The Bureau of Mines conducted laboratory and in-mine tests on the racal airstream helmet (model #AH21) to evaluate its protection efficiency. Laboratory tests indicated that as ambient air velocity increased, the protection offered by the helmet decreased. In tests conducted at several coal mine longwall sections with face velocities under 400 fpm, the helmet was 84 pct effective at reducing respirable dust in the miner's breathing zone. However, at a high-velocity section, 1,200 fpm, the efficiency of the helmet was significantly reduced, confirming our laboratory results.
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