UV radiation and sunburn

UVA rays

Between 10-50% of the UVA rays that reach the skin pass through the outer layers of the skin into the deeper layers. This activates the melanocytes that produce melanin pigment in reaction to UVA rays in the sunlight.

UVA and UVB and skin
Cross section of Epidermis with Melanocytes in
the middle layer.

 

This tans your skin but also speeds up the ageing of your skin. The pigmentation has the effect of absorbing the UV radiation in sunlight to protect the cells from UV damage. In other races than Caucasians eumelanin production is continuous, so the skin is always pigmented to some degree. People with red hair produce more phaeomelanin, which is why they usually do not tan very well.

 

UVB rays

UVB rays burn the skin and thus increase the risk of skin cancer. Only 1-10% of the UVB rays are able to penetrate the skin's outer layer because of their shorter wavelength. They do not reach the melanin producing cells. Burnt skin has to regenerate itself, and it is during this process that mutations that lead to skin cancer can occur. The more frequent and severe the burns are, the greater the risk of developing skin cancer. The earlier in life these burns occur, the greater the risk.
The ability of the UVA and UVB rays to penetrate the skin is illustrated to the left.

 

Sunburn

In human skin, UV penetration is actually very slight. Ultraviolet radiation usually doesn't make it past the Epidermis - the top layer of the skin. When you get sunburn, your skin is damaged by UVB radiation and your body is responding to the damage.
A sunburn is really a delayed UVB-induced erythema (redness and inflammation) caused by an increase of blood flow to the affected skin that begins 4-24 hours following exposure. The cause of this reaction is the reparation of the damage that has been caused to specific cells. Erythema is a marker for severe UVB damage.
Moreover, some medicines can cause light hypersensitivity reactions that cannot be prevented by using a sun protection product.

Did you know?
The sun's UV rays are reflected by snow, water and sand, thus exposing your skin to higher doses. Snow reflects up to 85%, sand reflects about 17% and water 10% of the sun's UV rays.