What is the initial percentage of air supply in an SCBA designated for working and exiting?

The initial percentage of air supply in a Self-Contained Breathing Apparatus (SCBA) designated for working and exiting is 66%. This percentage is critical in ensuring that the user has an adequate amount of breathable air for both performing tasks in potentially hazardous environments and for safely exiting the area.

The 66% air supply provides a sufficient buffer for working activities which may require additional oxygen due to exertion or heavy breathing. This is especially important in emergency situations where time is constrained, and respiratory needs may increase because of physical activity.

In contrast, options representing percentages lower than 66% would not provide an adequate margin for safety during extensive use and emergency egress, while percentages higher than 66% may not reflect the optimal operational level required for most standard SCBA operations. Thus, the selection of 66% represents the balance necessary for operational efficiency while ensuring safety in hazardous situations.