People often think that the only parts of their skin that are at risk of being damaged by the sun are the parts that aren't covered by clothing. They assume that clothing - of any kind - protects their skin completely from the sun's ultraviolet (UV) rays. But this is not true.
Some types of clothing provide much better protection against UV rays than others do. The weight of the fabric, how tightly the fabric is woven, the type of fiber that the clothing is made of, the color, and the amount of skin covered by the garment all influence the amount of sun protection that it provides.
A rating called UPF, for Ultraviolet Protection Factor, has been devised to compare the ability of various types of clothing to protect against the sun's UV radiation. The UPF is similar to, but not identical to, the SPF factor used to rate sunscreens. The UPF measures the amount of UV radiation that goes through the fabric. For example, a piece of clothing with a UPF of 10 allows one-tenth of the sun's UV rays to pass through it. Higher UPF ratings mean better protection against the sun.
Light-colored, lightweight, and loosely woven fabrics have low UPF ratings, while darker-colored, heavier-weight, tightly woven fabrics have higher UPFs. The Skin Cancer Foundation notes that you can judge, in a general way, whether a particular fabric will protect your skin adequately from the sun by holding it up to a light. If you can see through it, UV rays can penetrate it. If it's the only layer of clothing you're wearing, that means that the UV rays will reach your skin. An example of a kind of clothing that provides relatively little protection against UV rays and therefore has a low UPF value is a white T-shirt. People often put on white T-shirts when they are at the beach in the hope of protecting themselves against sunburn. But they would be better off with clothing made of a darker, thicker, heavier fabric.
What people do while they're wearing a piece of clothing can affect its UPF. For example, if a fabric gets wet, its UPF may decrease by half, according to the Skin Cancer Foundation. Thus, if you go swimming while wearing that white T-shirt, or if you play sports while you're wearing it and get sweaty, it is even less protective than it was when it was dry.
Some manufacturers are now making special sun-protective clothing, using tight weaves, fabrics that protect well against UV rays, and in some cases, added chemical sunscreens. Much of the clothing that is sold as sun-protective has a UPF of 50 or more, although the UPF may decrease if the clothing becomes stretched or wet. Laundry additives that increase clothing's ability to protect against the sun are also available. People who often spend time out in the sun may want to investigate these products.