Recently issued public health guidance on skin cancer prevention from the National Institute for Health and Clinical Excellence (NICE) recommends that, used properly, a sunscreen with a sun protection factor (SPF) of at least 15 is enough.1 On the face of it, this advice seems reasonable given the theory that sun protection, whether from a sunscreen, a shirt, or shade, need only have an SPF of 15 to prevent sunburn from all-day exposure to tropical sunshine.2 The problem, though, is that a sunscreen only …