The extent of damage depends on the duration and severity of the ischemia. If ischemia persists for more than 5 minutes with perfusion below 5% of normal, some neurons will die. However, if ischemia is mild, the damage will occur slowly and may take up to 6 hours to completely destroy the brain tissue. In case of severe ischemia lasting more than 15 to 30 minutes, all of the affected tissue will die, leading to infarction. The rate of damage is affected by temperature, with hyperthermia accelerating damage and hypothermia slowing it down and other factors. Prompt restoration of blood flow to ischemic tissues can reduce or reverse injury, especially if the tissues are not yet irreversibly damaged. This is particularly important for the moderately ischemic areas (penumbras) surrounding areas of severe ischemia, which may still be salvageable due to collateral flow.