Sun Protection Factor (SPF) a standard measurement to help quantify how long you can be exposed to UVB rays before getting burned. It is developed in a lab setting and true protection value depends on where you are, what you are doing and how well + how often you are applying / reapplying! So, always apply the recommended amount (1 oz - a shot glass full - for your whole adult body) and reapply every two hours or 40 minutes after swimming / sweating. It's important to know that no sunscreen can filter out 100% of the sun's UVB rays. That's why it's important to reapply regularly, wear protective clothing and seek shade when possible. For example: SPF 15 filters 93% of UVB rays, SPF 30 filters out 97% and SPF 50 filters out 98% … higher SPF than 50 give a false sense of security. SPF also does not indicate any protection from the damaging UVA rays, which cause premature aging. That’s why you must look for “Broad Spectrum” designation to ensure protection from both UVA + UVB. We believe ALL sunscreen should protect from UVA, UVB and even blue light/HEV/pollution as they can all lead to skin damage over time. Our products cover all of those bases.