Dirt Masks overlay a 2D image on to the screen, so won't work as standard in 360 content becasue they will repeat the image across each of the 6 faces that make up the 360 image.
When enabled in the 360 camera’s Post Process Settings, the Dirt Mask will be applied to all faces identically, so the image will be repeated. 
- To create a 360 compatible Dirt Mask, you can wrap the Dirt Mask texture around a sphere and positioning that around the 360 camera.
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The viewer’s field of view will be significantly more zoomed in, so dirt masks will need to be more intensely packed together to have a similar effect. The viewer will also be free to look around the whole map, so dirt masks with a central focal point may not be as effective.
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The viewer’s actual field of view will be more like this:
