It can be used either way. Checkout the diagram in the product pictures. I have a dust cover over it to filter dust. https://www.amazon.com/gp/product/B01DXYMHWS
One of the product pictures shows a diagram. Looks like the filter can be used either way. I also have a dust filter (not pictured) that I still need to put over it. https://www.amazon.com/gp/product/B01DXYMHWS