Numbering different grades of sunscreen can confuse users and even increase the risk of skin cancer, a researcher has warned.

Professor Brian Diffey, from Newcastle General Hospital, argues in the British Medical Journal that the current system should be scrapped and replaced by the use of terms such as "low", "medium" and "high" protection.

Professor Diffey argues that the numerical indicators lull people into falsely believing they are able to stay in the skin for long periods without getting burned.

He says that even those people who use high factor sunscreens get sunburned because they do not apply enough sunscreen.

Professor Diffey said that if sunscreens were applied properly, there would be no need for sun protection factors higher than 15 to prevent sunburn.

Children and people in strong sunshine for many hours would be encouraged to use high or ultra-high protection sunscreen.

Filtering out radiation

Those who wish to tan or are in shady areas might prefer medium or low protection products.

Professor Diffey argues that such a system would shift the focus more to protection, rather than encouraging prolonged exposure to sunlight.

Sunscreens increase the length of time someone can spend in the sun by filtering out the harmful forms of light in sunlight.

They are given sun protection factors (SPFs) that refer to the extra time one can spend in the sun before getting sunburn.

For someone who burns after 15 minutes exposure to sun, using a cream with a SPF of four would allow an hour in the sun before burning. Then it would be time to get out of the sun, not to re-apply cream.

Professor Diffey's warning follows research that found people who use sunscreen are more likely to develop skin cancer, both melanoma and the less-dangerous basal cell carcinoma.

There are three main types of skin cancer:

• Basal cell carcinomas, which are easily treated and rarely fatal
• Squamous cell carcinomas, which are also unlikely to lead to death
• Malignant melanoma, the least common but most aggressive form of skin cancer, which accounts for the vast majority of deaths from skin cancer

Dr John Buchan, a GP in Powys and member of the Primary Care Dermatology Society, said the current SPF system was unacceptable.

He said it was based on the false premise that people knew how long it would take their skin to burn without protection.

Different skin types burned at different rates, he said.

He said: "There is also an assumption that sunscreen is properly applied when there is plenty of work to show that people under-apply it, particularly in the UK where it is so expensive. Most people get 50% or less of the sun protection they think they are getting.

"I would welcome any move that would simplify and de-mystify the sun protection coding system that we have got at the moment and make it much more user friendly so people had more idea what they were doing."