According to all the information I can find, including some chemistry text-books and "processing guideline" manuals, bulk Osmium is considered generally safe, because it does not produce toxic osmium tetroxide at room temperature (only above 400C). Powdered osmium sponge, on the other hand, transforms into osmium tetroxide at room temperature.

My question is: Why does a block of osmium not constantly form a small amount of osmium tetroxide on its surface?

Osmium tetroxide can't form a protective layer on the surface, as it is a liquid (and it can evaporate), so why is oxidation not continually occurring?

I get that it has less surface area than a powder, but so does a chunk of iron vs iron powder, and iron chunks still rust, hence my confusion.