Aren't synthetic brushes more hygienic than animal hair brushes?

They certainly can be. In general, microbes prefer organic materials on which to multiply and grow. The microbes also prefer moisture and warmth. CosmeFibre®  inhibits water from entering the fibre, and the surface of the fibre is less hospitable to microbes than an animal hair. You still must regularly clean and sanitize all your makeup tools including your brushes. This will help maintain a hygienic application of makeup.