Human Exposure to UV

From the viewpoint of human exposure to UV, the maximum clear-sky UV irradiance and exposure occurs in the equatorial zone latitudes, ± 23.3°,following the seasonal sub-solar point, and at high mountain altitudes. In general, UV erythemal, UV-A, and UV-B irradiance decreases with increasing latitude outside of the equatorial zone, due to the decreases in maximum daily noon solar elevation angles and for UV-B increases in ozone amount. An exception occurs for UV-B wavelengths at southern mid- to high-latitudes when reduced ozone amounts from the Antarctic ozone hole remain late into the spring and are pushed away from Antarctica toward the lower latitudes. For example, UV measurements indicate that the equatorial UV-B irradiance levels can occur in the southern part of South America for several days.

Global images of daily-integrated UV erythemal exposure (kJ per m ), averaged during the months of January (Southern Hemisphere summer), July (Northern Hemisphere summer), and the two equinox months of September and March, are shown in Fig. 5.14 (based on WMO, 1999). Because of cloud cover, the high equatorial clear-sky irradiances do not translate into the highest monthly cumulative exposures. The maximum erythemal doses near the equator occur when the sun is directly overhead during March, which has lower cloud cover than during September. The difference is related to the annual cycle of the cloud cover associated with the Intertropical Convergence Zone (ITCZ), which is usually over the equator in September, but is south of the equator in March. Two examples of very high UV exposures occur in the South American Andes (e.g., the sparsely populated Atacama Desert in Chile at 4,400 m to 5,600 m altitude, and in the city Cuzco, Peru) during January and in the Himalayan Mountains (over 100 peaks exceeding 7,000 meters) during July, as shown in Fig. 5.14. In both Southern Hemisphere cases, the sun is nearly overhead in January when the earth is also closest to the sun. Excluding high altitude locations, the largest monthly UV exposures occur in Australia and South Africa during summer (January) because of the very low amount of day-today cloud cover from late spring to early autumn. Other mid-latitude, low altitude relatively cloud-free areas also receive high doses, e.g., summertime (July) in the southwest U.S. and the Mediterranean countries.

January 1988 March 1988

January 1988 March 1988

July 1988 September 1988

Figure 5.14 Erythemal exposure kJ per m2 for the months of January, March, July, and September 1988 (from WMO, 1999) based on Nimbus-7ITOMS ozone and reflectivity data. These extreme levels are not seen in the September 1988 panel because the sun is just beginning to rise over Antarctica and the 1988 ozone depletion was not extreme

Figure 5.14 Erythemal exposure kJ per m2 for the months of January, March, July, and September 1988 (from WMO, 1999) based on Nimbus-7ITOMS ozone and reflectivity data. These extreme levels are not seen in the September 1988 panel because the sun is just beginning to rise over Antarctica and the 1988 ozone depletion was not extreme

Other factors contribute to the high Southern Hemisphere UV doses. There is a five million km decrease (3%) in Earth-Sun distance (perihelion near January 3) during the Southern Hemisphere summer, as compared to the Northern Hemisphere summer (aphelion near July 4), causing a 6% increase in summer irradiance in the Southern Hemisphere for a couple of weeks around perihelion. Average summer ozone in the Southern Hemisphere (270 DU) is lower than the Northern Hemisphere (320 DU) by about 13% which would lead to a 13% increase in 310 nm and a 26% increase in 305 nm irradiance. The exact percentage of increase is a function of latitude. In general, the Southern Hemisphere has fewer pollution aerosols, which can cause a small increase in UV irradiance relative to the Northern Hemisphere.

After latitude, the biggest factor affecting mid- and low-latitude exposure to UV radiation is the amount of cloud cover during the summer months. Satellite measurements of reflectivity indicate that there are some regions where there are long-term reflectivity increases (more cloud cover) over land in a few places, such as over the Indian Ocean, parts of Morocco and Algeria, northern Mexico/ southern U.S., and Canada. However, there are decreases in cloud cover over central U.S., northern Europe (60°N, 20°E), Kazakhstan (80°E 45°N), ArgentinaChile, and also smaller decreases over Australia and New Zealand that produce corresponding increases in time-integrated solar irradiance (exposure) reaching the ground (Herman et al., 2009b). While the decreasing cloud reflectivity increases solar radiation reaching the ground, changes in ozone are important for UV-B since it affects the clear-sky days when the irradiance is at a maximum for that day and latitude.

In Australia and other countries, any increase in UV exposure is especially detrimental to the European portion of the population that has minimal natural UV protection for skin cancer (Diffey, 1991; and a more general reference for health impact, Lucas et al., 2006), eye cataracts (Taylor, 1991), suppression of the immune system (Vermeer et al., 1991), and to ecosystem biology (Ghetti et al., 2006). Based on U.S. National Institute of Health (NIH) data (Devesa et al., 1999), similar skin cancer problems are present in the U.S., with more skin cancer occurring at lower latitudes where the UV exposure is higher. The seriousness of the very high UV exposure problem is observed in Australia where skin cancer rates have increased dramatically (20% for basal cell, to 788 per 100,000, and over 90% for squamous cell, to 321 per 100,000 carcinomas) based on household surveys in 1985, 1990, and 1995 (Staples et al., 1998). This compares to the U.S. National Cancer Institute's estimate (Devesa et al., 1999) of 14.5 per 100,000 for the U.S. Skin cancer incidence by skin type has been estimated by the U.S. National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program, which states that Caucasian people have the highest melanoma incidence, followed by a much lower rate for Hispanics, African Americans, and with the lowest incidence for Asian Pacific islanders.

The effects of custom or culture can be seen in Europe with large populations of light-skinned people living at fairly high latitudes (e.g., Stockholm, Sweden at 59.3°N, London, England at 51.5°N). However, there is a major difference in non-melanoma cancer incidence rates between citizens of Sweden (15 per 100,000) and the citizens of the United Kingdom (5 per 100,000) based on data from 1995 (Qiu and Marugame, 2008).

At higher latitudes in the Northern Hemisphere, where there is much more cloud cover than in Australia (e.g., central Europe 50°N, northern U.S., and Canada), a small decrease in cloud cover and ozone may produce the beneficial effect of increasing natural vitamin D production from increased UV-B exposure during the spring and summer months (Grant, 2002; McKenzie et al., 2003; Holick, 2004) without producing greatly enhanced rates of skin cancer. In the U.S., NIH data shows (Devesa et al., 1999) that increased solar exposure appears to correlate by latitude with decreases in some internal cancers along with increases in skin cancer.

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