Sunscreens fails to prevent free radicals
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Sunscreens fail to offer full protection
17:30 29 September 03
NewScientist.com news service
Sunscreen creams do not adequately protect against all the Sun's harmful rays, warns a new study. Researchers found that current creams only blocked about 50 per cent of one type of ray thought to be linked to the deadliest form of skin cancer.
Sunblock creams do fully block ultraviolet-B rays, but not UV-A, say the team at the Restoration of Appearance and Function Trust (RAFT) based at Mount Vernon Hospital in Middlesex, UK.
UV-B causes sunburn and can lead to the most common types of skin cancer - basal cell and squamous cell carcinoma. UV-A increases the production of highly reactive chemicals known as free radicals, which are also known to have carcinogenic effects. Scientists believe UV-A exposure is responsible for the most lethal skin cancer - malignant melanoma.
"We basically saw the sunscreen only cut down the amount of free radicals produced [by UV-A] in the skin by half," says Claire Linge, one of the team and head of cell biology at the RAFT Institute. "If you expect UV-A protection to be similar to UV-B, you would expect it to be cut down to a thirtieth."
Mark Birch-Machin, a skin cancer expert at Cancer Research UK, says: "The message from this study is that sunscreens do not provide total protection against skin cancer. They are almost like a last line of defence. People should stick on a hat, a T-shirt and stay in the shade."
The team used pieces of human Caucasian skin donated by surgery patients and three commercial sunscreens with Sun Protection Factor's above 20. The SPF is a measure of how much longer a person can stay in the Sun without burning.
A little known star-rating system also exists in the UK to indicate levels of UV-A protection. The two UK products had four stars - the highest level of protection - and the third was believed to be equivalent.
The researchers placed the treated skin inside Electron Spin Resonance spectrometers - machines that can measure free radicals. The team looked for levels of a "telltale" buffer radical called ascorbate, which cells produce in response the presence of damaging radicals.
When the sunscreen creams were applied at the recommended level of two mg/cm² the UV-A induced radicals were reduced by only 55 per cent compared to unprotected skin. However, most people do not use sunblock at the recommended levels. When the screens were applied as they are commonly used (at 0.5 to 1.5 mg/cm²), free radical production fell by only 45 per cent.
Linge says an "absolute link" between UV-A exposure and melanoma has not been established. UV-B was traditionally viewed as more dangerous because its photons are more energetic than UV-A's and are thought to directly disrupt DNA, she says.
But now UV-A is thought to cause similar damage by interacting - via free radicals - with proteins, lipids and DNA.
Nonetheless, Linge stresses: "We are not saying 'don't use sunscreen' - it's as good as current technology will allow." She told New Scientist that the team is now collaborating with industry to develop a cream that will also shield against UV-A.
Journal reference: Journal of Investigative Dermatology (DOI: 10.1046/j.1523-1747.2003.12498.x)