The sun sends out different types of radiation that reach the earth –visible light that is seen as sunlight, infrared radiation which can be felt as heat and ultraviolet radiation (UVR), that we can’t see. Ultraviolet radiation can be classified as UVA, UVB, or UVC. The ozone layer absorbs some, but not all, of these types of UV radiation. UVA rays account for up to 95% of the UV radiation reaching the earth’s surface, and can penetrate through clouds and glass. These UVA rays play a major part in photodamage and skin ageing, while UVB rays are the main cause of skin reddening and sunburn, damage which can lead to premature skin ageing.

Both UVA and UVB rays have the potential to cause skin cancers. Skin cancer is a disease of the body's skin cells and occurs when skin cells are damaged by UV radiation penetrating the skin. UV damages skin cell DNA, causing cells to mutate and grow abnormally. If these mutant cells are not destroyed through the body’s own processes or other means, they will continue to develop into skin cancers.

Sun Protection Factor (SPF) is a measure of a sunscreen’s ability to prevent UV radiation from damaging the skin. SPF measures the amount of UVB radiation transmitted to the skin after sunscreen has been applied, compared to the amount of UVB transmitted to the skin without sunscreen. A sunscreen with an SPF of 30 provides 30 times more protection than if not wearing a sunscreen. A sunscreen with an SPF of 50 provides 50 times more protection than if not wearing a sunscreen. SPF 50 provides around 98% protection, and an SPF 30 provides roughly 96.7% protection, 

There are many misconceptions about how much and how often we need to apply sunscreen. Even when we are indoors, in the office, or running daily errands, a minimum of SPF 30 sunscreen should be applied on a daily basis after cleansing. This is because UVA rays play a major long-term term premature skin ageing and can even initiate the development of skin cancers. Sunscreen should be applied at least 20 minutes before going outdoors and reapplied every two hours, or immediately after swimming or working up a sweat. A minimum of 5ml (approximately one teaspoon) of sunscreen should be applied to each limb, plus the face, and ears.