You say, 'the number on the bottle is not directly proportional to the amount of radiation screened out. Factor 50, for example, is not twice as effective at blocking UVB radiation as factor 25'. But if factor 50 is letting through half the amount of UVB, does that not make it, in a sense, twice as effective? In terms of skin damage, the 98% versus 96% of radiation blocked feels like the wrong comparison: Surely what counts is the 2% versus 4% let through?
You say, 'the number on the bottle is not directly proportional to the amount of radiation screened out. Factor 50, for example, is not twice as effective at blocking UVB radiation as factor 25'. But if factor 50 is letting through half the amount of UVB, does that not make it, in a sense, twice as effective? In terms of skin damage, the 98% versus 96% of radiation blocked feels like the wrong comparison: Surely what counts is the 2% versus 4% let through?
Great explanation, Kit. Some research has also found benefits of UV exposure in low sunlight countries (like Scotland): https://www.ed.ac.uk/news/2024/uv-rays-may-boost-health-in-low-sunlight-countries