I'm a really white guy and appreciate sun protection. I'm sitting in the shade right now. But ask yourself, when you take your shirt off after a day in the sun, have you tanned under your shirt? Is you nut sack tanned (or burned) after a day at the beach? All fabrics have UV protecting properties- some are better than others, some are very good. But, not having a UPF rating does NOT mean that the garment does not protect from the sun. It just means that the fabric has not been tested.
Farmer tan is the effect of UV protective clothing, and has been a thing for way longer than UPF has been a thing (testing invented in 1996). It used to be that SPF 15 sunblock was sufficient and anything over 30 was hype, now you ain't got shit unless it's SFP 50. Did the earth drift closer to the sun in the last 30 years?
Your bodies reaction to UV exposure is melanin production. If the melanin in your skin is insufficient, you burn. A friend of mine learned many years ago that "black don't crack" is just a cute rhyme.
I'm not saying that UPF is not necessary, just that the UPF tag is something you are paying a premium for, but it's a lot of marketing too. Tight weave, synthetic, reflective fabric will have a high UPF (double layer even more so)- with or without the tag. Heavy cottons like denim does too. If your clothing is giving you tan lines, you are probably wearing enough, even without the UPF tag.
