These portable Grade-D filtration systems are designed to provide breathing air for a specific number of workers. The system filters inlet air to provide respirator users with Grade-D quality air and monitors for CO and/or Oxygen. The first stage element filters bulk water and particulate, and has an auto drain. The second stage coalescing filter eliminates atomized oils, mists, ultra-fine particulates and has an auto drain. The third stage filter removes organic vapors and odors and has a manual drain. Filter change indicators are standard on all three stages of filtration. Filtration efficiency is 99.99% at .01 micron. All Breather Boxes™ are designed to be used in the upright and closed position. The standard carbon monoxide monitor operates on disposable 9-volt batteries, 12-volt DC line voltage or 115 VAC for continuous detection. An external alarm and light signals the workers of high CO content.