Sunscreen: you need a lot of it!

Published Dec 22, 2006

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A top radiation oncologist has raised a red flag on sunscreens, warning that only meticulous application will guarantee safety against skin cancer.

Professor Ben Smit, head of radiation oncology at Tygerberg Hospital in Cape Town, says that most people apply about half of the sunscreen needed.

Smit and his colleagues, writing under the Forum of Radiation Protection on Sunscreens and Ultraviolet Radiation, said the active ingredients in topical sunscreens could absorb up to 95 percent of UVA and UVB radiation, but laboratory testing was different from using sunscreen "in the field".

Variables that would alter efficacy include:

- the SPF (sun protection factor)

- the binding power of the sunscreen to the skin when a person sweats

- the amount applied per square centimetre of skin

- regular re-application every three to four hours

- how well it is applied.

Incidences of melanoma correlate with the number of hours spent in the sun and the country's latitude.

Smit said that while the SPF factor of sunscreen indicated protection under laboratory conditions, the "human factor" was all-important.

SPF values are obtained by applying 2mg per square centimetre of skin under experimental conditions, "quite a generous application".

To cover the body of a large adult weighting around 76kg, 800ml of sunscreen would be needed, and should be re-applied regularly.

"Studies have shown that the average individual applies no more than 1mg per square centimetre, effectively halving the SPF," said Smit.

He said the regular use of sunscreen with an SPF of 15 during the first 18 years of life reduced the lifetime incidence of basal cell carcinomas and squamous carcinomas by 78 percent.

"However, the proper application of sunscreen is assumed," he said.

And Smit warns that apart from skin cancers, an added complication in fair-skinned people is the development of cataracts.

Smit suggests that sunscreens with high SPF ratings "may offer some help", if applied "generously, thoroughly and judiciously".

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