Re-entrainment refers to what phenomenon?

• A. Fresh outdoor air entering the building
• B. Air from outdoors leaking around seals
• C. Exhaust air drawn back into a building
• D. Recirculation of indoor air through filtration

Answer: (C)

Re-entrainment is when the air that has been exhausted from a space doesn’t fully escape to the outdoors, but is pulled back into the building. This can happen when wind and pressure differences near a building cause the exhaust plume to linger near intakes, so the exhausted air gets drawn back in rather than dispersing away. That’s why the correct description is exhaust air being drawn back into the building.

The other scenarios describe different airflow phenomena: fresh outdoor air entering is ventilation, air leaking around seals is infiltration, and recirculation of indoor air through filtration is internal air cycling. Re-entrainment specifically involves the returned intake of exhaust air into the building.