The primary concern when using a nonrebreathing mask for O2 delivery is:

The primary concern when using a nonrebreathing mask for oxygen delivery is mask fit and patient comfort. A nonrebreathing mask is designed to provide high concentrations of oxygen, and for it to be effective, it must fit snugly on the patient's face to prevent any leakage of ambient air. If the mask does not fit properly, the delivery of oxygen can be compromised, potentially reducing the effectiveness of the therapy.

Patient comfort is also crucial because if the mask is uncomfortable or difficult to tolerate, the patient may refuse to wear it or may not be able to keep it on for the required duration. This can lead to insufficient oxygen delivery, undermining the therapeutic goals.

Proper flow rate, reservoir capacity, and concerns about oxygen toxicity are important considerations in oxygen therapy in general, but they are secondary to ensuring a proper fit and comfort for the patient when using a nonrebreathing mask specifically.