WHY IS NEW ZEALAND S MELANOMA PROBLEM SO BAD?

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1 WHY IS NEW ZEALAND S MELANOMA PROBLEM SO BAD? Gerald Smith Industrial Research Limited PO Box , Lower Hutt The annual incidence of melanoma amongst the non-maori population in New Zealand has doubled over the last ten years 1 and one of the factors which could be contributing to this alarming trends is an increase in the level of solar erythemal UV radiation. There has long been indirect, but somewhat anecdotal, evidence of the harshness of solar UV radiation in New Zealand, eg. the rapid deterioration of materials exposed to sunlight. However, until ten years ago there were very few data relating to carcinogenic solar UV radiation because of the technical difficulty of this type of measurement. What is Erythemal/Carcinogenic Radiation? The ultraviolet radiation responsible for erythema is only a fraction of a percent of the total radiation from the sun. It extends from the shortest wavelength radiation which reaches the earth s surface, ie about 300 nm (nanometres) to about 340 nm 2 (cf. violet light with a wavelength of 400 nm and red light with a wavelength of 600 nm). The same ultraviolet radiation is responsible for non-melanoma skin cancers 3,4. This radiation is particularly insidious because you cannot feel it as heat, nor can you see it. Indeed, its intensity bears little relationship to that of visible sunlight. Erythemal/carcinogenic ultraviolet radiation is also responsible for producing a sun-tan and the action spectrum for tanning 5 is distinguishable from that for tanning; thus if you want a sun-tan it is necessary to expose yourself to potentially dangerous ultraviolet radiation. Erythemal Radiation Reaching the Earth Erythemal radiation is extensively scattered in all directions by minute particles in the atmosphere such as water droplets and airborne pollutants. It is also absorbed by the ozone in the upper atmosphere. Therefore the amount of this radiation which reaches the earth s surface is very dependent on the distance it has to travel through the atmosphere. When the sun is high in the sky at midday, in summer, and particularly at low latitudes, the radiation has to traverse less atmosphere and is therefore more intense. Figure 1 illustrates this point by showing the variation of erythemal UV dose at midsouthern latitudes with month of the year. The way in which ultraviolet radiation interacts with the atmosphere and the earth s surface has important consequences.

2 a) Ultraviolet scatter UV radiation is scattered by particles in the atmosphere more than visible light. As a result of this, erythemal UV comes not only direct from the sun but also as scattered radiation from the whole sky. Consequently it is possible to get sunburnt while in the shade on a clear day, although the risk is much lower than is the case in direct sunlight. Also industrial pollutants or aerosols from natural sources (eg. Mt Pinatubo eruption) can reduce the amount of UV radiation. As a consequence, carcinogenic UV radiation in New Zealand can be more than 50% greater than at some industrialised northern hemisphere locations at the equivalent latitudes during summer because of our clear, unpolluted atmosphere. 6,7 b) Ultraviolet reflection UV radiation is reflected well by surfaces such as snow, water and sand. Therefore, there can be unexpectedly high levels of erythemal UV radiation in these environments. c) Effect of clouds Clouds usually attenuate UV radiation. For example, there are lower levels of erythemal UV in North Queensland than might be expected at low latitudes during the summer because it coincides with the wet season 8. This contrasts with summer in New Zealand which is characterised by a combination of high sun angles (relatively short atmosphere pathlength) coupled with relatively clear skies. Erythemal radiation measurements carried out during the 1984/85 summer in New Zealand and North Queensland showed that Hamilton and Lower Hutt experienced as much erythemal UV during February as Rockhampton and Townsville (see Figure 2). 9 d) Effect of ozone Ozone in the upper atmosphere absorbs much of the erythemal UV radiation from the sun. The effect of changes in the amount of stratospheric ozone on the intensity of UV at the earth s surface depends on sun angle: for example, during summer, changes in the amount of ozone result in relatively larger changes in the levels of UV than is the case in winter (see Figure 1). The amount of ozone in the upper atmosphere is subject to naturally occurring seasonal cycles. The timing and magnitude of these cycles depends on latitude. In New Zealand there is a peak in ozone levels in spring and a trough in autumn with as much as a 30% change in the amount of ozone between peak and trough (ie (15% swings from the mean). The timing of these cycles is such as to slightly shift the period of the year when erythemal UV radiation is most intense towards late summer (ie from December to January). Much recent publicity has surrounded the Antarctic ozone hole and the depletion of the ozone layer by synthetic chemicals such as aerosol propellants or refrigerants. Measurements of ozone over New Zealand show there has been about a 7% decrease in the average ozone level since the decline began in the late 70 s. 10 (The rate of decline has slowed during the 90 s). It is important to appreciate that it is difficult to measure such reductions in average ozone on top of the (15% naturally occurring

