Yep. Knew it had to be there somewhere on the website.
Just too lazy to go looking
And yes... like all optical surfaces, the advice is don't touch unless it's absolutely necessary.
You could try aiming a very low jet of distilled water at the surface/s showing dust. You could perhaps soak a few pure cotton balls in distilled water and gently swirl those around the filter and then rinse under distilled water or gentle running tap???