SPF stands for sun protection factor, which is a relative calculation of how long sunscreen will protect you from ultraviolet (UV) radiation. UVB rays primarily impact the epidermis, the skin’s outer layer. They cause sunburns and certain skin malignancies on the surface. The sun also releases UVA rays, which can enter into the dermis, the bottom layer of skin. UVA photons are commonly linked to “tanning.” The darker tone of the skin, on the other hand, indicates dermal cell injury. SPF ratings are normally only for UVB radiation, although certain sunscreens can also protect against UVA rays.
How does the SPF tell you how long you’re safe? Assume you generally burn after 30 minutes of being outside and plan to apply an SPF 15 sunscreen. In this scenario, you’d increase 30 by the SPF, which is 15. That implies you’re theoretically safe for 450 minutes, or 7.5 hours—but keep in mind, this is just technically true. Most sunscreens will fade more sooner due to exposure to the outdoors as well as incorrect application. It is advised to reapply every two hours. The quantity of UVB rays you are protected against rises somewhat with SPF. SPF 15 filters out 93% of UVB rays, SPF 30 filters out 97%, and SPF 100 filters out 99%.
A simple formula is employed to calculate the SPF number. The time it takes for a patch of skin to mildly redden while coated with sunscreen is divided by the time it takes to slightly redden when no sunscreen is applied. Assume that it takes 300 seconds for skin to burn with sunscreen and 10 seconds without. 300 divided by ten equals 30. The SPF rating is 30.
There are a few more elements to consider when it comes to the SPF of a sunscreen. The first is that no matter how high the SPF, some UV rays can always penetrate the skin. No sunscreen can ever entirely prevent sunburn and skin damage; sunscreen is merely one component of good sun protection, not the first line of defense.
The second is that SPF is calculated by tests that use a large amount of sunscreen. That implies that in order to get the protection specified on the label, you must apply sunscreen liberally enough—which many people fail to do. You should also reapply sunscreen every two hours since it might be rubbed off by clothes or sweat.
Finally, before using any sunscreen, make sure to check the ‘use by’ or ‘expiry’ date. If it is out of date, some of the active components may have degraded and the sunscreen will no longer function properly. Also, if it’s not a thick cream, shake it before applying.