How safe is it to shelter in place?
A 2001 report by The National Institute for Chemical Studies cited several studies that demonstrate the value of sheltering in place during a chemical emergency.
In 1996, the U.S. Army did a study on how fast the air exchanges in the average older home. In older homes, the average house changed its air at a rate of less than 1/3 change per hour. A second study found if one room in the house was sealed up with duct tape and plastic, the amount of chemical that was in the room after one hour was between 1/7 to 1/17 of what was outside.
Another study found that sealing up a house also filtered out some of the chemicals. Not only did a sealed-up house limit the amount of air coming into a house, the walls actually filtered the air that did seep in. Sheltering in place cannot completely eliminate all exposure to the chemical, but it can keep the exposure below dangerous levels.
In conclusion, the report said:"For the vast majority of events that have led to the public sheltering in place, there have been no reported injuries. In fact, for a very few cases, clouds of toxic materials of sufficient concentration to cause harm have entered communities and, because sheltering in place has been accepted by the community and was successfully implemented, no one was injured. The body of evidence suggests that if there is insufficient time to complete an evacuation, or the chemical leak will be of limited duration, or conditions would make an evacuation more risky than staying in place, sheltering in place is a good way to protect the public during chemical emergencies."