What UPF Ratings Mean

UPF is the rating system used for apparel. It’s similar to SPF (Sun Protection Factor), the rating system used for sunscreen products. SPF pertains only to a sunscreen's effectiveness against UVB rays, considered to be the more damaging type of light. UPF, though, gauges a fabric's effectiveness at filtering both ultraviolet A (UVA) and ultraviolet B (UVB) light.

When you shop, things are relatively simple: Look for a higher UPF rating number and greater skin coverage in order to get better sun protection.

A UPF rating of 30 indicates the fabric of a garment will allow 1/30th (roughly 3 percent) of available UV radiation to pass through it. A garment rated UPF 50 permits only 1/50th (roughly 2 percent) UV transmission. Any fabric that allows less than 2 percent UV transmission is labeled UPF 50+.

Why you don’t see ratings below UPF 15: Fabrics rated below UPF 15 are not considered UV-protective. A typical white cotton T-shirt, for example, offers about a UPF 5 rating, which means that 1/5th (roughly 20%) of available UV radiation passes through it.

Factors that enhance UPF ratings: As a shopper, simply look for the UPF rating on garment hangtags or labels. Clothing makers will already have considered the following factors in trying to achieve that level of sun protection:

• Construction: Dense, tight construction minimizes the amount of UV light that can pass through. Other construction details being equal, thicker fabrics may have reduced UV transmission versus thinner ones.
• Color: Generally, darker colors prevent more rays overall (including UV rays) from reaching your skin. Within the same color, more saturated hues outperform paler ones.
• Treatments: Chemicals and dyes that are effective at absorbing UV light can be added to enhance UPF.
• Fiber type: Polyester does an excellent job at disrupting UV light, as does nylon. Wool and silk are moderately effective. Cotton, rayon, flax and hemp fabrics often score low without added treatments.

Factors that reduce the effectiveness of UPF-rated clothing: Whether you’re considering the UPF spec on new clothing or packing your favorite rashguard for a trip to the beach, be aware of the following:

• Fabric wetness: For many types of materials, wetness can cause a significant reduction in a fabric's UPF rating. Some studies, though, suggest that polyester may actually protect slightly better when wet.
• Fabric wear: As a fabric becomes worn or faded, it also becomes less effective at blocking UV light.
• Fabric stretch: Stretched fabric can lose a significant amount of its UPF, so consider replacing any item that simply fits too tightly.

How laundering affects UPF ratings: Washing your clothing can either increase or decrease its UPF, depending on several factors:

• Detergents with brighteners: While most detergents contain them and most brighteners do enhance UPF, there’s no way to tell for certain if a given detergent will enhance your garment’s rating.
• Shrinkage: All else being equal, shrinking a garment gives it a tighter weave, which can increase its UPF.
• Clothing that relies on finishes for its UPF rating: Finishes can vary in durability. If a finish washes out, UPF diminishes. Check to see if your garment states that its UPF rating is good for a specific number of washes.
• Clothing that relies on inherent fabric properties for its UPF rating: UPF should remain relatively unchanged as a result of washing—until it diminishes simply because the fabric becomes worn or faded.