Exhaust Recirculation Percentage is the amount of air removed from a space and discharged to outside the building by means of mechanical or natural ventilation systems. According to ASHRAE 62.1.

      a. None: No exhaust air is recirculated.
       b. Exhaust Air Recirculation 20%: 20% Exhaust air is recirculated along with fresh             air to maintain indoor comfort.
       c. Exhaust Air Recirculation 40%: 40% Exhaust air is recirculated along with fresh              air to maintain indoor comfort.
       d. Exhaust Air Recirculation 60%: 60% Exhaust air is recirculated along with fresh               air to maintain indoor comfort. 

Understanding HVAC Concepts
The control of air flow or ventilation in buildings is important for maintaining suitable indoor air quality and subsequently maintaining a comfortable and healthy interior environment. Outdoor air enters a building through its air intake to provide ventilation air to building occupants. Likewise, building exhaust systems remove air from a building and expel the contaminants to the atmosphere. If the intake or exhaust system is not well designed, contaminants from nearby outdoor sources (e.g., vehicle exhaust, emergency generators, exhaust stacks on nearby buildings) or from the building itself (e.g., laboratory fume hood exhaust, plumbing vents) can enter the building before dilution. Poorly diluted contaminants may cause odors, health impacts, and reduced indoor air quality. 

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