https://blendermarket.com/products/procedural-air-and-surface-dust

This unique tool allows you to quickly generate aerial dust particles.  Works pretty well with Depth of Field and gives you that extra realism to your shots. As you are emitting from a mesh, you doesn't even need to simulate, just create a box in the area want the dust particles, and you are ready to render. As it uses instances, you can use an insane amount of particles without using too much memory. It also allows you to simplify how those particles is shown on the viewport, so you don't have a huge impact on the performance while working on the scene