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I read on https://en.wikipedia.org/w/index.php?title=Sun_protective_clothing&oldid=930941768:

A relatively new rating designation for sun protective textiles and clothing is UPF (Ultraviolet Protection Factor), which represents the ratio of sunburn-causing UV measured without and with the protection of the fabric. For example, a fabric rated UPF 30 means that, if 30 units of UV fall on the fabric, only 1 unit will pass through to the skin. A UPF 30 fabric that blocks 29 out of 30 units of UV is therefore blocking 96.7%. Unlike SPF (Sun Protection Factor) measurements that traditionally use human sunburn testing, UPF is measured using a laboratory instrument (spectrophotometer or spectroradiometer) and an artificial light source, and then applying a sunburn weighting curve (erythemal action spectrum) across the relevant UV wavelengths. Theoretically, human SPF testing and instrument UPF testing both generate comparable measurements of a product's ability to protect against sunburn.

From my understanding, this means that SPF should always be equal to UPF, i.e. that they measure the exact same thing.

However, both https://www.blockislandorganics.com/Blog/post/2018/07/11/Is-There-A-Difference-Between-SPF-UPF.aspx (mirror) and https://blog.coolibar.com/whats-the-difference-between-spf-and-upf/ (mirror) claim that UPF measures UVB and UVA protection whereas SPF only measures UVB (which contradicts Wikipedia, since the latter claims that both SPF and UPF are erythemally weighted). Also, if SPF is equal to UPF, then why having two metrics?

This confuses me. What is the difference between SPF (Sun Protection Factor) and UPF (Ultraviolet Protection Factor), if any?

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SPF and UPF both have very similar purposes.

But the are metrics for different things - the first is for rating sunscreen creams, they latter is for apparel. The measurement method is different in both (one measures how much gets through, the other measures how much is filtered out).

Sunrays have two types of UV rays (UV-A and UV-B, the former causes aging, the latter causes skin burn, and they both increase the risk of skin cancer). UPF measurement considers both UVA and UVB, whereas SPF considers only UVB.

The first mirror link in the answer already has sufficient information to answer the question, as I understand it. So I am just going to copy the relevant section below.

SPF

  • Only measures UVB protection (make sure the sunscreen is labeled “broad spectrum” to also get UVA protection)
  • Is the percentage of UVB rays filtered out
  • Applies to sunscreens

UPF

  • Measures UVB and UVA protection
  • Is the fraction of the sun’s UV rays that reach the skin
  • Applies to clothing
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  • Thanks! In your answer: "UPF: Measures UVB and UVA protection" -> doesn't that contradict the Wikipedia quote in the question, especially the part "UPF is measured using a laboratory instrument (spectrophotometer or spectroradiometer) and an artificial light source, and then applying a sunburn weighting curve (erythemal action spectrum) across the relevant UV wavelengths.", since from my understanding the erythemal mostly care mostly about UVB and barely about UVA?
    – Franck Dernoncourt
    Jan 27, 2020 at 7:20
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    After erythemal weighting, the proportion of UV A is 17%. The weighting is based on a model for the susceptibility of the caucasian skin to sunburn (erythema) temis.nl/uvradiation/info/uvindex.html
    – dakini
    Jan 27, 2020 at 12:28
  • thanks, according to en.m.wikipedia.org/wiki/Sunscreen#Sun_Protection_Factor SPF also seems to be using erythemal weighting, i.stack.imgur.com/v0QQl.jpg, doesn't it?
    – Franck Dernoncourt
    Jan 27, 2020 at 12:41
  • Yes it is. Oddly enough, the same weighting function is also sometimes used for non-dermatological situations.
    – dakini
    Jan 27, 2020 at 13:16
  • thanks, if both SPF and UPF use erythemal weighting, then aren't they equivalent?
    – Franck Dernoncourt
    Jan 27, 2020 at 13:20

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