In order to have the device compliant to the European Regulation 2017/745 we must be compliant to ISO 16900-2:2017 and ISO/CD 16975-3.

ISO 16900- 2:2017 - Determination of breathing resistance.
The device should not cause any breathing difficulty. It should allow the gases to pass through it smoothly.

ISO/CD 16975-3 - Physiological responses and limitations of oxygen and limitations of carbon dioxide in the breathing environment.
The mask should not limit the flow of oxygen in to the body and carbon dioxide out of the body.