What is UPF, and How Does it Work?

UPF means Ultraviolet Protection Factor, and rates a fabric or garment’s ability to stop UV rays from reaching your skin. Fabrics with a UPF rating are, essentially, a physical sunblock. A fabric’s UPF potential relies on a few main properties:

Material: Happily for cyclists, synthetic fabrics like Lycra and polyester, common material for most cycling kits, often score well on UPF tests—particularly if the fabric is tightly knit or has a shiny surface. Among the worst is cotton. Wool usually performs well, in part because the thick, textured yarn blocks more radiation.

Construction: Woven fabrics typically have a higher UPF rating than knits. Weaves are typically tighter than knits, and weaves by definition have very little stretch, so they let in less light. Simon Huntsman, head of product development and R&D for Rapha, notes that knits can vary widely; complex double-knit fabrics can have very high UPF ratings, while lighter single-knits generally offer less protection.

Color: Very dark colors absorb more UV radiation; light colors perform worse and white offers the least protection, because it’s the most transparent, says Carol Little, Pearl Izumi’s senior materials development manager. Red shades can do surprisingly well at blocking most UV radiation, with dark blue and green not far behind.

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Fit: Stretching a fabric can greatly reduce its UPF rating; a 2014 study at Hong Kong’s Institute of Textiles and Clothing found that even stretched 10 percent (that is, to 110 percent of normal fabric length) can reduce UPF by up to two-thirds, depending on knit structure. Wet fabrics also typically drop in effectiveness.

Wear and Tear: Garments typically don’t begin to lose UPF effectiveness from wear until they are highly worn or faded. In fact, according to Rigel, most fabrics used in your cycling kit actually initially improve in UPF as they’re washed and worn because of “microfraying” of the yarn and garment shrinkage, which tightens the knit.

Chemical Treatments: Some textile brands are incorporating physical or chemical treatments to increase UPF. However, some may wash out long-term, much like how a durable water-repellant finish will degrade over time.