Using human hair for amigurumi stuffing
Human hair is primarily made of keratin and is usually somewhat oily (a result of the sebaceous glands lubrication to help the follicles grow out of their subepidermal sheathing). Although keratin also forms the basis of horn, claws and even fingernails, it is an organic protein that can serve as food for microorganisms. It has a long history in art, which is why I think it is a good idea to look at what professional art restorers have to say:

Human hair is a fragile organic material, which is sensitive to light, temperature and relative humidity.
... common degradations are: abrasion / breaking / undulation / infestation by lepidoptera and dermatophytes

So if you are going to use it as a material, I would recommend that you take a couple precautionary risk-minimization approaches to support its longevity by reducing its stress factors.
Dermatophytes are fungal colonies that on humans are pretty hard to treat, but you can take a trick from Ayurveda for treating water: get some uncoated and very thin copper wire from an electronics store (or harvest some from an old cable by defraying it and washing it in acetone). Copper ions are antifungal and antibacterial.
Next, I would recommend occasionally misting the object with a diluted (1:10) lavender essential oil to distilled water. This will not only impart a pleasant and sweet smelling note to the object, but it will also inhibit mite infestation.
Finally, you will want to consider adding a desiccant to remove ambient humidity inside the object (unless it will always stay in a dry, arid region). If you are not worried about the object being accidentally consumed by small children or pets, you might add a sachet of silica-gel inside (like the ones you can find in new shoeboxes or leather gloves).
Let us know how it goes!!!