Many materials like cotton will let sunlight through quite readily, especially when thin or wet. A sun hoodie has 50+ UPF. A cotton shirt that has a similar low thickness will have a mere 5 UPF, making it basically worthless for sun protection. Sun hoodies also often sport features like vents to catch breezes.

Negative, cotton is basically like wearing nothing. This user have a good explanation.

https://lemmy.ml/comment/2295421