Chris de Freitas: Burn Time Index best under the sun

COMMENT

New Zealanders are risking serious sunburn and skin cancer caused by UV weather forecasts they do not understand.

Summer burn-time warnings have been replaced with a new UV Index, a decision in keeping with an international trend. But among experts the choice of index is a divisive issue.

The pros and cons are particularly important since UV radiation is exceptionally high over New Zealand and skin cancer is by far the most common form of cancer affecting us.

Some 68,000 new cases of skin cancer are diagnosed every year, compared with a total of 16,000 cases for all other forms of cancer. Two out of three New Zealanders will develop skin cancer.

We have the highest rate of melanoma and general skin cancer in the world - 19 per cent more than Australia.

Auckland has become known as the melanoma capital of the world; an estimated one in 10 of its population suffers a cancerous growth at some time. On cloudless summer days, Auckland receives about 10 per cent more sun-burning UV than Dunedin.

Three factors together cause the exceptionally high UV intensities. The first is clear air. A smaller land area in the Southern Hemisphere means there is also less dust in the atmosphere, except over parts of Africa and Australia.

The atmospheric window through which the sun shines down on New Zealand is clearer than almost everywhere else in the world.

Secondly, the Earth is closer to the sun during summer in the Southern Hemisphere than it is during the Northern Hemisphere summer. Because of this, we receive up to 4 per cent more sun-burning UV than places at equivalent latitudes in the Northern Hemisphere.

Thirdly, the protective UV shield in the air over New Zealand has been weakening. It is estimated that ozone losses since 1980 have caused our UV rate to increase by 15 per cent in the past 20 years.

The net result is that UV intensities here can be 15 to 40 per cent higher 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.

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 outdoors, or under the brim of your hat.

The new UV Index supersedes the Burn Time Index which has been used previously. As an approximate guide, a UV Index of 12 corresponds to a burn time of about 12 minutes; a UV Index of 6 corresponds to a burn time of 24 minutes, and so on.

The new UV index, like air temperature, is a physical term expressed as a number on a simple linear scale, but interpretation is not intuitive. UV is not warm - we neither feel nor see it. A UV Index of less than 2 is defined as a low exposure level and 11 or above extreme.

Critics of the UV Index cite numerous reasons why the Burn Time Index is preferable. It is well established that weather factors expressed as abstract physical units are more difficult to interpret than indicators that reflect the significance or impact of weather conditions on people, such as the Burn Time Index or Windchill Index.

The UV Index is difficult to translate, thus increasing the uncertainty. On the other hand, the Burn Time Index is easy to understand, in both a relative and absolute sense.

It requires no translation or conversion. Its meaning is intuitive and better suited to the effect UV radiation has on people, one that is both physiological and cumulative.

It is more intuitive to have a measure for a potentially harmful conditions expressed in terms of how long before damage takes place. A burn time is a good indicator of relative risk.

There are large uncertainties in converting UV Index to skin response. "Burn times" are calculated for the worst-case scenario - fair skin and clear skies - without the implicit assumption that the UV will not change over the period.

Critics of the UV Index also point out that the Burn Time Index has been in widespread use internationally for some time and enables comparison between UV levels in New Zealand with other places.

On balance, both indices have their merits and each is a reliable scientific indicator that tells us in one way or another how much ultraviolet radiation there is in the atmosphere. Yet despite the care and good intentions of weather forecasters and years of campaigning by the Cancer Society, so many of us rush carelessly into the sun every summer.

So this summer, as you enjoy the season of sunshine and outdoor living, pay heed to the UV forecast and, above all, make sure you understand what it means.

* Chris de Freitas is an associate professor of geography and environmental science at Auckland University.