UPF stands for Ultraviolet Protection Factor — a rating assigned to fabrics that measures how effectively they block the sun’s broad-spectrum UV rays before they reach your skin. Like SPF for sunscreen, UPF measures protection, and every fabric has a rating whether listed or not. For instance, a standard white t-shirt has a UPF of just 3. Shade’s proprietary CottonUPF+ formula, however is certified UPF 40+ under AATCC’s TM183 standards —making it the only 100% cotton white tee on the market with built-in sun protection.

Several factors affect the sun protection level of clothing: the tightness of the weave, the type of fiber, color, and any special treatments applied to the fabric. Some materials, like unbleached cotton or certain polyesters, naturally block more UV, while others are enhanced through non-toxic treatments to boost their protective qualities.