3 seasonal cycles. In addition, there are other substantial, naturally occurring variations in stratospheric ozone which accompany the 11 year solar cycle and short-term climatic changes. e) Temperature Although temperature has no effect on the levels of UV carcinogenic radiation reaching the earth it does indirectly affect the risk of over-exposure to solar UV. Many parts of New Zealand are cooled by wind which allows people to stay in the sun for longer than they would do if it was hotter. However, despite this absence of uncomfortable heat, the levels of carcinogenic radiation can still be dangerously high. Trends in Solar UV Radiation in New Zealand and the Consequences for the Incidence of Skin Cancer Ten years ago we established (with funding from the Ministry of Health) a network of solar UV radiometers in centres of population in New Zealand which operates continuously during daylight hours. At this time we are unable to unequivocally state that there has been any upward trend in UV levels across the whole of New Zealand over the past ten years. A method of analysis we have found to be successful in looking for trends is to compare UV levels at the same sun-angle. There were reduced UV intensities during the summer of 1991/92 due to the Mt Pinatubo eruption which produced significantly elevated levels of aerosols around the world. Although UV levels have risen since then in Wellington no significant upward trend has been apparent in Christchurch and Auckland. The UV levels in Christchurch are markedly lower than in Auckland and in Wellington and this may be due to higher levels of aerosols in the Christchurch metropolitan area although this is still to be proven by scientific measurements. The impact of increasing UV levels on the incidence of skin cancer has been derived from laboratory experiments on mice and analysis of a large epidemiological study in the US. It has been estimated that for every 1% increase in UV radiation there is a 2.5% increase in the incidence of squamous cell carcinoma and a 1.4% increase in basal cell carcinoma While such predictions are of wide interest it is important not to lose sight of the fact that we already have a major skin cancer problem and any exacerbation of this problem by increasing levels of solar UV radiation will not be apparent for some years. Human Exposure to Carcinogenic Solar UV Radiation The UV intensities measured by our radiometers provides good information about the UV environment or climate at a particular location but using this information to estimate the exposure of an individual or even a human population is exceedingly complex. How does one handle human behaviour quantitatively/statistically? Despite the difficulty, one way of using our data has proved very successful in the public health setting. During the summer months, in many cities throughout New Zealand, in association with the Ministry of Health we regularly broadcast

4 measurements of solar UV levels and burntimes in real time via local radio stations. In this way the listening public are informed of dangerous levels of solar UV as they occur. An alternative to this method of providing UV levels to the public would be to have personal dosimeters which individuals could wear. However, to my knowledge, no reliable, easily used device has been invented. A film badge devised by Davies and Diffey has been used for a number of studies of UV exposures experienced by selected occupational groups, but the badges are not suitable for use by the general public. Although marrying UV radiation level data with the UV exposure of humans is very complex because of the idiosyncracies of human behaviour, there may be a simplifying factor which we are currently investigating. Most individuals have the capacity to repair a certain amount of UV damage to skin. As a result, exposure to levels of solar UV below some (as yet to be determined) dose can be accommodated by the skin s repair system. We are determining the period during the day when the UV intensity exceeds a given level and from this analysis we will rank days during the year according to the risk of encountering a dangerous dose of radiation. Concluding Remarks 1. A suntan can only be acquired by exposure to potentially harmful UV radiation. 2. Humans have no built-in UV sensor and therefore we are unaware of how harsh the UV radiation is in many situations, for example: a) Cool conditions such as in the snow or in cooling breeze, are not necessarily associated with, or an indicator of, low levels of UV radiation. b) UV is reflected well by water, sand and snow. c) We can be sunburnt from scattered UV radiation on light overcast days or even in the shade. d) Under clear sky conditions, the intensity of erythemal ultraviolet radiation peaks quite sharply at noon. Therefore the two hours either side of noon is the period of greatest risk of sunburn. 3. Although ozone has attracted considerable publicity at present as an important determinant of the intensity of erythemal radiation, cloud and other airborne particles are equally important attenuators of this radiation. Our relatively clear atmosphere in New Zealand results in very much higher levels of carcinogenic UV radiation here compared with industrialised areas in the Northern hemisphere. Recommendations to the Public Treat exposure to sunlight sensibly and with some caution. This is particularly true on holidays in regions where you are unfamiliar with local UV conditions. In many parts of New Zealand, as little as 15 minutes of exposure to midday, summer sunshine, can produce sunburn in untanned skin. Ideally, avoid the sun in the two hours before and after noon, (1.00pm during daylight saving).

5 If not, wear hats, appropriate clothing and apply a sunscreen to unprotected skin. References 1. JM Elwood, KR Cooke, BD Coombs, B Cox, JE Hand and DCG Skegg. A strategy for the control of malignant melanoma in New Zealand, NZ Med J, 101 (1988) PM Farr, BL Diffey. The erythemal response of human skin to ultraviolet radiation, Brit J Dermatol, 113 (1985) CA Cole, PD Forbes, RE Davies. An action spectrum for UV photocarcinogenesis, Photochem Photobiol, 43 (1986) JC van der Leun. UV carcinogenesis, Photochem Photobiol, 39 (1984) JA Parrish, KF Jaenicke, RR Anderson. Erythema and melanogenesis action spectra of normal human skin, Photochem Photobiol, 36 (1982) GJ Smith, unpublished results, Thessaloniki, Greece (1993). 7. G Seckmeyer and RL McKenzie, Elevated UV radiation in New Zealand compared with Germany, Nature, 359, 135 (1992). 8. GW Paltridge, IJ Barton. Erythemal ultraviolet radiation over Australia, Search, 9 (1978) GJ Smith, A solar erythemal monitoring programme in New Zealand and Queensland, N.Z. Med. J., 103, 5 (1996). 10. WA Matthews and DJ Keep, Ozone trends and variability, globally and over New Zealand, Proc. NSSCCC Workshop, Roy. Soc. N.Z. Misc. Ser. 25 (1993). 11. A Kricker, BK Armstrong, ME Jones and RC Burton, Health, Solar UV radiation and environmental change (Int. Agency Res. Cancer, WHO, Lyon 1993). 12. J Scotto, TR Fears and JF Franmeni, Incidence of non-melanoma skin cancer in the United States, NIH Pub. # , US Dept. Health H Slaper, AA Schothorst and JC van der Leun, Risk evaluation of UVB therapy. Photdermatol. 3, 271 (1986).

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