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A New Zealand engineer has invented a "cutting-edge" device to calculate just how much sunshine we need to stay healthy.

Too much ultraviolet radiation and we risk skin cancer; too little and we are at risk of the effects of vitamin D deficiency such as bone deformities, fractures and possibly heart disease.

Dr Martin Allen, of Canterbury University, invented the badge-sized UV "dosimeter" with the National Institute of Water and Atmospheric Research and is developing it with Oamaru manufacturer Scienterra.

It sells for $300 but is only available for research - such as a ground-breaking New Zealand study to measure vitamin D production - and it is being developed for schools to use in overcoming many teenagers' reluctance to avoid sun, wear a sunhat or apply sunscreen.

Dr Allen said yesterday it was not feasible to add a sunburn warning buzzer and sell the device for individuals to use at the beach.

This would invite lawsuits in the United States from sunburned customers who had not placed it on the part of their body that received the most sun.

Health authorities have had trouble balancing sun-avoidance skin cancer messages with the need to spend some time in the sun during safer times to ensure adequate vitamin D levels.

For New Zealanders, most vitamin D is produced by the effect of ultraviolet rays on substances in the skin. A small amount comes from food and the vitamin can be taken in tablets.

Around half of New Zealand adults have insufficient vitamin D and 4 per cent of women and 2 per cent of men are vitamin D-deficient.

There is growing evidence having an adequate level can prevent or improve the outcome of many diseases including breast, prostate and bowel cancer, cardiovascular disease, diabetes and multiple sclerosis.

There is even a possible association between low levels and obesity.

Associate Professor Robert Scragg, of Auckland University, leads a study in which 500 volunteers in Auckland and Dunedin wore a UV dosimeter, which he said was a cutting-edge device, daily for eight to 10 weeks.

"The aim is to find out how much sun exposure we need to increase vitamin D levels by a certain amount."

Darker skin produced less, but the factors affecting the rate of production were not fully understood.

Dr Allen said UV dosimeters would be offered to high schools for use in science experiments to demonstrate the effects of sun-protection measures.

"If they can work out that the daily UV cycle around 12.30 to 1.30pm, it really is intense, it may work out better at changing behaviour," he said.

The Dosimeter:

* Can measure level of the sun's burning rays every few seconds.

* Electronic components convert ultraviolet light to electrical signals which are stored for later downloading to a computer.

* Mounted in a teflon case.

* Can be pinned on clothing or attached to a wrist-strap.