What determines ultraviolet protective factor?

January 8, 2026

What does UPF mean and how is it measured?

UPF stands for Ultraviolet Protective Factor and refers to how much a garment protects
your skin from the sun’s rays. It tells you how much sun reaches your skin through clothing. It is not the same as SPF (Sun Protective Factor), which is used to indicate how well sunscreens protect the skin from ultraviolet (UV) radiation. SPF tells you how long you can stay in the sun without burning. A sunscreen with SPF 70 means you can stay in the sun 70 times longer without burning than you would without sunscreen.

Measuring UPF can be done in one of two ways. In one method (in vitro method) a lab
technician uses a machine called a spectrometer to shine UV rays on a fabric to see how much penetrates through. UVB (280-320 nm) or both UVA + UVB (280-400nm) can be tested. However, only UVB is taken into account when providing the UPF number.

A second method (in vivo), shines UV rays on a piece of fabric on the skin. The end UPF
number is calculated with the complicated equation below. Stay with me, I’ll explain why it’s important that you know how UPF is calculated.

So what does this equation actually mean?

The top of the fraction represents the “sunburning power” of sunlight, or how well it is
able to cause redness in the skin. The bottom of the fraction represents how much of that total sunburning power passes through the fabric. This “sunburn power” is a direct reflection of UVB ray power, with minimal contribution from UVA (see E(λ) from the equation).

For example, if the complete UV intensity (top number) is 100 units, and the fabric only allows 2 units (bottom number) of that to reach the skin: 100/2 = 50 UPF

That translates to the familiar UPF 50 (blocks 98% of UV).

UPF measurement is heavily weighted toward the UVB wavelengths because UVB is primarily what causes redness in the skin.

In the U.S., there is currently no standard to measure UVA protection in clothing. Brands are required to list the UPF number if they claim that it is “sun protective clothing” but this number, as explained above, does not take the full UVA spectrum into account.

How is UPF measurement regulated?

Unlike the United States, countries within the European Union are also required to test
sun-protective garments for how well they block UVA. It is required that sun-protective
garments have less than 5% transmittance in the UVA region. This is important because like UVB, UVA rays also have the potential to cause skin aging and skin cancer.

When you’re looking for UPF swimwear and clothing, you want to make sure it is
protecting you from both UVA and UVB and that those fabrics have been lab tested. In 2019, an Australian study found that over half of the hats at a shopping center that were labeled with UPF rating of 50+ did not meet that rating.

UPF ratings were once regulated by the FDA but this was discontinued in the 1990s.
Now, we have guidance from the American Association of Textile Chemists and Colorists
(AATCC) and the American Society for Testing and Materials (ASTM). False UPF claims are managed by the Federal Trade Commision.

Helioshield fabrics have been tested for UVA and UVB and block >99% of UVA and UVB rays.

What affects UPF rating in clothing?

The degree of protection depends on fiber structure, fiber type, color, dryness, and
treatments.

Fabric Structure

Fabrics that are weaved more tightly offer more protection than loosely weaved fabrics.
This is referred to as the “porosity” of the fabric. If a fabric has larger “holes” (looser weave, stretched from wear and tear) then more UV rays can sneak through.

Fiber Type

UV protection is highly dependent on the physical and chemical structure of the fibers.
Synthetic fibers like nylon and polyester typically offer the best protection from the sun’s rays. Natural fibers like cotton, silk, and wool absorb less UV radiation than synthetic fibers.

Fiber Color

The dyes used to color fabrics have a large influence on how well they block UV rays.
Dyes act as UV absorbers and increase the UPF of a fabric. Generally, for the same fabric
structure, the darker the fabric the higher the UPF. Darker linens/cottons often have higher UPF ratings. For example, a dark, tightly woven cotton shirt can potentially block 98-99% of UV rays, whereas a light colored cotton shirt blocks only 5-15%.

Fabric Dryness

The UPF of wet clothing is lower than the same piece of dry clothing. Water increases
UV permeability and can cause 30-50% reduction in the fabrics UPF rating.

Fabric Treatments

Clothing can be treated with nanoparticles composed of titanium dioxide and zinc oxide
that absorb and scatter UV rays. These treatments can fade with laundering, but mixing these nanoparticles with binders and creating bonds between them and corresponding fibers are two ways in which this type of clothing can maintain its UPF rating despite washing. Iron oxide, copper oxide, and graphene oxide can also be used as finishing agents to increase the UPF.

What are the benefits of wearing UPF swimwear?

UPF swimwear is an excellent addition to any sun-protection plan. Sun-protective
behavior has been shown to decrease rates of skin cancer and signs of skin aging. While
sunscreen is great, it has its limitations and should not be used as your only form of protection. Sunscreen needs to be applied in the proper amount, can be difficult to apply to hard-to-reach areas, needs to be re-applied, and may not protect well against UVA1 and visible light. Using both sunscreen and UPF clothing together is ideal. When thinking about sun protection, I often use the “Swiss cheese” analogy. One slice has holes, which represent the weaknesses of any single method. Stacking multiple slices reduces the chance that something will slip through. The best approach is to combine a UPF hat, UPF swimwear, sunglasses, sunscreen of at least SPF 30, seek shade when available, and avoid peak UV hours from 10 a.m. to 2 p.m. when possible.