Some BSCs exhaust their air through a HEPA filter directly in the cleanroom without interfering with the cleanroom pressure while other fume hoods and BSCs must exhaust the air to the outside (hazardous drugs or other chemicals), which unbalances room pressure. To maintain the correct pressure in the cleanroom, we must compensate the air output with fresh air. To do so we must know the quantity of air exhausted by the fume hood or biosafety cabinet.