Dr Richard McKenzie from the National Institute of Water and Atmospheric Research (NIWA) at Lauder in Central Otago is conducting research in many areas of UV, including UV radiation and vitamin D production. He recently started a research project in collaboration with researchers at the University of Auckland and the University of Otago looking at the relationship between UV and vitamin D production to find out which parts of New Zealand and in which seasons the UV intensity is too low to maintain adequate vitamin D levels in the body. The UV wavelengths needed to produce vitamin D are roughly the same as the UV wavelengths that cause skin damage and cancer.

This results in particular problems in New Zealand because of our unique UV radiation patterns. To better understand these problems, Dr McKenzie has been measuring the intensity of UV radiation throughout New Zealand and throughout the year. His recent research has showed that New Zealand’s peak summer UV Index values are 40% greater than corresponding North American latitudes – due to our low ozone, clear atmosphere, higher sun and correspondingly shorter atmospheric path of radiation and the Earth’s elliptical orbit – but our winter peak UV Index values are much the same as corresponding North America latitudes. What this means is that, in New Zealand, we have strong seasonal variations between summer and winter UV peak. Our winter UV peaks are only about 10% of our summer UV peaks.

This large range of UV peaks has significant implications for the health of New Zealanders. In summer, we have a high risk of skin damage, but in winter, in many parts of New Zealand, we have the risk of levels of UV intensity being too low to produce sufficient levels of vitamin D. This situation is made even worse because the tan picked up over the summer months means less vitamin D producing UV radiation can penetrate the skin. For example, a 2004 study conducted by Dr McKenzie and colleagues showed that most, if not all, apparently healthy Christchurch people did not have sufficient vitamin D in their bodies at some stage during the year. Even in summer, only 12% had the optimal level. During July to August, 35% were vitamin D deficient.

Murray Skeaff, Jennifer Rockell and Timothy Green from the University of Otago recently carried out research that tested the levels of vitamin D in the blood serum of 2,948 New Zealanders aged 15 years and over. They found that 3% were deficient in vitamin D and 48% had insufficient levels for optimal health – particularly in winter. Vitamin D insufficiency and deficiency were highest amongst Pacific Island adults, followed by Māori. This is because the darker the person’s skin, the longer exposure to UV radiation is required to maintain adequate production of vitamin D. Other research that has been done at the University of Otago tested 1,585 children and found that 32% had insufficient vitamin D levels, but amongst Pacific Island children, this level was 60%. People with darker skin have a higher risk of vitamin D deficiency, but a lower risk of skin cancer. One conclusion of this research was that, for many people, vitamin D supplements may be advisable.

Dr McKenzie and colleagues Paul Johnston and Ben Liley have suggested that health advisors use the idea of a ‘safe zone’ so that people can avoid the risk of skin damage and skin cancers, but still get sufficient vitamin D for best health. His research suggests that, throughout New Zealand, exposure to the Sun should be avoided during summer for the five hours around solar noon (11am to 4pm before daylight saving). Summer sun should be enjoyed in the morning or late afternoon. However, in the winter, especially in the South Island, some midday sun may well be needed to produce sufficient vitamin D. The best UV Index range that produces vitamin D but not skin cancer is between 1 and 3. This is the ‘safe zone’. Further research is still being carried out in this area so that the public may be well informed.

    Published 29 July 2008