Indicators of sun screen protection should be qualitative rather than quantitative, a photobiology expert has suggested. Professor Brian Diffey (professor of photobiology, Newcastle General hospital) says that numerical labelling has led to "more confusion than clarity".
He suggests a system where sun protection factors (SPF) 4 to 7 would be labelled as providing low protection, factors 8 to 14 medium protection, 15 to 24 high protection and 25 and above ultra-high protection. Products with an SPF of less than 4 would not be classified as sunscreens. High and ultra-high sunscreens would be recommended for children and people who planned to be in strong sunshine for a long time (British Medical Journal 2000;320:176).
Professor Diffey, who invented the "star" rating system for measuring UVA protection, says that the reason that people who use high factor sunscreens still get sunburnt is that they do not apply the creams appropriately. If people applied sunscreens uniformly "there would be no need for sun protection factors higher than 15". Sunscreen protection is tested at an internationally agreed thickness of 2mg/cm2. However, in practice, people apply creams much less thickly, typically between 0.5 and 1.3 mg/cm2, he says. This means that consumers have a mean effect of 20 to 50 per cent of that expected.
A manufacturer would be reluctant to test sunscreens at a reduced thickness, say 1mg/cm2, because, for example, creams currently labelled as factor 20 would become factor 10, putting the manufacturer at a commercial disadvantage, Professor Diffey explains.