KEY POINTS:
Recent research suggests modest sun exposure to ultraviolet-rich sunlight (UV) promotes the production of vitamin D in the body, which helps us live longer.
A number of studies, including a highly publicised paper published in the Proceedings of the National Academy of Sciences by researchers from the Institute for Cancer Research in Oslo, have found protective effects from higher vitamin D intake for some cancers and illnesses such as rickets, osteoporosis and diabetes.
But reliance on the sun as a source of vitamin D carries increased risk of skin cancer and eye damage, especially in New Zealand. A key consideration is the large seasonal harmonic of UV.
Peak UV intensities in the New Zealand summer are more than 40 per cent greater compared with corresponding sites at equivalent latitudes in North America or Europe, whereas in winter they are quite similar. Our wintertime peaks of UV are typically only 10 per cent of those in the summer.
As winter sets in, our suntanned skin tends to reduce penetration of the increasingly low levels of vitamin D-producing UV radiation. By the end of winter, the protective effects of a tanned skin have all but disappeared. With the onset of high UV in summer, the pallid skin is susceptible to damage, especially in early spring when the ozone layer is at its thinnest.
Three factors together cause the exceptionally high UV intensities we experience. The first is clear, clean air.
Second, the Earth is closer to the sun during summer in the Southern Hemisphere than it is during the Northern Hemisphere summer. Because of this, New Zealand receives up to 4 per cent more sunburning UV than places at equivalent latitudes in the Northern Hemisphere.
Third, the protective UV shield of ozone has been thinning.
The net result is that UV intensities can be 40 per cent higher in New Zealand than that for equivalent locations in the Northern Hemisphere. You can effectively double this UV loading if you are in highly reflective surroundings, such as on a white sand beach, or on the water in an open boat.
Research shows peak UV values on a South Island skifield are approximately 30 per cent more than at sea level and that the UV dose during skiing is comparable with that received during a summer round of golf.
A large portion of UV reaches us as so-called "diffuse" radiation, which does not come directly from the sun. It gets to us indirectly as light scattered by the atmosphere and through clouds. This is why you can be sunburned on overcast days or even in the shade, or under the brim of a hat.
New Zealanders may also be risking serious sunburn and skin cancer caused by UV weather forecasts they don't understand. The new UV index is a physical term expressed as a number on a simple linear scale, but interpretation is not intuitive. UV isn't warm - we neither feel nor see it.
The linear UV index scale does not reflect the cumulative effect of the erythemal process.
There is much to consider in the UV-vitamin D issue. Messages to the public must recognise what is involved.
* Chris de Freitas is an associate professor in the School of Geography, Geology and Environmental Science at the University of Auckland.
