Joe Campbell reports on the need for a re-think on sun protection

Research published this week suggests that the molecule, found in the upper layer of the skin, reacts with both UVB and UVA light to produce the leathery, wrinkled look associated with old age.

The trans-ucranic acid molecule was an ingredient in sun tan lotions in the 1950s.

The discovery, made by Dr John Simon and Dr Kerry Hanson of the University of California in the USA, brings scientists a step closer to knowing why exposure to sunlight causes premature ageing.

Normal exposure

About 90% of the visible signs of ageing are thought to be due to sunlight.

Around 95% of the sunlight which reaches the earth's surface comes from UVA light.

UVB light, which is more damaging, is mostly blocked by the ozone layer.

However, the new research shows that the trans-urocanic acid molecule, formed when the outer layer of skin cells die, reacts with normal exposure to UVA light to produce the oxygen free radicals that damage skin.

The amount of damage done increases with UVA wavelengths.

Models like Claudia Schiffer stay away from the sun's damaging rays

The scientists say absorption is often weak, which could explain why the signs of ageing take a long time to appear.

"The initiating molecule has a weak efficiency for absorbing the light, but when it does, it is highly efficient in causing the damaging event," they write in the Proceedings of the National Academy of Scientists.

Drs Simon and Hanson say their research shows that people need to think about protecting themselves from exposure to UVA light as well as UVB.

No standard Trans-urocanic acid was described as a natural sunscreen in the 1950s as it was thought to protect the skin from UV light damage.

It was used in sun tan lotions until it was found to cause immune system damage.