The decision between sheltering in place and evacuating depends heavily on the predicted duration of the fallout and the proximity of the radiation source. During the initial phase of a nuclear event, radioactive particles settle on surfaces and decay rapidly. In these early stages, sheltering in place is often the safest option because the shielding provided by thick walls can significantly reduce your radiation dose while the most intense radioactivity decays.
Sheltering in place is most effective when the expected duration of high radiation levels is relatively short. If the radioactive cloud is expected to pass quickly, staying inside a sturdy building provides immediate protection from inhaling or touching particles. However, if the contamination is widespread and expected to persist for many days or weeks, the buildup of radiation inside a building could eventually make it unsafe.
Evacuation becomes the preferred choice if local authorities issue an order or if your current shelter cannot provide adequate protection against long term exposure. Moving away from the contaminated area reduces the total time you spend in a high radiation zone. Always follow the specific instructions of emergency management officials regarding when to stay put and when to move to a cleaner area.
