When sheltering people in place, the citizens should be directed to:

• A. Close all heating, ventilation, and air conditioning systems
• B. Open windows for air exchange
• C. Keep all heating, ventilation, and air conditioning systems running
• D. Periodically open the doors to relieve the pressure

Answer: (B)

When sheltering in place, the goal is to reduce people’s exposure by controlling how air moves between indoors and outdoors. Opening windows for air exchange creates a dilution effect: outdoor air replaces indoor air, lowering the concentration of any contaminant that might be present inside. This helps prevent stagnant indoor air from concentrating hazards and keeps people safer than keeping the air completely contained or relying on systems that could recirculate contaminated air.

Keeping heating, ventilation, and air conditioning systems running or closing them off entirely can either pull in contaminated outdoor air or trap contaminants inside, leading to higher exposure. Periodically opening doors can cause unpredictable air currents and pressure shifts, which may draw in more contaminants rather than achieving steady dilution.

So, directing people to open windows for air exchange models the practical idea of diluting indoor air with fresh air to reduce exposure during sheltering in place, assuming outdoor air is not worse than indoors.