Basic Life Support (BLS) Course B Progress Indicator: Question 2 of 25 2.  Overcasts and undercasts are used to permit the two air currents to cross without the intake air short-circuiting into the exhaust.  Undercasts are seldom used in a mine because _________. Choices: they are difficult to construct it is cheaper to build overcasts they are apt to fill with water and debris None of the above