D production has been termed a conundrum (Webb and Engelsen, 2005), and is indeed considered as such by many in the medical research community. However, some believe there is no conundrum. Their recommendation would be to avoid sun exposure and acquire vitamin D from a proper diet and supplements (Gilchrest, 2007). Others recommend that humans allow for moderate sun exposure, sufficient to photosynthesize adequate vitamin D, yet not to the point where skin damage occurs (Webb, 1993; Garland et al., 2002; Holick and Jenkins, 2003; Dowd and Stafford, 2008). Taking the opinion that a balance of exposure is best, McKenzie and Liley (Chapter 2 this volume) examine the time required to achieve balance in a range of climates. The balance is possible because only low levels of UV-B exposure, far less than one Minimal Erythermal Dose (MED) for the initiation of sunburn in light-skinned individuals, defined as 200 Jm , is needed in the vitamin D synthesis process (Webb, 1993), although the time needed for adequate vitamin D varies widely depending on the levels of irradiance and spectrum, skin type, and clothing (Webb and Engelsen, 2005; McKenzie and Liley, Chapter 2, this volume).

Many articles suggest that vitamin D deficiency is currently not a common problem in North America, partly because of fortification of foods (Simard et al., 1991). However, a panel of experts convened by the U.S. Centers for Disease Control and Prevention (U.S. CDC) in 2001 indicated concern about recent rickets cases (Scanlon, 2001). Even though vitamin D supplements have apparently been a general public health benefit, irregular application of the supplements has been noted. Survey results have shown that many samples contained either much less or much more vitamin D than stated on the label (Chen, 1999). During the 1940s in Europe, foods fortified with excess amounts of vitamin D caused intoxication in infants, leading to hypercalcemia and irreversible brain damage (Chen, 1999). A 1995 study reported rickets to be a problem in Mexico City, because UV radiation was greatly reduced by pollution (Galindo et al., 1995). Low vitamin D is much more prevalent in dark-skinned populations in the U.S. In one study, hypovitaminosis D occurred in 40.4% of black women compared to 4.4% among white women (Giovannucci, 2005). Low levels of vitamin D are also common among women in religious groups that require most of the body to be covered (Gannage-Yared et al., 2000; Glerup et al., 2000). Low vitamin D can also be a serious problem for those who have medical conditions that require avoidance of sun exposure, such as organ-transplant recipients, those with xeroderma pigmentosum, or people who must take medications that increase sensitivity to UV radiation (Reichrath, 2007).

The 2002 United Nations assessment of the effects of ozone depletion devoted a single paragraph to UV effects on vitamin D. While noting some evidence for high UV exposure or high levels of vitamin D in reducing the risk of some non-cutaneous cancers, it asserted that there is "no simple direct relationship between the vitamin D hormone and UV exposure because of the many regulatory feedback mechanisms," (De Gruijl et al., 2002). The 2006 United Nations assessment (Norval et al., 2007) has a more detailed treatment with about 50 citations dealing with vitamin D benefits for the immune system, internal cancers, autoimmune diseases, and possible infectious diseases.

