How UPF clothing works

UPF protection comes from three things: how tightly the fabric is woven or knitted, what fiber it is made from, and how dark and saturated the color is. In a well-made garment those are structural properties of the fabric itself, which is why the protection does not wash out. Cheaper garments sometimes use a chemical UV absorber applied to the surface, and that does degrade with laundering.

Understanding the mechanism makes the buying decision obvious, and it explains why an ordinary cotton T-shirt — which feels like clothing and looks opaque — offers so much less protection than people assume.

The three structural factors

1. Weave and knit density

The dominant factor. UV passes through the gaps between yarns, so fewer and smaller gaps means less transmission. You can assess this yourself in a shop: hold the fabric up to a window or a light. Visible pinholes are UV getting through, and a fabric you can see light through clearly is not a UPF 50 fabric whatever the tag says.

2. Fiber type

Polyester and nylon block more UV per unit weight than cotton or linen, because the polymers themselves absorb in the ultraviolet range while cellulose fibers largely do not. That is why purpose-built sun shirts are almost always synthetic, and why a linen shirt — which feels protective because it is opaque and cool — performs poorly.

3. Color

Darker and more saturated colors absorb more UV than pale ones, because the dye itself absorbs. The practical caveat: a purpose-built UPF 50+ fabric already blocks 98% or more in any color, so color becomes a heat decision rather than a protection one. Dark fabric in still air gets hot.

What an ordinary T-shirt actually offers

That gap between everyday cotton and a rated garment is the whole reason this category exists. It is also why the Skin Cancer Foundation's guidance treats sun-protective clothing as a distinct recommendation rather than as a subset of wearing clothes.

Fabric-based versus treatment-based protection

Some garments achieve their rating through a chemical UV absorber applied to the fabric rather than through construction. Those work when new and degrade with laundering — typically stated as effective for a number of washes.

Fabric-based protection, achieved through weave density, fiber and color, does not wash out because there is nothing to wash out. It is the property of the material. When buying, prefer brands that describe protection as being in the fabric rather than applied to it — and treat a stated wash count as a warning that it is the other kind.

What genuinely degrades UPF

• Stretching. Pulling fabric taut opens the gaps. A shirt straining across the shoulders transmits more UV than the same shirt relaxed.
• Wetness, in cotton. Water fills the interstitial spaces and reduces scattering. Purpose-built synthetics are tested to hold their rating wet. Does UPF clothing work wet.
• Wear and thinning. A garment worn to translucency has lost protection regardless of what it started as.
• Chlorine and hot drying, which degrade elastane and leave the fabric slack — which then stretches more easily.
• Loss of color over many seasons, which reduces the dye's contribution.

How the rating is tested

Fabric is placed in a spectrophotometer and total ultraviolet transmission is measured across the UVA and UVB ranges. The ratio of incident to transmitted UV becomes the UPF number, and testing standards specify assessment of the fabric in a new state and, for many certifications, after simulated wear and laundering.

Because the measurement covers both UV ranges, there is no separate broad-spectrum claim on clothing the way there is on sunscreen. UPF versus SPF.

Caring for it

1. Rinse in fresh water after salt or chlorine exposure.
2. Wash cool, which protects the elastane that keeps the knit dense.
3. Line dry. Hot tumble drying is the fastest way to ruin a technical garment.
4. Skip fabric softener, which coats fibers and interferes with moisture movement.
5. Replace when it goes thin or slack, which is a wear question rather than a washing one.