I'm trying to untangle the relationship between the UPF/SPF rating of clothing or sunscreen and its effectiveness at reducing health risks from exposure to sunlight.

The first complication is that a UPF/SPF rating represents the denominator of the fraction of UV radiation that passes through the protective barrier. This means that there are diminishing returns on higher UPF/SPF ratings. For example, a mere UPF 5 blocks 80% of UV radiation. Doubling the UPF rating blocks half of the remaining radiation, which is only 10% of the original amount. Doubling it again only blocks another 5%. Et cetera.

The second complication is that the health effect of UV radiation may be non-linear. In other words, an 80% reduction in exposure may result in more or less than an 80% reduction in risk.

Ideally, I'm looking for an answer that takes these factors into account, expressing health risk as a function of UPF/SPF while holding exposure time constant.

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