Understanding the Prickle Factor

For a lover of wool yarns, it can be painful—yet oddly exciting—to hear someone utter, “Wool is itchy.” The phrase hurts because it’s not wholly true, so why do we hear it repeated so often? Fortunately, these words can spark an opportunity to change minds and recruit new wool enthusiasts!

While plenty of people have personal horror stories about hideous, itchy handmade sweaters gifted by grandmothers and great-aunts, there exists an entire world of ultra-soft wools that challenge this tired narrative. With minimal background knowledge about why a wool may feel “itchy” to the wearer, plus an understanding of the variety of wool available to us, we can turn wool doubters into believers.

What Is Prickle Factor?

The wool industry has even come up with a measurement to predict which fibers will be uncomfortable: Prickle Factor.

The fiber diameter of one strand of wool is measured in microns (µm), and a micron (µ) is one-millionth of one meter. These measurements are taken with computer imaging, so they are incredibly precise, according to The Spinner's Book of Fleece by Beth Smith.

Because sheep across the world are bred for myriad distinct qualities and in a range of climates, sheep’s wool can span anywhere from about 10µm (fine) to over 80µm (coarse), depending on the breed, age, and other characteristics that farms have intentionally bred for. (For comparison, human hair averages around 100µm but can span anywhere between 20 and 180µm.)

A scanning electron microscope reveals the difference in diameter and texture between superfine Merino wool (top) human hair (bottom). Photo by CSIRO; CC-BY-3.0; courtesy of Wikimedia Commons

Prickle Factor, or PF, is determined by the percentage of fibers measured within a sample that have a micron count of higher than 30µm. A PF of around 5% or more indicates a coarser fiber with a higher likelihood of feeling “prickly” to the wearer. Merino is a famous example of a wool with a micron count of less than 30µm; its popularity has grown in recent years due to its softness and availability. Fortunately, there are many wool breeds available that fall under that 30µm—sheep and otherwise—but this isn’t the only determining factor of “softness.”

Wool Allergies

A human allergy to wool is somewhat of a myth, because wool is made up of the exact same stuff as human hair—proteins and fats. If someone has an allergic reaction to wearing something wool, it’s most likely they are actually reacting to the chemicals and dyes used in wool processing, or even the naturally occurring lanolin (the fatty substance that gives wool a waterproofed quality), according to The Fleece & Fiber Sourcebook by Deborah Robson and Carol Ekarius. Essentially, the “itchiness” of depends very much on the wearer, usually not the wool itself.

Targhee sheep, well suited to the weather of the Mountain West, have a micron count lower than 30. Photo by Kristen McRae Bieber

However, knowing what breed of wool you are purchasing or using can be incredibly helpful in avoiding an itchy feeling in a sweater. In addition to Merino wool, there are many wool breeds that have a low Prickle Factor, or a micron count of less than 30µm. Some widely available breeds include Bluefaced Leicester (BFL), Cormo, Polwarth, Rambouillet, Shetland, and Targhee. Keep in mind that these are averages. You could very well find wool from a generally coarser breed with a lower micron count, depending upon the genetics of the particular flock. (For an in-depth listing of wool fibers and their micron counts, staple lengths, best uses, and more, refer to Deborah Robson and Carol Ekarius’s excellent books The Fleece & Fiber Sourcebook and The Field Guide to Fleece.)

Keep these and other breeds you come across in mind when looking at yarn labels. Labels that say “100% wool” do not indicate which breeds are included, and chances are it’s a blend of many different types of breeds. If a yarn label specifies which breeds were used to create the yarn, you’re on the right track!

Beyond Sheep’s Wool

In addition to fine-wool sheep breeds, there are also many non-sheep wools out there with low micron counts. They can be harder to source and more expensive, but they truly show off wool’s ability to be incredibly soft while still maintaining its other important qualities including strength, lightness, warmth, and moisture-wicking ability. Many fiber enthusiasts are familiar with alpaca, which is known for its softness, but there are some other very interesting and lesser-known animals that yield very soft wool, including cashmere, yak, and others. If the wool has been well dehaired to remove coarser fibers, these should provide an itch-free experience.

Goat’s wool is some of the finest found in the world, and cashmere has a well-deserved reputation for its softness and warmth. Camels and llamas are in the Camelid family with alpacas, but alpaca and llama fibers have a much wider range in diameter than camel or yak down, both from animal to animal and within the same fleece, which might point to why even ultra-soft alpaca can sometimes feel prickly against the skin.

Vicuñas, not only soft but adorable too! Photo by Wildcr via Pixabay

A very special animal in the Camelid family is the vicuña, which is found in alpine highlands of the Andes in South America. This endangered species is not bred on farms, but rather lives in the wild. The animals are rounded up once every two to three years to be sheared, then released back into the wild. Because of these factors, this fiber is very expensive, but it is some of the softest wool in the world, and is definitely worth a try if you can get some. Vicuña fiber could change anyone’s mind about the itchiness of wool! More available to knitters is paco-vicuña, which combines properties of alpaca and vicuña. (For more on paco-vicuña, see “Jefferson Farms: The Search for Wild Luxury.”)

A musk ox showing signs of shedding their coat. The downy light-brown fiber is the soft qiviut. Photo by Annapersson via Pixabay