I just got a few Thuja occidentalis and notice that they have some brown on the inner parts of the leaves/needles. All of it is at the inner part of the plant. There does not seem to be any dying of the leaves. I am not sure if it is simply dirt/dust that has collected over time or if it is some type of fungal/smut type issue?

I can't find any disease that show up as a dust like cover on inner needles.
Picture shows it best on the leaf/branch in the middle of the image.

It looks like normal browning of interior leaves due to lack of light caused by new growth. Unless there is some actual spore material as well.

It's mainly the powder/dust that is ON the leaves that I am wondering about. It is a physical powder that I can wipe/flake off. Right now I am leaning towards it just being dust/dirt build up on the leaves.

It look like the fungal disease that effect thuja are less dangerous, and also don't seem to have the same symptoms that I see here.