Shut off the ventilation system and latch all doors and windows to reduce airflow from the outside.
Go to your shelter-in-place room (where your precut plastic, duct tape, radio, and other supplies should be stored).
Use precut plastic sheeting and duct tape to cover openings where air can enter the room, including doors, windows, vents, electrical outlets, and telephone outlets.
Listen to Emergency Alert System broadcasts on a battery-powered radio for the all clear. You will generally not need to stay in a sealed room for more than a few hours.
After contaminants have cleared, open windows and vents and turn on fans to provide ventilation.