The difference between SPF vs. UPF is this:
SPF stands for Sun
Protection Factor. It is the ratio of UV radiation dose required to produce
recognizable redness on skin that has been protected with a sunscreen from the
UV radiation dose required to produce the same redness on unprotected skin
(using a clinical test procedure on human skin). SPF tells you how much longer
it will take for your skin to begin to redden with the product on than if your
skin was unprotected. The percentage of burning UV rays that a product blocks
does not increase proportionally with the SPF value. So SPF values indicate a
function of skin reddening.
The Australian Goverment, a leader is UV research for over two decades, has
stated that, "Over-exposure to solar ultraviolet radiation (UVR) can cause
sunburn, skin damage and an increased risk of developing skin cancer. Clothing
provides one of the most convenient forms of protection against UVR but not all
garments offer sufficient sun protection"
UPF stands for
Ultraviolet Protection Factor. It is the ratio of the average effective
ultraviolet radiation (UVR) irradiance transmitted and calculated through air to
the average effective UVR irradiance transmitted and calculated through fabric.
In other words, it is the amount of ultraviolet radiation that a fabric blocks.
Hence UPF values are used in the Fabric's UV transmittance test by