An air-entrainment mask will deliver an FIO2 higher than intended when:

An air-entrainment mask is designed to deliver a specific fraction of inspired oxygen (FIO2) by mixing oxygen with room air. The principle behind its operation relies on the air-entrainment ports that allow ambient air to mix with the oxygen flow, achieving the desired FIO2.

When the air-entrainment ports are blocked, the mask relies solely on the oxygen supply without the necessary mixing with room air, resulting in a higher concentration of oxygen being delivered than intended. This situation effectively reduces the amount of room air that is supposed to dilute the oxygen flow, thus increasing the FIO2 beyond the set level.

In contrast, if the flow is set too high, the mask is still functioning as designed by mixing oxygen with room air, although it may deliver a different concentration than intended. Increasing humidity through nebulization can have varying effects but typically does not directly affect the FIO2 delivered by an air-entrainment mask. Adding corrugated tubing can lead to resistance but does not block the air-entrainment ports; thus, it would not directly cause a higher-than-intended FIO2.

In summary, the critical factor is that blocking the air-entrainment ports removes the necessary source of ambient air,