Does clothing block UVA? UPF ratings and what actually works
Clothing is the oldest and most reliable form of sun protection: it does not wear off, does not need reapplying, and covers evenly. But "wearing a shirt" spans an enormous range of actual protection — from roughly a factor of three to a factor of a thousand — and the rating system used to describe it has the same UVB bias as the SPF on a sunscreen bottle.
What UPF measures
UPF — Ultraviolet Protection Factor — is the ratio of ultraviolet that would reach your skin without the fabric to the amount that reaches it through the fabric. UPF 20 transmits one twentieth, or 5%. UPF 50 transmits one fiftieth, or 2%.
The measurement is made instrumentally rather than on volunteers: a spectrophotometer measures transmission wavelength by wavelength through the cloth, and the result is weighted by the CIE erythemal action spectrum — the same sunburn-weighting curve behind the UV Index — then integrated.
Standards bodies are aware of this. The Australian and New Zealand standard AS/NZS 4399, which established the UPF system, rates on the weighted number alone. The European standard EN 13758-2 does not: to be labelled as UV-protective clothing, a garment must reach UPF 40 and keep mean UVA transmission below 5%. The existence of that second criterion is an admission that the first one does not cover UVA on its own — the same structural fix the EU applied to sunscreen with its circled-UVA mark.
| UPF | AS/NZS 4399 category | UV transmitted |
|---|---|---|
| 15–24 | Good protection | 6.7–4.2% |
| 25–39 | Very good protection | 4.1–2.6% |
| 40–50+ | Excellent protection | 2.5% and below |
Fabrics below UPF 15 are not rated at all under the standard — which quietly excludes most ordinary summer clothing.
What ordinary clothes actually rate
The figures below are typical values from textile testing. Real garments vary widely, and the point of the table is the spread rather than any single number.
| Garment | Approximate UPF | UV transmitted |
|---|---|---|
| White cotton t-shirt, dry | ~5–7 | ~15–20% |
| White cotton t-shirt, wet | ~3 | ~30%+ |
| Black cotton t-shirt, dry | ~10–20 | ~5–10% |
| Light open-weave linen shirt | ~3–5 | ~20–35% |
| Unbleached or dyed dense cotton | ~20–40 | ~2.5–5% |
| Polyester or nylon activewear | ~30–50+ | ~2–3% |
| Denim, heavy twill | Effectively total | Negligible |
| Purpose-made UPF 50+ garment | 50+ | Under 2% |
The white t-shirt result surprises people, and it is the single most useful fact here. At UPF 5 a shirt is doing about as much as a very thin application of a low-SPF sunscreen. It is not nothing — but a full day at the beach in a wet white tee is not a protected day.
The five things that decide a fabric's performance
- Weave density, above all. Ultraviolet passes through the gaps between yarns far more readily than through the yarns themselves. Hold a garment up to a light: the amount of light you see through it is a decent proxy for the UV getting through. This is why a thin dark shirt can underperform a thick pale one.
- Fibre type. Polyester is a genuinely good ultraviolet absorber — its aromatic structure absorbs in the UV — which is why synthetic sportswear rates well despite being thin. Nylon is decent. Bleached cotton and untreated linen are poor; unbleached cotton retains natural pigments and lignins that help.
- Colour. Dark and intense colours absorb across the spectrum, including the UV. Black, navy and deep red typically double or triple the UPF of the same fabric in white. Optical brightening agents in white fabrics absorb some UV and re-emit it as visible light, which helps a little.
- Moisture. Water fills the interstices between fibres and reduces scattering at the fibre surfaces, making the fabric more transparent — visibly so, and to ultraviolet as well. A wet cotton shirt can lose half its already-modest UPF. This matters most exactly where it is worst: at a beach or pool, under a high UVA Index, with water and sand reflecting from below.
- Stretch and wear. Pulling a knit fabric taut opens the gaps; a garment stretched by 10% can lose a substantial fraction of its rating. Ageing, thinning and repeated laundering all reduce it too, though the first few washes often raise UPF slightly as cotton shrinks and the weave tightens.
Hats, and the parts of you a hat misses
A hat's performance depends far more on geometry than on fabric. The brim has to intercept light arriving at the angle the sun is actually at, and it has to do so for the parts of your face most exposed.
- A brim of at least 7.5 cm all round is the usual recommendation, and it meaningfully reduces dose to the nose, cheeks and — importantly — the eyes and eyelids.
- A baseball cap protects the forehead and little else. Ears, neck, jawline and lower cheeks receive essentially no benefit, and those are common sites for skin cancers.
- No hat helps much against reflected UVA. A brim blocks light from above. Ultraviolet bouncing up from snow, water, sand or pale concrete arrives from below, straight under the brim — which is why skiers and boaters burn on the underside of the chin and nose.
Shade is a partial measure
Shade removes the direct solar beam. It does not remove the sky. Because a large share of surface UVA arrives as diffuse skylight scattered from the whole dome rather than straight from the solar disc, sitting under a beach umbrella typically leaves you receiving on the order of half the ultraviolet you would in the open — and more than that if bright sand is reflecting into your shade from the side.
Deep shade with a large amount of sky blocked — a dense tree canopy, a covered terrace, the inside of a building — performs far better than a small parasol, because the amount of visible sky is what actually matters. The same principle explains the numbers in UVA on cloudy days: what you can see of the sky is roughly what you are getting from it.
Practical summary
- Assume ordinary summer clothing is weak UVA protection unless it is dark, dense or synthetic.
- Treat a wet shirt as roughly half the protection of a dry one.
- For anything sustained outdoors, a purpose-made UPF 50+ garment removes the guesswork entirely and never needs reapplying.
- In Europe, look for the EN 13758-2 claim rather than a bare UPF number — it carries the separate 5% UVA limit.
- Cover the areas clothing habitually misses: back of the neck, ears, tops of the feet, hands on a steering wheel.
- Clothing and shade handle the direct beam; broad-spectrum sunscreen and UV400 lenses handle what arrives from everywhere else.