Understanding the Ratings: What's the difference
between UPF & SPF?

SPF -
SPF is the ratio of time required to produce minimal erythema (redness) when a sunscreen products has been applied compared to the time required to produce the same amount of erythema without the sunscreen. This means, if skin reddening takes 20 minutes with a person who is using no protection, theoretically, the use of a sunscreen with an SPF of 15 would prevent reddening 15 times longer (about 5 hours).

UPF -
Australian researchers introduced the term Ultraviolet Protection Factor (UPF) in 1996. It defines the amount of Ultraviolet light (UVL) that penetrates a fabric. UPF is a ranking of fabrics according to how much UVL penetration occurs based on standardized criteria. For example, a UPF rating of 30 would indicate that 1/30 of the UVL hitting the fabric acturally penetrates it. Therefore, fabric with tighter weaves and thicker fibers will have a higher UPF.

Factors Affecting the Rating -

• Tightness of knit or weave: basically the tighter the weave, the higher the SPF/UPF. When a fabric is stretched, the tightness of the weave diminishes, resulting in increased UVL transmission. * Thicker fabrics habe higher SPF/UPF than thinner fabrics.
• When fabric gets wet, its SPF/UPF can decrease dramatically. Wet cotton can lose up to 50% of its SPF/UPF. This is because water reduces the scattering of UVL, thereby increasing its transmission of harmful ultraviolet rays.
• Dark colors are more protective than white. In a study of identical fabrics, which were either white or dyed, it was found that white cotton fabrics had an UPF of 12, whereas a similarly constructed black fabric had UPF of 32. In testing polyester, the studies showed that a white polyester was a 16 UPF and black polyester was a 34 UPF. The popular view that white is more sun protective than dark colors is erroneous.