(Ultraviolet Protection Factor) The UPF rating indicates how effective a fabric is at blocking out solar ultraviolet radiation from reaching the skin. UPF ratings range from 15 to 50 with higher ratings indicating more effective blocking and therefore better protection for the wearer of a garment. Fabrics that test higher than UPF 50 are rated as UPF50+. UPF testing involves exposing a fabric to ultraviolet radiation (UVR) and measuring how much is tranmitted through the sample. Different wave-lengths of radiation in the UVR spectrum have different effects on human skin and this is taken into consideration when calculating the UPF rating. Factors that contribute to the UPF rating of a fabric are:
Composition of the yarns (cotton, polyester, etc)
Tightness of the weave or the knit (tighter improves the rating)
Color (darket colors are generally better)
Stretch (more stretch lowers the rating)
Moisture (many fabrics have lower ratings when wet)
Condition (worn and faded garments may have reduced ratings)
Finishing (some fabrics are treated with UV absorbing chemicals).