# Including air as part of mesh

**URL:** <https://discourse.slicer.org/t/including-air-as-part-of-mesh/27439>\
**Category:** Support\
**Created:** [January 24, 2023, 12:23pm UTC](https://discourse.slicer.org/t/including-air-as-part-of-mesh/27439 "2023-01-24T12:23:32Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![ascam](https://avatars.discourse-cdn.com/v4/letter/a/b487fb/32.png) [@ascam](https://discourse.slicer.org/u/ascam)\
**Post date:** [January 24, 2023, 12:23pm UTC](https://discourse.slicer.org/t/including-air-as-part-of-mesh/27439/1 "2023-01-24T12:23:32Z")

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Hi there.

I’m looking to run a FEM model of light propagation across biological tissue (including free space / pockets of air), and am using Slicer to create a volumetric mesh. I need to include air as part of the mesh. Does anyone know how to include air / free space as part of the mesh?  
Thanks

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [January 24, 2023, 12:26pm UTC](https://discourse.slicer.org/t/including-air-as-part-of-mesh/27439/2 "2023-01-24T12:26:57Z")

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If you use “Cleaver” mesher in SegmentMesher extension then it can add such background material. You can also add a background segment on Segment Editor by using any editing tool (e.g., Paint or Scissors) with choosing “outside all segments” as editable region.