Shelter in place (also known as a Shelter In-Place Warning SAME code: SPW) is to seek safety within the building you already occupy, rather than to evacuate the area or seek a community emergency shelter. The American Red Cross says the warning is issued when "chemical, biological, or radiological contaminants may be released accidentally or intentionally into the environment." and you should "select a small, interior room, with no or few windows, and taking refuge there."
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Radiological and chemical defense
It entails closing all household doors, windows and vents and taking immediate shelter in a readily accessible location that puts as much indoor air and/or radiation shielding-mass between the individual and the hazardous outside air, such as a basement or centrally located medium to small room, and trying to make it as airtight as possible by shutting off all ventilation/HVAC systems and extensively sealing the shelter's doors and windows from all outside air contaminants with damp towels, or if available, plastic sheeting and adhesive tape. Diagrams of what sheltering in place entails following a chemical, biological, radiological or nuclear (CBRN) threat, and how long it is advised to be done for, is provided by the U.S. Federal Emergency Management Agency-affiliated website Ready.gov.
Shelter-in-place effectiveness has been evaluated and experimental results show that proper sealing can make a substantial difference to a normal home shelter, finding it to be at least twice as effective against a host of airborne substances when compared against simply staying inside and not implementing the countermeasure, and in most airborne contaminant cases, it is usually much more effective, depending on the particle size of the substance in question. If the occupant's breathing is the only consumer of oxygen and producer of carbon dioxide in the room, then carbon dioxide levels would not begin to reach dangerous values until 3+ hours had passed, in most likely, 4-person home scenarios.
In the military, "Shelter-in-Place" is comparable to "buttoning up" and has proved life-saving in certain nuclear fallout instances.
The danger of radiation from radioactive precipitation/"fallout" decreases with time, as radioactivity decays exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. Creating the following 7-10 rule-of-thumb after a typical nuclear detonation while under the conditions that all fallout that will fall on the land has done so completely and no further deposition in the area will occur - After 7 hours, the average dose rate outside is reduced by a factor of ten; after 49(7x7) hours, it is reduced by a further factor of ten (to a value of 1/100th of the initial dose rate); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared to the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.
If an individual finds themselves outside during an emergency that calls for shelter-in-place, then effective but low-tech decontamination is required before entering into the shelter.
The phrase "Shelter-in-place" has also erroneously been used, instead of the more accurate lockdown, to describe precautions to be taken by the public when violence has occurred or might occur (particularly in shootings) in the area and the perpetrator is believed to still be in the area but not apprehended. The public in the area is advised to carry out all the same tasks as a typical shelter-in-place but without the key step of sealing the shelter up to prevent outside air from circulating indoors, in this scenario people are simply urged to lockdown — stay indoors and "close, lock and stay away from external doors and windows."
Reasons
In light of active shooter events, and the panic that can ensue, a lockdown alert, although often erroneously issued as "shelter in place", is also being implemented as a response to armed events, such as the 2014 Fort Hood shooting. Similarly a lockdown alert was likewise issued as a "Shelter-in-place" alert by the Massachusetts Emergency Management Agency over the cell phone Wireless Emergency Alerts service for local residents during the manhunt for the Boston Marathon bombing suspects.
Implementation
Residents of an affected community might be informed that shelter in place is being implemented through the news media, Emergency Alert System, Reverse 911, warning sirens or horns, National Oceanic and Atmospheric Administration weather reports, and announcements from vehicles equipped with public address systems. In the United States, facilities like nuclear power plants are required to be equipped with audio alert systems that can be heard within a 10-mile (16 km) radius.
Once a shelter in place is called, residents are expected to immediately go indoors, bring all children and animals with them, and to close and lock windows and doors. All ways in which outside materials may enter the shelter area should be eliminated, including closure of fireplace dampers, shutting off ventilation or climate control systems, and prepare an area for pets to eliminate waste that does not require allowing them outside. If told to do so via television or radio, those sheltering should seal their rooms with duct tape and plastic. Upon reaching shelter, those who were outside for a period of time seeking shelter after the shelter in place was called and who may have been exposed to chemical contaminants should remove all outer clothing, put it in a plastic bag, and wash with warm water. After an announcement that the shelter in place is over, residents should go outside and open all doors and windows to ventilate the shelter. Similar processes should be followed in cars, workplaces, or schools.