UV Dosimetry and Minimization Strategies

UV radiation incident on the earth's surface is comprised of both a direct component and a diffuse component. The combination of the diffuse and direct UV is termed the global UV. As the direct component is incident directly from the sun, it is easier to minimize by simply blocking its path. However, the diffuse UV component is incident from all directions due to atmospheric and environmental scattering and can constitute a significant proportion of the UV exposure to the human body. The relative amounts of direct and diffuse UV compared to global UV depend on the solar zenith angle (SZA). For example, the ratio of the diffuse UV to global UV increases with increasing SZA (Blumthaler and Ambach, 1991). This is due to the longer path through the atmosphere. Furthermore, the ratio of diffuse UV to global UV is higher at the shorter wavelengths due to the higher degree of scattering at the shorter wavelengths.

Numerous studies have employed UV dosimeters to investigate the efficacy of different strategies utilized to minimize exposures to diffuse UV and direct UV. Tree shade is widely employed and Parisi et al. (2000a, 2000b) utilized manikin forms with dosimeters placed at specific anatomical sites in tree shade to measure the UV exposure ratios under Australian gum trees. The exposure ratios of global UV radiation in a shaded environment to an unshaded environment ranged from 0.16 to 0.49 for the different anatomical sites. It was also found that exposure ratios for the legs ranged from 0 to 0.75 for the different anatomical sites for a sitting posture in summer compared to 0.14 to 0.39 for a standing posture. Furthermore, tree shade provided reductions in personal annual erythemal UV exposures by a factor of 2 to 3 and 4 to 6 in the contribution to the risk of basal cell carcinomas and squamous cell carcinomas, respectively, compared to not employing the protection of tree shade.

Erythemal UV exposures to the faces of school children wearing hats while playing sports were measured with polysulphone dosimeters over the period of an hour. The mean facial exposures of unprotected students (no hat) to protected students (hat) varied from 140 ± 82 Jm to 99 ± 33 Jm , respectively (Downs and Parisi, 2008). The cosine response of the polysulphone film used in these dosimeters has been found to be within approximately 20% of the cosine function for angles of incidence up to 70° (Krins et al., 2000). Parisi and Wilson (2005) measured erythemal UV exposures beneath different types of clothing with dosimeters and found that the highest exposure under a high ultraviolet protection factor (UPF) knitted garment was only 1.5% of that of full sun exposure. Parisi et al. (1999)

employed dosimeters to measure the solar UPF for different stocking thicknesses and colors and found that the highest UPF of 4.6 was provided by 50 denier stockings with the lowest UPF of 1.4 provided by 15 denier stockings. Dosimetric measurements behind different thicknesses of glass (Parisi et al., 2007) found that the glass filtered solar UV ranged from 59% to 70% compared to the unfiltered UV and was only influenced to a small extent by the thickness of the glass and the solar zenith angle. It was also found that laminated window glass only transmitted 12% of incident UV radiation and that windscreen laminated glass transmitted approximately 2.6%. All new cars use laminated windscreen glass that transmits minimal UV. Turnbull and Parisi (2005) measured exposures to the human form while using shade structures. An example of polysulphone dosimeters employed on a human form manikin to measure the UV protection of a shade device is provided in Fig. 7.1. This research found that during summer and winter, significant decreases in exposure of up to 65% for summer and 57% for winter can be attained when comparing the use and non-use of polycarbonate sheeting for side-on UV protection.

Figure 7.1 A human form manikin with dosimeters placed at various anatomical sites measuring the UV protection provided by a shade umbrella

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