Although a tan may look healthy and attractive, it is in fact a sign that the skin has been damaged. A sun tan is developed when the skin absorbs UV (ultraviolet) radiation from the sun rays, causing the skin to darken due to higher levels melanin. The darkened skin therefore becomes less prone to burning at the same level of UV exposure.
A higher level of UV radiation results in a higher risk of skin burning and damage. You can usually find out what the level of UV radiation is likely to be by looking at the weather forecast which should show the UV Index. The index starts at 0 and goes to 10+, with a higher index rating indicating a higher level of UV radiation. The BBC Weather Centre web site has a great explanation of the UV Index.
Sun screens have a Sun Protection Factor (SPF) which can range from 4 upwards. The higher the SPF, the greater protection you will achieve. Dark skinned people and those which do not burn may find a low SPF is suitable for their needs, but it is worth noting most skin experts recommend a sun screen with a SPF of at least 15 so you limit your risk of skin damage. Pale skinned people and those prone to burning may wish to opt for a much higher SPF, such as 35-40.
Sun screen will not prevent you tanning; it will offer some protection so you avoid sun burn and excessive skin damage. This is assuming you pick a sun screen with a suitable SPF, however.
Sun block is different to sun screen – as its name suggests, sun block is designed to avoid any skin damage by totally blocking UV radiation (and therefore tanning).
It is a myth that sunburn will fade into a tan. Sunburn is visible damaged blood vessels on the surface of the skin, and should be avoided at all costs due to the risks involved. Sunburn can lead to sun spots, skin inelasticity, and skin cancer. So, how can we avoid sun burn?