Ever seen a sheep with sunburn?
Merino wool has the property of absorbing UV radiation and thus keeping away from your skin. Merino wool fabrics have a sun protection factor of 40+. The time you can stay in the blazing sun with a Prosa merino shirt without getting a sunburn will be 40 times longer. Synthetic fibres and other natural fibres cannot offer comparable protection without having to rely on the use of questionable ingredients.
Merino wool has a UPF of 40 and more. UPF stands for Ultraviolet Protection Factor. Just like a sunscreen with the abbreviation SPF, the Sun Protection Factor. These terms indicate how much of the UV radiation is retained. Conventional cotton T-shirts have a UPF of about 10, which means that approximately 10% of UV radiation can penetrate your skin. A UPF value of 40 means that more than 97.5% of the radiation is retained. So you are well protected with your prose shirt.