When you apply sunscreen you might not be as well protected from the sun as you think, say researchers in Italy.

Consumers have come to rely on the protection provided by sunscreen to prevent skin damage from the sun's harmful ultraviolet (UV) radiation. 

But chemists led by Nick Serpone and Angelo Albini at the University of Pavia have discovered that compounds routinely used as UV filters in commercial sunscreens do not have the protecting power previously thought. Exposing these compounds to UV light of the same wavelengths found in sunlight causes the molecules to degrade and lose their protecting abilities.  And the researchers found that using certain combinations of compounds actually worsens this effect.

Most sunscreens with high sun protection factor (SPF) contain two organic chemical filters, one for UVA and the other for UVB, which absorb the harmful rays. They also have a reflective component such as titanium dioxide. 

Two of the most widely used UV filters in commercial sunscreens, cinnamates (for UVB) and dibenzoylmethanes (for UVA), break down irreversibly when exposed to sunlight. The reaction is caused by UV light and forms products that do not have the filtering properties needed for sunscreen. When these two common filters are combined together, they are even more sensitive to light than either of the compounds alone.

'Much more detailed investigation on the use and regulation of sunscreens must be carried out on these and other compounds,' said Albini.

 'These results are important for the evaluation of how efficient these compounds are as protectors against UV radiation, and also how fast they disappear from the skin surface,' said Axel Griesbeck, an expert in photochemistry at the University of Cologne, Germany. 'It's a factor that is often ignored in the application of these sunscreens.'

Sophia Anderton