Avoiding halo's when compositing volumetrics?

Hope someone can help with an issue I have composting a smoke/dust pass over a scene. I have rendered the scene with transparancy on, seperate Env pass and seperate volumetrics. When I try to re-build the shot in the compositor, I’m getting a nasty halo around elements that appear in front of the environment. If I turn off the environment, then it seems to composite fine…so thinking I should have set something somewhere to prevent what I’m seeing…anyone have any ideas? Thanks

With Environment:

Without Environment: