To an extent, any piece of clothing protects against the sun’s UV rays. How successful the clothing is at doing this is expressed in a number called the Ultraviolet Protection Factor (UPF). This number reflects how much of the UVA and UVB rays are blocked. As an example, a UFP of 50 indicates that the cloth only allows 1/50th of the UV radiation to pass through it. For comparison, a regular white cotton t-shirt has a UPF of about 5, whereas some specially designed clothing has UPF of 50+.

To enhance the clothing’s UPF, manufacturers can do several things. Simply making the weave of the fabric tighter will block more rays than a loose weave. The material can make a difference as well (typically polyester has a higher UPF than cotton). Additionally, chemicals can be added, such as titanium dioxide, that serve to block the UV radiation in a similar manner as sunscreen. The process and chemicals involved in making the sun protective clothing are fairly well established- so as long as the company is trustworthy (i.e. not intentionally lying about what their clothing is made of), then I would say you can certainly trust their advertisements!