there are two types of UV light - UVA and UVB light. over exposure to UVB light is known to cause skin cancer and cataracts. repeated exposure to UV light can damage fibres called elastin in the skin, leading to premature ageing. UV light is partially filtered by haze or clouds in the atmosphere; the UV content is greater at high altitudes, where the cloud cover is thinner. However, you can still burn on a cloudy day at lower altitudes because some UV light penetrates clouds. both UVA and UVB rays age the skin but it is UVA rays which cause tanning. sunscreens protect the skin by absorbing UVB rays, thereby reducing the amount that enters the skin; sunblocks provide a physical barrier that reflects or scatters UVA and UVB rays. Sunscreens are graded using a sun protection factor (SPF), which is a measure of the level of protection they provide against UVB rays. the higher the rating, the greater the protection. basically it is UVA rays that r transmitted by sunbeds to give u the tan so a normal SPF doesnt do much as they r designed to protect from UVB rays