Does a T shirt block the sun? What a UV card shows

What the research found

When German researchers measured 236 summer fabrics bought from shops in 2000 and 2001, about a third of them gave too little protection to reach even the lowest band now used in Australia and New Zealand. Only about half reached the rating that the European standard of the time recommended. Most of the wool, polyester and blended fabrics got there, while most of the cotton, linen and viscose fabrics did not.

The authors drew a plain conclusion. It is difficult for a careful shopper to choose the right garment, and summer fabrics should be measured and labelled to a standard.

Why a thin T shirt lets the sun through

Ultraviolet passes through the gaps between the yarns. ARPANSA, Australia’s radiation protection agency, lists what makes the difference: how tightly the fabric is made, whether it is woven or knitted, its thickness, its colour and dye, and its condition. Many everyday tees are made thin and light on purpose, which is exactly what makes them comfortable on a hot day.

Knits have a second weak point. ARPANSA notes that stretching a knitted fabric opens the spaces in it, so a tee that is snug across the shoulders or back lets more through than the same tee lying flat on a table.

Wet, stretched and worn

Water changes things, though not always in the same direction. A 2002 study of 69 summer fabrics found that soaking lowered the protection of the cotton fabrics, while the linen, viscose and polyester fabrics often did better when wet. SunSmart, the sun safety programme in Victoria, keeps its advice simple: fabrics generally offer less protection when wet, and a fabric that dries quickly spends less time that way.

Age matters as well. SunSmart notes that a first wash can improve cotton by shrinking the gaps in it, and ARPANSA adds that after many washes a garment slowly fades and wears, and protects less than it did.

What a UV card shows

You cannot see ultraviolet, but a UV card can. The card carries a dye that darkens in ultraviolet light and fades again away from it. A 2024 study of these indicators found that they change colour almost at once in simulated sunlight, and take a minute or more to fade back to their starting colour.

Hold the card in full sun and watch it darken. Then lay one layer of your T shirt over a pale card, in the same sun, and compare the two. If the card under the shirt darkens too, ultraviolet is getting through the fabric. If it stays pale, the fabric is stopping most of what the card responds to.

What a UV card does not show

A card gives a yes or no answer, not a measurement. The same study found that these indicators reach their darkest colour at low levels of ultraviolet, so a card cannot tell a good fabric from an excellent one. An earlier study of UV indicator cards found that temperature changes their colour too, so a card left on hot stone can mislead you.

For a rating, fabric goes to a laboratory and is measured to a standard such as AS/NZS 4399:2020. We explain how that works in what UPF is and how clothing is tested, and how to run the card test properly in how to do the shirt test at home.

Why we put a card in every order

Most shirts let the sun through. Rezzella is made not to. We would rather you see that for yourself than take our word for it, so every order comes with a UV card, and the shirt test shows how to use it.

Try it with the Sun Tee and with a tee you already own, side by side in the same sun. Our fabric goes to ARPANSA, Australia’s radiation protection agency, for testing to AS/NZS 4399 before the first run. The full report goes on the shirt test page.