However, if such a shelter is unavailable, the person should then be advised to follow the Shelter in Place protocol, or if given, emergency evacuation advice. Evacuation orders would entail exiting the area completely by following a path perpendicular to the wind direction and therefore perpendicular to the path of the fallout plume. Taking upper atmospheric winds into account, surface winds alone are not to be depended upon as indicative of the direction of fallout movement. "Sheltering in place" is staying indoors, in a preferably sealed tight basement, or internal room, for a number of hours, with the oxygen supply available in such a scenario being more than sufficient for 3+ hours in even the smallest average room, under the assumption that the improvised seal is perfect, until carbon dioxide levels begin to reach unsafe values and necessitate room unsealing for a number of minutes to create a room air change.