There is new evidence that the sun's UV rays are even more damaging than once thought.

The sun emits two kinds of UV rays to the earth's surface: UVA and UVB.

It had been thought that those rays do not damage the deeper layers of the skin as much as they damage the top layers.

New research from Kings College London has found that is the case for UVB rays, but not for UVA rays.

The study has found UVA rays are more carcinogenic than previously realised - a finding scientists say underscores how important it is to limit exposure to the sun and to tanning studios.

The study was led by Antony Young, Professor of experimental photobiology at King's College.

"The damage seemed to increase as it went through the epidermis and we think that is due to a form of backscatter in other words, the damage goes through and is somehow reflected back," he said.

That is significant because the deepest layer of the skin, the basal layer, is where cells divide.

"Dividing cells or cells that divide to produce the rest of the epidermis, if they carry mutations that has possible consequence in terms of skin cancer. We must try to protect the basal layer," Professor Young said.

The research suggests that UVA rays are more carcinogenic than previously realised.

The problem is, they are also more prevalent.

Sorry, this audio has expired New warning over sun's dangers

"In sunlight, UVB represents at most 5 or 6 per cent of the UV rays so the vast majority is UVA, whereas with UVB the intensity is very dependent on the height of the sun," Professor Young said.

"We advise people to avoid the sun at noon for example. With UVA, there is much less variation with the height of the sun so the strength of UVA is reasonably constant throughout the day and doesn't change so much with season compared to UVB.

"So we are exposed to quite a lot of UVA."

Lines of defence

Professor Michael Kimlin, the director for the Centre for Research Excellence in Sun and Health, says it is also more evidence of the dangers of sunbeds.

"Depending on which brand and style of solarium, it certainly does alter what type of UV is emitted but typically, typically it's UVA exposure and a little bit of UVB," he said.

He says most sunscreens on Australian shelves are broad-spectrum, which protect against UVB and UVA rays.

"The Australian standard does acknowledge that UVA can be harmful and most of the sunscreens used in the market today are actually what they call broad-spectrum including UVB and UVA protection," he said.

"But I guess the message is that sunscreens shouldn't be the first line of defence.

"Really, this study shows we do need to look at limiting our exposure and trying to stay out of the sun where possible."

The research has been published in the Journal of Investigative Dermatology.