For example, if a sunscreen is rated SPF 2 and a fair-skinned person who would normally turn red after ten minutes of exposure in the sun uses it, it would take twenty minutes of exposure for the skin to turn red. A sunscreen with an SPF of 15 would allow that person to multiply that initial burning time by 15, which means it would take 15 times longer to burn, or 150 minutes. Dermatologists strongly recommend a sunscreen with an SPF of 15 or greater year-round for all skin types . SPF protection does not actually increase proportionately with a designated SPF number. One study determined that skin protected by an SPF 15 sunscreen and then exposed to 15 times the minimum dose of sunlight normally required to cause redness produced 2.5 times the number of sunburn cells seen in SPF 30 protected skin with the same dose of sunlight.
More research is currently underway on the protective effects of sunscreens on different skin types.
They also contribute to skin burning and skin cancer. Both UVA and UVB rays can cause suppression of the immune system which helps to protect you against the development and spread of skin cancer. The SPF number on sunscreens only reflects the product's screening ability for UVB rays. At present, there is no FDA-approved rating system that identifies UVA protection. Scientists are working to create a standardized testing system to measure UVA protection.