Which is a design feature of an oro-nasal oxygen mask?

An oro-nasal oxygen mask is specifically designed to cover only the mouth and nose of the wearer. This targeted coverage is essential for delivering supplemental oxygen directly to the respiratory system while allowing for normal communication and facial expressions. The design ensures that the mask fits comfortably and effectively, providing optimal oxygen flow without obstructing the entire face.

The other options do not accurately describe the primary function and design of an oro-nasal mask. For instance, a mask that covers the entire face would be classified as a full-face mask and is used in different applications, such as in anesthesia or certain types of mechanical ventilation. Similarly, ear coverage is not a functional requirement for oxygen masks. The inclusion of a built-in oxygen monitor is more characteristic of advanced medical devices rather than standard oxygen-delivery masks, which focus primarily on the delivery of oxygen. Thus, the defining feature of an oro-nasal mask is its design that covers just the mouth and nose.