The action spectrum for the formation of pre-vitamin D by sun exposure in human skin is based on a single study published in Science by MacLaughlin et al. (1982), but there are somewhat differing interpretations of this graphically represented spectrum. The spectrum has an approximate bell-shaped curve with a maximum near 295 nm and tails that approach 0 near 255 nm and between 315 nm and 320 nm. The original spectrum published in Science and republished at a larger size by Chen (1999) is plotted with a linear scale of response over one order of magnitude. It is not clear from the figure that the longer-wavelength end of the response actually reaches an absolute value of 0 by 315 nm, as has been assumed by some users (Webb et al., 1988; Webb, 1993; Kimlin, 2004; Engelsen et al., 2005), or whether it extends to longer wavelengths. A close perusal of the action spectrum curve suggests it approaches 0 in the vicinity of 320 nm asymptotically, and may extend beyond 320 nm. Michael Holick (pers. comm., July 22, 2005), one of the original authors of the Science article, expressed his belief "that the limit of 315 nm is correct." Even a small response at 320 nm could be significant under natural solar radiation because irradiance is typically three orders of magnitude larger at 320 nm than at 295 nm or 296 nm where the vitamin D response apparently peaks. Webb and Engelsen (2005) extrapolated the MacLaughlin relative vitamin D action spectrum to 0.001 at 320 nm, which would somewhat affect the action spectrum convoluted with a solar irradiance spectrum. Figure 2.1 shows the relative action spectrum for vitamin D interpreted from the figure in Chen (1999), with the exception that the Webb and Engelsen (2005) extrapolation to 320 nm is used. The convolution of the action spectrum with a typical solar irradiance spectrum in Fig. 2.1 shows the wavelength most important for vitamin-D to be about 310 nm. A 2006 report by the International Commission on Illumination (CIE) (Bouillon et al., 2006) tabulated a re-interpretation of the vitamin D action spectrum from MacLaughlin et al. (1982) and extrapolated the tail to a value of 0.000,078 at 330 nm. The new interpretation of the vitamin D action spectrum is contrasted to the CIE erythema action spectrum in Fig. 2.5 of McKenzie and Liley (Chapter 2, this volume).

0.001

0.0001

- ■ - EflfectiveVitD '

y ■

* t t

^-

290 300 310 Wavelength(mn)

Figure 12.1 Action spectrum of pre-vitamin D3 formation in human epidermis (PREVITD) from Webb et al. (1988), typical response of International Light UV-B radiometer SED240/UV-B/W (ILresp), and relative effectiveness of typical solar irradiance spectrum in formation of pre-vitamin D3 (adapted from Heisler, 2005)

290 300 310 Wavelength(mn)

Figure 12.1 Action spectrum of pre-vitamin D3 formation in human epidermis (PREVITD) from Webb et al. (1988), typical response of International Light UV-B radiometer SED240/UV-B/W (ILresp), and relative effectiveness of typical solar irradiance spectrum in formation of pre-vitamin D3 (adapted from Heisler, 2005)

Figure 12.1 also shows the typical manufacturer-provided response function of an International Light SED240/UV-B/W filtered vacuum photodiode UV-B sensor (IL)1. The response was shown by the manufacturer only for every 5 nm, with a finite response at 315 nm and zero response at 320 nm. Up to 315 nm, the relative IL response is nearly identical to the MacLaughlin et al. (1982) vitamin D action spectrum, and depending upon the interpretation of that action spectrum, the similarity may be even closer between 315 nm and 320 nm than Fig. 12.1 suggests. The IL sensors were used in a series of measurements of tree influences on UV-B irradiance at Purdue University (Grant and Heisler, 1996; Grant, 1997; Grant and Heisler, 2001; Heisler et al., 2003a). Thus, the results of those measurements should relate closely to vitamin D synthesis.

Early evidence that UV-B exposure in mid and high latitudes during winter was limiting for vitamin D photosynthesis was provided by high incidence of rickets in winter (Chen, 1999). A model recently available online (http://zardoz.nilu.no/%7Eolaeng/fastrt/VitD.html) (Engelsen et al., 2005) provides easily obtained estimates of the length of time during a day that UV-B irradiance would be sufficient to photosynthesize pre-vitamin D from 7DHC, which is one of the important roles of UV-B radiation in regulating vitamin D (Holick, 1999).

A CIE standards committee issued a report with a slightly revised interpretation of the MacLaughlin et al. (1982) vitamin D action spectrum (Bouillon et al., 2006). This spectrum is used by McKenzie and Liley (Chapter 2, this volume) to explore diurnal, seasonal, and latitudinal differences between erythemally weighted and pre-vitamin D-weighted irradiances. McKenzie and Liley point out

1 Company names are provided for the convenience of the reader and do not constitute an endorsement of a product by the USDA or the Forest Service.

that because pre-vitamin-D weighting depends more strongly on the shorter UV wavelengths, it is more dependent on ozone and solar zenith angle (SZA) than erythemal weighting.

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