Fiber Facts: Mercerized Cotton

In 1851, British chemist John Mercer discovered that treating cotton yarn with sodium hydroxide brought about changes in the fiber. Mercerized cotton is stronger, shinier, and smoother than untreated cotton. You can see the difference in the microscopic images of regular cotton and mercerized cotton below.
Regular cotton (above), appears coarse and uneven.

Mercerized cotton (above) appears smoother.

Unfortunately for John Mercer, mercerized cotton didn't really take off until later in the 19th century, when Horace Lowe added an extra step to the process and drummed up interest in the British cotton industry. Mercerized cotton is now widely available to knitters.

I don't want to scare anyone, but the sodium hydroxide used to make mercerized cotton is highly toxic. This is not to say that mercerized cotton yarn *itself* is bad for you, but it might be worth checking where and how these yarns are manufactured, since prolonged exposure can negatively affect workers. The CDC lists a wide array of health effects from exposure to sodium hydroxide:

Sodium hydroxide is very corrosive and can cause severe burns in all tissues that come in contact with it. Inhalation of low levels of sodium hydroxide as dusts, mists or aerosols may cause irritation of the nose, throat, and respiratory airways. Inhalation of higher levels can produce swelling or spasms of the upper airway leading to obstruction and loss of measurable pulse; inflammation of the lungs and accumulation of fluid in the lungs may also occur.

Ingestion of solid or liquid sodium hydroxide can cause spontaneous vomiting, chest and abdominal pain, and difficulty swallowing. Corrosive injury to the mouth, throat, esophagus, and stomach is very rapid and may result in perforation, hemorrhage, and narrowing of the gastrointestinal tract. Case reports indicate that death results from shock, infection of the corroded tissues, lung damage, or loss of measurable pulse.

Skin contact with sodium hydroxide can cause severe burns with deep ulcerations. Pain and irritation are evident within 3 minutes, but contact with dilute solutions may not cause symptoms for several hours. Contact with the eye may produce pain and irritation, and in severe cases, clouding of the eye and blindness.

Long-term exposure to sodium hydroxide in the air may lead to ulceration of the nasal passages and chronic skin irritation.

The U.S. Occupational Safety and Health Administration (OSHA) limits exposure in the workplace 2 milligrams of sodium hydroxide per cubic meter of air (2 mg/m³) per 8 hour day/40 hour work week. Nonetheless, in a study published in the American Journal of Industrial Medicine, textile workers in the cotton and wool industry in Croatia had higher rates of respiratory problems. In addition to sodium hydroxide, these workers were exposed to the following:

• direct dyes (sulfonated azo compounds)
• reductive dyes (indigo and indigo disodium salts, or anthraquinone derivate)
• disperse dyes (azo and anthraquinone structure of low molecular weight)
• naphtol dyes (azo with azochromophorm components)
• reactive dyes (azo and anthraquinone derivate-Cibakon E, Cibakon F)
• cation dyes (diphenylmethane derivate, triphenylmethane derivate, or triazine colors)
• sulfur dyes (sulfur compounds)
• acetic dyes (sodium salt of organic acids)
• acetic acid (CH3COOH)
• formic acid (HCOOH)
• sodium hydroxide (NaOH)
• sodium hydrosulfide (NaHS)
• potasium bicarbonate (KHCO3)
• chromium salt or ormaldehyde (HCHO).

Yikes! We might question the environmental effect of all these chemicals... In fact, after researching this I'm starting to wonder about non-organic yarns in general, as much as I love the wide array of intense colors and interesting fibers we can buy. Good thing there are plenty of organic yarns out there, not to mention ways of recycling yarn from sweaters and such!