UPF 50+, explained.

UPF stands for Ultraviolet Protection Factor. It is a rating applied to fabric, and unlike most of the language on a clothing tag, it is the result of a laboratory test rather than a copywriter's judgement.

What the number means

A UPF rating describes the fraction of ultraviolet radiation a fabric lets through. UPF 50 means roughly one fiftieth of the UV hitting the fabric reaches the skin beneath it — about two percent. UPF 50+ is the highest rating that gets printed, and it means the fabric measured at 50 or better.

The published bands are straightforward. Ratings of 15 to 24 are labelled good, 25 to 39 very good, and 40 to 50+ excellent. Anything under 15 is not rated at all.

UPF is not SPF

The two get used interchangeably and they should not be. SPF is measured on skin and describes how long a sunscreen delays burning, which makes it primarily a measure of UVB protection. UPF is measured on fabric and accounts for both UVA and UVB.

The practical difference is durability. Sunscreen degrades — it sweats off, rinses off in salt water, and gets wiped away by a towel, and reapplication is the step almost everyone skips. A fabric's rating does not decline over an afternoon. It is the same at four in the afternoon as it was at nine in the morning.

Sunscreen protects for as long as you keep reapplying it. Fabric protects for as long as you are wearing it.

What makes a fabric protective

Several things, and they compound:

• Construction. A tight weave or dense knit leaves less space between yarns for UV to pass through. Hold a fabric to the light — if you can see through it easily, so can the sun.
• Fibre. Polyester performs well because it absorbs UV rather than transmitting it. Untreated cotton and linen perform poorly.
• Colour. Darker and more saturated colours absorb more UV than pale ones, all else being equal.
• Stretch. Pulling a fabric taut opens the spaces between yarns and lowers its effective protection. A garment stretched over the body protects less than one that skims it.
• Moisture. Many fabrics lose protection when wet, as water fills the gaps and helps transmit light.

The cotton t-shirt problem

This is the part that surprises people. A standard cotton t-shirt measures somewhere around UPF 5 to 8 — meaning it lets through something in the region of an eighth of the UV hitting it. Wet, that number gets worse.

For an hour outside it is fine. For a full day on open water it is not doing the job most people assume it is doing. If you have ever come home from a boat day with a sunburn in the shape of your shirt, that is the explanation.

How to read a label honestly

Look for a specific number. A garment that says UPF 50+ is making a measurable claim. A garment that says sun-protective, beach-ready, or UV-friendly is making no claim at all, and the absence of a number is usually the answer.

Be slightly sceptical of protection that comes from a chemical finish rather than the fabric itself, since finishes can wash out over a garment's life. Protection built into the construction and the fibre does not.

Keeping the rating

Wash cold and hang dry. Heat is what breaks down performance knits over time — a hot dryer will age a sun shirt faster than a season of wearing it will. Replace anything that has gone thin or visibly translucent, because at that point the fabric is telling you what its rating has become.