I've found some information about Fujix, the manufacturer of the threads I use. The company has been assessed by the Japan Chemical Quality Assurance (JCQA) and is certified which means it practices in an environmentally responsible manner. It is based in Kyoto and the Shiga area of Western Japan.
It has also gained the right to use the Oeko-Tex Standard 100 label which means it has been tested for harmful substances.
Here's an excerpt from the site: Textiles with this mark are
- Textiles that do not contain allergenic dye-stuffs and dye stuffs that form carcinogenic arylamines of the MAK-groups III A1 and III A2.
- Textiles, that had been tested for pesticides and chlorinated phenoles.
- Textiles that have been tested for the release of heavy metals under artificial perspiration conditions.
- Textiles free from formaldehyde or containing trace amounts significantly lower than the required legal limits.
- Textiles with a skin friendly pH
- Textiles free from chloro-organic carriers
- Textiles for garments free from biologically active finishes
I've also seen this Oeko-Tex label on clothing for newborns at Uniqlo. Reassuring isn't it.