Everywhere scientists look, from the deep sea to coastal waters to the surface of the Atlantic, they find these tiny fibers, which are less than 5 millimeters long. All along, the hope among scientists and environmentalists has been that organic fibers, like the cotton from your jeans, might not be as persistent in the environment as plastic ones, and would dissolve over time however in reality this does not seem to be the case, they are everywhere in our water.

Angelina W.M.G. Souren

Thanks, guys. Particularly that Revisto Bionatura article is interesting. Turns out that the answer I was looking for - as I am not a textile scientist - is this (with thanks to Sam Nicole Athey):

"Fibers consisting of natural materials can be further categorized as proteinaceous, cellulosic, or mineral. “Natural” fibers used in textiles are modified by the addition of chemical additives and treatments. We define cellulosic fibers with evidence of anthropogenic modification, such as the presence of chemical additives (including dyes) or manufactured morphology, as anthropogenically modified cellulose (AC). For example, while indigo denim is composed of cotton, a natural cellulosic fiber, it is highly processed and contains synthetic indigo dye as a colorant of the warp thread and other chemical additives that improve textile durability and performance. Denim is also constructed of weft thread that is commonly un-dyed."

Further reading:

• Article The Widespread Environmental Footprint of Indigo Denim Micro...

• Xue, J.; Liu, W.; Kannan, K. Bisphenols, Benzophenones, and Bisphenol A Diglycidyl Ethers in Textiles and Infant Clothing. Environ. Sci. Technol. 2017, 51, 5279−5286.
• Lacasse, K.; Baumann, W. Textile Chemicals: Environmental Data and Facts, 1st ed.; Springer: Dortmund, Germany, 2004.