At what percentage of air does the vibralert sound?

The vibralert is a crucial safety feature in certain firefighting equipment, specifically designed to enhance situational awareness for firefighters in environments with potentially hazardous air conditions. It is calibrated to activate when the air supply in a portable breathing apparatus (PBA) reaches a critical level, alerting the user as well as nearby personnel of the need for immediate action.

The correct percentage for the vibralert to sound is 33%. This specific threshold allows firefighters to be notified before their air supply is critically low, providing them with an appropriate warning to retreat or replenish their air supply. This is vital because situations during fire and rescue operations can change rapidly, and having an early warning can make a significant difference in safety and effectiveness on the scene.

While the other percentages listed are common levels of interest in other contexts, they do not represent the specific setting at which the vibralert is designed to operate, thus making 33% the correct choice for this question.