Hazardous Materials: What You Need to Know
Federal Emergency Management Agency (FEMA)
Shelter Safety for Sealed Rooms
Ten square feet of floor space per person will provide sufficient air to prevent carbon dioxide build-up for up to five hours, assuming a normal breathing rate while resting.
However, local officials are unlikely to recommend the public shelter in a sealed room for more than 2-3 hours because the effectiveness of such sheltering diminishes with time as the contaminated outside air gradually seeps into the shelter. At this point, evacuation from the area is the better protective action to take.
Also you should ventilate the shelter when the emergency has passed to avoid breathing contaminated air still inside the shelter.