It’s not hard to predict which sweater or scarf will be itchy when you see fibres under the microscope.
Here are a few details about the significance of the scales that you can see and the absence of them, on protein, plant, and synthetic fibres.
Animal fibres have scales that protrude; the larger the scales and the more they stick out, the itchier the fibre will feel. Alpaca and cashmere, with smoother, flatter scales are the easiest to wear of the animal fibres.
Sensitivity to animal fibre may be due to those scales or can be caused by a sensitivity to lanolin, found in wool.
Silk worms extrude a single length of scale-free fibre, hence, no itch.
The plant fibres, linen and cotton, are free of scales, but linen, made from the flax plant’s stalk, can feel harsh to wear at first. It softens when washed. Cotton, made from the downy fibres of the cotton boll, is comfortable to wear.
Synthetic fibres, like acrylic and polyester, are chemically created, liquified and exuded in continuous fibres on machines. There are no scales to cause itching, but synthetics have less breathability than the nature-made fibres.
Choose carefully if itchy yarns bother you. Years ago a knitting teacher recommended tucking a bit of a fibre into your bra to test sensitivity to it, but I’ve heard since that the neck is a better testing site. Not easy to do when you are standing in your favourite yarn store, smitten with a yarn, so information on the ball band is your friend. What is in that yarn?