Venting: 

When a key is pressed in a membrane switch, the air pressure within the switch cavity increases. In order for the switch to close properly, air within
a switch cavity must be displaced, equalizing the internal pressure. There are two standard venting methods that can solve this issue:

Internal Venting: 

Designing narrow channels between key location cutouts (in the spacer layer) allowing the air from one key to move to another key locatoin when that key is pressed. 

External Venting: 

Similar to internal venting, narrow channels between keys are cut into the spacer layer. These channels then exit through the sides, rear, or internal cut-outs of the membrane switch. External venting is not recommended for membrane switches exposed to harsh environments, as there is a greater risk of contamination.