There are a number of reasons;

Food production produces micro fibre or duct i.e. Flour

The operative has facial hair

The operative may have a potential germ transmission issue

There are airborne micro organisms that can effect health

Rayon is a synthetic fibre.

natural fibre

An acrylic fibre is a synthetic fibre made from a polymer.

Yes, flax is a fibre crop (hence linen). However, not all flax is grown for its fibre; some is grown for its oil. Hence, you have linseed flax and fibre flax. Flax fibre is primarily phloem fibre, meaning it's bast or soft fibre.