The Sun's UV rays are even more damaging than once thought, UK researchers say.
The Sun emits two kinds of UV rays that reach the Earth's surface: UVA and UVB.
It has long been thought that these rays don't damage the deeper layers of the skin as much as they damage the top layers.
But new research from King's College London has found that while it's the case for UVB rays, it isn't for UVA.
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 salons.
"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," ABC Science quoted Dr Antony Young, a professor of experimental photobiology at King's College, as saying.
That is significant because the deepest layer of the skin, the basal layer, is where cells divide.
"If they carry mutations that has possible consequence in terms of skin cancer. We must try to protect the basal layer," said Young.
Adding concern to this latest finding, is that UVA rays are more prevalent than UVB.
"In sunlight, UVB represents at most 5 or 6 per cent of the UV rays, so the vast majority is UVA," said Young.
The study has been published in Journal of Investigative Dermatology.