Mechanical Ventilation Pitfalls

Once contaminated air has entered a building, the HVAC system can transport it rapidly throughout via vents and ducts. What's more, the concentration of toxins in this air can be very high, especially if the incident occurred nearby and has not been diluted by natural dilution and other barriers.

Chimney effect
Within each ventilation zone, contaminated air can be drawn through return ducts and distributed through supply ducts at high rates of flow. It can also be driven and distributed by the chimney effect of stairwells and elevator shafts, especially during the winter when the difference in temperature between the inside and outside air is at its greatest. At ground level, the flow due to chimney effect is inward in winter and outward in summer so a ground level release is more likely to be drawn into a building during the winter.

In fall and spring, buildings that use economizer systems typically introduce much greater volumes of outside air to reduce the operating costs related to air-conditioning. Contaminated air can also be transported between ventilation zones of a building through hallways and occupied spaces.