A shirt feels like the problem is handled. Shoulders covered. No sunscreen smell. Then the burn shows up through the fabric—or on the neck and arms you never treated because the outfit “counted.”

Clothes help. They are not automatic sunscreen. Thin cotton lets a large share of UV through. Wet fabric lets through more. The label that tells you the difference is UPF—ultraviolet protection factor—not the fact that you got dressed.

What the numbers say

The Skin Cancer Foundation puts hard figures on common clothes:

• White cotton T-shirt — about UPF 7 when dry; about UPF 3 when wet
• UPF 50 fabric — blocks about 98% of UV; only 1/50 of the rays that hit the cloth reach the skin underneath
• Dark, long-sleeved denim — can reach on the order of UPF 1,700 in published examples (essentially full cover for the skin it actually covers)

UPF 7 means about one-seventh of the UV still reaches the skin—roughly 14%. Wet, that leak jumps to about one-third. Same tee. Same sky. More than twice the dose through the cloth.

Denim is the other end. Dense, dark, long-sleeved cloth can sit three orders of magnitude above a white tee. Gauze and open knit sit the other way: pinprick sky, lots of UV. The Skin Cancer Foundation’s construction line is that contrast: densely woven cloth—denim, canvas, wool—beats sheer fabric. Cancer Council Australia’s rule is weave first. Darker dyes of the same fabric absorb more UV than white or pastel. Closeness of the weave still beats the dye.

Cancer Council Australia’s rated-garment scale under AS 4399:2020 has only four clothing UPF ratings: 15 (minimum; about 93.3% blocked), 30 (good; about 1/30 / 3.3% through, 96.7% blocked), and 50 / 50+ (excellent; about 98% blocked)—the band they recommend when clothing should carry the load. Below 15 is partial cover, not a free pass. ARPANSA’s outdoor-work line wants materials rated 50. The Skin Cancer Foundation’s Seal on fabric starts at UPF 50; they call 30–49 very good and 50+ excellent.

UPF is not SPF

SPF is a lab test for a sunscreen film on skin. UPF is a lab test for fabric.

The Skin Cancer Foundation splits them this way. UPF measures how much UVA and UVB a fabric lets through. SPF is built on time to redden: if unprotected skin burns in 20 minutes, an SPF 15 sunscreen used correctly may stretch that about 15 times. SPF’s core test is the burn—mostly UVB. FDA’s rule is the same split: the SPF test is a clinical measure of sunburn protection (primarily UVB); a separate Broad Spectrum test is what speaks to UVA.

A UPF 15 shirt is not “SPF 15 in cloth form.” Rated clothing is usually tested dry and unstretched. The label does not describe the wet or tight version you put on after lunch.

Why the shirt fools you

Three traps stack on ordinary outdoor days:

1. “I’m covered.” Coverage and protection are different. A sheer tee casts a shadow on your torso and still passes a meaningful UV dose. Cancer Council NSW’s field check is the hold-to-light test: less glow through the weave usually means better cover. If you can see pinpricks of sky, UV is using those holes.
2. Heat as the cue. Light colors and thin weaves feel cooler, so people choose them on High and Extreme UV days. Darker, denser cloth usually absorbs more UV. Comfort and cover pull opposite ways. Stretch opens the gaps. Cancer Council: stretched fabric—common in knitted or elasticised tees—is less protective. Pick the right size, or offset stretch with a denser weave and a darker color.
3. Wet, stretch, wear. Sweat and pool splash open the weave. The same Skin Cancer Foundation white-tee example drops from about UPF 7 to about UPF 3 when wet. That is why a damp cotton T-shirt fails in the water too—covered separately in UV still hits you underwater.

Shade is a different dose path: shade does not cancel UV. This piece is the cloth on your skin.

Before you pack the outfit

Use the live UV index first, then judge the fabric:

1. Check today’s peak. If it is High (6–7) or above, do not treat a thin tee as enough for a long midday block.
2. Hold the cloth to light. Less pinprick glow through the weave usually means better cover.
3. Prefer darker, denser layers on the torso and shoulders when you can—or a labeled UPF 30+ (ideally 50+) shirt or rash guard for all-day outdoor work and sport.
4. Sunscreen what cloth misses — face, neck, ears, hands, and any gap at the collar or sleeve. Clothing only protects the skin it covers.
5. Recheck after you get wet or change layers. A soaked cotton shirt is a different garment than the dry one you left the house in.

A brimmed hat and UV-blocking sunglasses still matter. The shirt does not cover your eyes or the rims of your ears.

Sources

Everyday-tee UPF 7 dry / 3 wet, UPF 50 blocking 98%, denim ~UPF 1,700, UPF vs SPF, Seal at UPF 50, dense vs sheer: Skin Cancer Foundation. AS 4399:2020 bands (15 / 30 / 50 / 50+), weave, wetness, stretch, color: Cancer Council Australia. Hold-to-light: Cancer Council NSW. Outdoor-work UPF 50: ARPANSA. SPF as a sunburn (primarily UVB) test: FDA sunscreen labeling. Ratings are for dry, unstretched fabric unless the label says otherwise.