SPF, or Sun Protection Factor, is a measure of how well sunscreen protects your skin from the sun's harmful UVB rays. It's calculated by comparing the amount of time it takes for protected skin to redden to the time it takes for unprotected skin to redden.
Here's a simplified explanation:
- Scientists test sunscreen on human volunteers in a lab. They apply sunscreen to one patch of skin and leave another patch unprotected.
- They expose both patches to controlled UV radiation.
- They measure the time it takes for each patch to turn slightly red.
- The SPF number is calculated by dividing the time it takes for the protected skin to redden by the time it takes for the unprotected skin to redden.
For example, if it takes 10 minutes for unprotected skin to redden and 100 minutes for sunscreen-protected skin to redden, the SPF would be 100/10 = 10. This means the sunscreen provides 10 times the protection against UVB rays.
It's important to note that SPF only measures protection against UVB rays, which are responsible for sunburn. It does not fully protect against UVA rays, which can cause premature aging and skin cancer.
