According to this article we should buy sunscreen with at least a factor 30, in order to prevent skin cancer. If the article is to be believed current guidelines from NICE (National Institute of Clinical Excellence) which recommend using at least facto 15 sunscreen are flawed. It seems that many people simply do not apply enough sun screen, often enough to be protected when using the lower factor. For factor 15 sun screen to be effective, it needs to be applied at a thickness of at least 2mg per cm2 when in reality most people apply less than than half this amount. it would also need to be reapplied frequently, at least every two hours, meaning for it to be fully effective you would need to use a 200ml bottle every 2 to 3 days. I tend to always buy the highest factor on offer, I’m not taking any chances with my pasty white skin.