SPF is short for Sun Protection Factor. Many consumers will read a high SPF written on pack to be a high sun protection factor, and this is true but remember that applies for the vast majority products claiming SPF at time immediately following application and for maybe a couple of hours following. Note that in order for SPF indication to be a real guide to the effectiveness of the screen, the SPF needs to not erode quickly off the skin. The SPF rating would be best if it’s long lasting. So say if you expose skin to the sun for four hours and you are looking for SPF 30 protection for that time, you would require the screen to be effective at SPF 30 for all that time (filtering approximately 97% of burning UVB rays) , or you may well risk skin damage with rapid erosion of product from skin. Sunscreen manufacturers are instructed to state re-apply often on pack in order to comply with the ‘standard’ of the region, which you would have to say is wise for all water based sunscreens that are naturally repelled by the skin. For example if you are fair skinned you may have an unprotected non-burn time of 10 minutes. Apply SPF 30 and your protected non-burn time goes to 10 X 30 = 300minutes. You need to assess if the screen you use remains effective at SPF 50 after application for the period of time your skin is exposed to the sun, or re-apply often. Rule of thumb: SPF 15 filters 97% burning UVB rays. Jump to SPF 30 and you filter at 97%. SPF 50, that goes to 98%. So what is SPF 100? We think the answer is ‘marketing’ ! How it applies to you lies mostly with the screens ability to not erode with water, not sweating off the skin, not rubbing off under clothes and just being long lasting.