SPF

SPF stands for sun protection factor. SPF is a number that represents how long a person can stay out in the sun without burning while using the product. It refers to the amount of ultraviolet energy required to produce a minimal redness or minimal erythema dose(MED) on unprotected skin. If a person normally burns in 10 minutes without sunscreen, an SPF-15 sunscreen will protect your skin for 150 minutes. However, there is no such thing as total protection from the sun, you will still get sunburn if you do not continuously reapply sunscreen while your skin is exposed to the sun.

You may start to think using a high number of SPF, you will never need to reapply sunscreen, but that simply isn't the case if you want to protect your skin. A higher SPF also means a greater concentration of chemicals, which can lead to a greater risk of allergic reactions.

Sun Damage

The sun projects three basic kinds of ultraviolet rays, or UV rays. UVC rays are short rays that are absorbed by the earth's ozone layer before they can enter the atmosphere. UVB are the next longest UV ray. It cause most skin cancers and are responsible for sunburns. They penetrate the epidermis to the basal layer. Although UVB rays are the ones responsible for sunburns, UVA rays penetrate deeper and are associated with breaking down collagen and elastin and leading to wrinkles. Although UVA don't cause sunburns, they do stimulate melanin production that can lead to pigmentation irregularities and cause long-term damage to skin.
People whose skins have been overexposed to sunlight during their younger age usually start to show signs of aging between the ages of 35 and 45, this is due to structural damage that affects the skin's elasticity. Once the skin has lost its elasticity, it cannot be restored. Many people may think that a deep tan is a sign of good health, but overexposure can be harmful.