Table of contents:

Part 1: In which I use my optical physics background to share some hopefully-uncontroversial observations
Part 2: In which I boldly defy Public Health Orthodoxy on the whole UV situation

Part 1: In which I use my optical physics background to share some hopefully-uncontroversial observations

1.1 UV depends a lot on “solar zenith angle” [a.k.a. “angle of the sun away from directly overhead”], not on how hot it is outside

That means: you should mainly be thinking about UV exposure in proportion to how close it is to (1) the summer solstice and (2) solar noon.

Here, I made this handy widget.[1] Select a city in the drop-down at the bottom, and mouse over (or tap) the colored area for specific datapoints:

There's an interactive widget here in the post.

I find that people intuitively judge sunburn risk based on temperatures being high, instead of shadows being short. So they worry about UV too much in the hot late summer, and/or not enough in the cool early spring; and they worry about UV too much in hot late afternoons, and/or not enough in cool late mornings.

(Of course, temperature matters indirectly, because if it's hot [...]

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Outline:

(00:25) Part 1: In which I use my optical physics background to share some hopefully-uncontroversial observations

(00:35) 1.1. UV depends a lot on solar zenith angle \[a.k.a. angle of the sun away from directly overhead\], not on how hot it is outside

(02:16) 1.2. Other things matter too, so just check your local UV index

(02:57) 1.3. Around half of UV is diffuse (mostly coming from the blue sky) not direct

(03:40) Part 2: In which I boldly defy Public Health Orthodoxy on the whole UV situation

(05:17) 2.1. I lean towards: (1) sunburns are bad, (2) tans are neutral (in themselves), (3) tans are good all things considered (because they prevent sunburns), (4) Sunscreen is for sudden transitions in sun exposure, and then you should try to wean off it

(08:23) 2.2. Wear sunglasses for comfort if you want, but theyre not a health product

(09:34) 2.3. An appropriate effective SPF in most situations is usually like 3, maybe up to 10 tops

The original text contained 6 footnotes which were omitted from this narration.

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First published:
April 9th, 2026

Source:
https://www.lesswrong.com/posts/t7GeZngqtzW49HceY/some-takes-on-uv-and-cancer

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Narrated by TYPE III AUDIO.

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“Some takes on UV & cancer” by Steven Byrnes

LessWrong (30+ Karma)