UPF (Ultraviolet Protection Factor) rates how effectively a fabric blocks UV radiation. UPF 15-24 = good, 25-39 = very good, 40-50+ = excellent. Dense weaves and darker colours increase UPF. Linen (UPF 15-40 depending on weave), polyester (UPF 30-50+), and denim (UPF 1700) vary widely. Relevant for outdoor, hiking, and beach contexts in high-UV climates (tropical, arid-desert, high-altitude).