The SPF has noting to do with the amount of time to re-apply. Instead, SPF ratings reveal how effectively they shield your skin from harmful ultraviolet (UV) rays. UV rays come in two main types, UVA and UVB, each penetrating the skin differently. SPF, or Sun Protection Factor, specifically denotes protection against UVB rays, which are known for causing sunburn and skin damage.
Here's the breakdown: SPF 15 blocks about 93% of UVB rays, while SPF 30 ups the protection to around 97%. Moving higher, SPF 50 reaches approximately 98%, and SPF 100 offers about 99% protection against UVB. Beyond SPF 30, the incremental increase in protection becomes minimal.
It's crucial to note that SPF ratings are determined under controlled conditions in labs, where a specific amount of sunscreen is evenly applied and exposed to a standardized light source. In real-world scenarios, variables like sun intensity and how much sunscreen is applied make exact protection levels harder to predict.
In general, SPF 30 is often considered sufficient for daily use, offering robust protection against UVB rays when applied correctly. Pick a brand you trust and get used to how it works and its protection. Application of a SPF 30 every couple of hours is the current advice for someone staying out in full sun.