There are two types of UV rays that can damage your skin: UVA and UVB. Both cause skin cancer, but UVB rays also cause sunburn. SPF, or sun protection factor, is a measure of how well a sunscreen will protect your skin from UVB rays. UPF, or ultraviolet protection factor, is a measure of how well a fabric will protect your skin from both UVA and UVB rays.
SPF is measured on a scale from 2 to 50+. The higher the number, the more protection the sunscreen provides. For example, SPF 15 blocks 93% of UVB rays, while SPF 30 blocks 97%. UPF is also measured on a scale from 15 to 50. A UPF of 15 means the fabric will allow 1/15th of the UV rays that hit it to reach your skin. So, a UPF of 30 will allow 1/30th of the UV rays to reach your skin.
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