Pilling is caused by the way yarn is made. In a yarn numerous wool fibers are spun together in a long thread. Washing and wearing a wool garment can cause loose fibers to push out from the surface and form into a small ball or pill.
Friction is the main factor in causing pilling. Repeated friction from, for instance, seat belts or shoulder bags, can provoke pilling. Likewise, overfilling the washing machine can cause similar friction. By airing the garment rather than washing it, the risk of pilling is reduced. However, it can be a good idea to wash the wool garment before first-time use. This removes loose fibers on the surface, thus reducing the risk of pilling.
The degree of pilling is dependent on type of yarn and knitting technique. The looser the yarn is spun, and garment is knitted, the higher risk of pilling.
If your garment is pilling, it is important to know that it doesn’t reduce the quality of your garment. The pills can simply be removed with a pilling comb/wool comb. This can be purchased online and in specialized yarn and fabric stores.