How does sun protective clothing work?

Clothing which provides sun protective properties does so, either through the "density"/"weight" of the materials used, or, with a chemical coating applied to the cloth materials used.

The attached graphic illustrates several relevant sun protective considerations:

1. the weight of the textile used is related to the content & density.....the more dense the textile, the closer together the filaments making up the textile.  The closer together the filaments, the less ultraviolet light can pass through the textile, therefore, the higher the Ultraviolet Protection Factor (UPF).  fyi:  a UPF of 230 correlates to an SPF of 230
2. the older a garment, the more likely it is that stretching of the fabric has occured (i.e. filaments pulled apart leading to reduced density, leading to a lower UPF
3. the UPF of a dry textile is typically reduced by 50-60% when wet

Therefore, low density textiles (e.g. cotton t-shirt) only have a UPF of 10-12, which when wet provides protection of spf <8.