Wool has been a precious raw material for people for a long time. As the range of available fibres was limited in the past, wool used to be a very valuable commodity. Today we are able to select between a huge variety of fibres with varying properties, but nevertheless, the continuing use of wool — in spite of the competition with other natural fibres and new synthetic fibres — can be attributed to the unique properties of wool:
thermal regulative due to high amounts of air embedded in the fibre,
high absorption of moisture,
low tendency to creasing,
However, a disadvantage, which emerges during washing, is the felting tendency.
Small, barbed scales cover the surface of wool fibers. When wool is machine-washed and dried, these scales can become interlocked, causing the wool to felt and shrink. To prevent interlocking, wool is usually dry-cleaned or hand-washed.
To prevent felting of wool and to make wool washable in normal household washing machines, several methods have been developed. The most popular one is to treat the barbed scales with chlorine, then, apply a thin polymer coating. This makes wool fibers smooth and allows them to slide against each other without interlocking.
Talking simple, washable wool is made by exposing the fiber to a chlorine that erodes the scales and then it is treated with a sort of plastic, a polymer resin called Hercosett 125. Hercosett 125 is a polyamide-epichlorohydrin polymer. While the chemicals that comprise this polymer are themselves highly toxic, that does not mean that the polymer itself is. Many plastics that are harmless once combined together into long chemical chains are made from smaller molecules that are toxic before they are combined. However, it is hard to find any information on the safety of Hercosett-treated wool. Since washable wool is coated with plastic, can we really consider it a 100% natural fiber?
Millions of pounds of wool are processed each year in this way. Unfortunately, this method results in wastewater with unacceptably high levels of adsorbable organohalogens (AOX) — toxins created when chlorine reacts with available carbon-based compounds. Dioxins, a group of AOX, are one of the most toxic substances known. They can be deadly to humans at levels below one part per trillion. Wastewater from the wool-chlorination process contains such high concentrations of chlorinated chemicals, that most wastewater treatment facilities in the United States do not accept it. Therefore, most chlorinated wool is processed in other countries, then, imported.
Sources: http://www.patagonia.com/pdf/en_US/chlorine_free_wool.pdf; http://woolful.com/fiber-conscious-superwashwool/; http://www.pburch.net/dyeing/dyeblog/C1691090068/E20091010162851/index.html; http://www.drpetry.de/fileadmin/user_upload/petry/pdfs/Lanazym_Wool.pdf