SPF 15 means that the skin can be exposed to 15 times more UVB radiation without being sunburned, compared to when you are not SPF protected. SPF 30 means that your skin can tolerate 30 times more UVB radiation without being sunburned, compared to when you are not SPF protected. You should choose SPF based on how much sun your skin can tolerate without wearing sun protection. As an example, if you have darker skin that is good at tolerating the sun and wear the recommended amount of sunscreen as well as reapply frequently, an SPF 15 might be enough. However, if you have lighter skin or are buying sunscreen for your kids, then you need higher protection, and you need to reapply more often.
Sunscreen should not be used to extend the time spent in the sun, but as a protection against the UV rays when being exposed to the sun.
It can easily be misunderstood that SPF 30 protects twice as long as SPF 15. However, the difference in how much the SPF protects against the UV rays is not that big. Sunscreen with SPF 15 protects against 93 % of the UVB radiation, while SPF 30 protects against 97 % of the UVB radiation, and SPF 50 protects against 98 % of the UVB radiation. Karmameju SUN uses broad-spectrum sun filters from Titanium Dioxide, which protects against both UVA and UVB.
NOTE! To achieve the correct sun protection, it is essential to apply a generous amount of sunscreen. As such, you have to apply a properly covering layer on the skin, as a layer too thin can reduce the level of protection significantly