CONTENTS

Introduction
Human Skin Cancer
Sunscreen and Fabric
The Mouse Model of Cancer
Studies Using Skin Tissue
Drugs and Sunlight
Plant and Algae Growth
Conclusion
Glossary
Bibliography

Sunscreen Testing

Sunscreens are required by law to be labelled with the sun protection factor (SPF) they provide. The sun protection factor, which is the number you see on the label of a sunscreen, indicates how many times longer it will take a person to get a minimal skin reddening or sunburn whilst they are wearing the sunscreen than if they were not wearing any protection. If a sunscreen has a SPF of 10 then a person wearing it would take ten times longer to get sunburnt than if they were unprotected. (It is most important that people use sunscreens according to the instructions on the packaging which includes application at least 15-20 minutes prior to exposure and repeat applications at least every 2 hours.)

A sunscreen is not like a battery that runs out after a certain amount of time or irradiation, rather it is just like a filter on the skin so it is a letting ultraviolet light thorough from the moment you put it on. This is why Greenoak argues that people should use a 15+ sunscreen all the time. “Say two people go out in the sun, one with an SPF 4 on and the other with an SPF 15+ on and they're only out in the sun for twenty minutes. Neither of those individuals will be sunburnt. The person with the 4 on will have more ultraviolet damage than the person with the 15+ on.” This is because the 15+ sunscreen will be blocking out more ultra violet light than the 4 sunscreen from the moment that the person steps into the sunlight.

Gavin Greenoak and Jules Martin run the Australian Photobiology Testing Facility which tests sunscreens to work out their sun protection factor. They test these sunscreens mainly on student volunteers who are paid for this. “All they have to do is sit still and if the sunscreen is being tested for its water resistance then they get to sit in the spa and have a lovely time,” says Greenoak. “They can't come back until at least 6 weeks later, and often they want to return earlier because they enjoy it so much. They usually come back for more.”

When a student volunteer first comes to the testing facility their backs are studied to see whether they are suitable. A good back is relatively unmarked, and has not already been exposed to too much sunlight. No one is tested who has any personal or family history of skin cancer. The student is then given a consent form which tells him or her what the testing involves. Before they sign it students can ask any questions they like about the testing and its health effects.

The requirement for students to give consent comes about because they are exposed to UV light during the testing and, although it is only a small amount on a small patch of skin, it is still adding to the cumulative dose of ultraviolet exposure that each student is exposed to. They have to be informed of that. “They probably get as much UV radiation as they would if they were just walking out to get their sandwiches on a sunny day,” Greenoak explains.

Once they have agreed to go ahead they are tested to find out the amount or dose of simulated sunlight that they can be exposed to before their skin begins to turn slightly red. This is called their “minimal erythemal dose”. The minimal erythemal dose will vary from person to person depending on their skin type. Fairer people tend to go red more quickly than darker people who have more pigment in their skin.

To find out how long it takes for the volunteer to turn red, he or she sits in front of the solar simulator which is designed to emit only a centimetre diameter of radiation. The solar simulator consists of a 150 watt xenon arc beam which is passed through filters and lenses to take out the infrared light and the shorter UV wavelengths that are not found in sunlight. The remaining light waves are then emitted as a one centimetre beam focussed on the skin through a template that the person’s back rests against. The radiation from the solar simulator is stronger than sunlight although the distribution of the various frequencies of light (the spectrum of light) is similar. If a fair skinned person sat on the University oval at midday in Summer, it would take them about 9 to 15 minutes to start to get sunburnt. With the solar simulator it takes between 10 and 16 seconds.

Because Greenoak and Martin have tested so many people they are pretty good at judging how many seconds it will take before the skin begins to redden with their solar simulator. They do a series of exposures (for each one a centimetre diameter circle of skin is exposed) within the range of time that they have reckoned, on different parts of the back so they can work out what that person's minimal erythemal dose is. This forms the basis for testing any sunscreen on that person.

Sunscreens which are to be tested by Greenoak and Martin are submitted by sunscreen companies with an estimated sun protection factor. The estimate is based on the ingredients of the sunscreen and an analysis of its probable screening ability. The sunscreen is put on a volunteer (who has had their minimal erythemal dose measured) and they are given a series of exposures from the solar simulator on one centimetre circles on their back. If the estimated SPF is 10 and the volunteer has an estimated minimal erythemal dose of 11 seconds then after 110 seconds (10x11 seconds) Greenoak and Martin would expect that the skin would redden.

They do five exposures with an average time of 110 seconds, some a bit more some a bit less. The volunteer is also tested in the same way with a sunscreen which has a known SPF (the control sunscreen). The volunteer then goes away and comes back within 16 to 24 hours so the result of the exposures with the test sunscreen can be compared against the control sunscreen. They come back 16-24 hours later because that is how long it takes for the redness to show. This procedure is normally carried out with 10 volunteers so that the real SPF of that sunscreen can be worked out.

It is assumed that the cause of sunburn is the same in everyone and that a sunscreen with an SPF of 15 allows a person to spend 15 times as long in the sun before becoming red, whatever their skin type.