They are ordinary, biodegradable, organic plant material, things to which our bodies are accustomed to for millions of years. You might as well ask whether grass is toxic.
> The human body does not contain enzymes that digest cellulose. So how is this degradation to occur?
Mostly outside the body, before ingestion occurs.
> Being accustomed to something doesn't mean the thing doesn't hurt us.
I disagree. Evolution had the last ~360 million years (that's about how old cellulose is I believe) to evolve mechanisms to protect our body from cellulose particles.