Nutritional emergencies arise from situations of mass starvation caused by the interruption of food supplies to the population over a long period. Unusual food shortages may be caused by major crop failures, war and civil conflicts, or natural disasters. According to the World Health Organization, the emergency subsistence level is the level below which large-scale starvation and death should be expected if the people are of normal body size and are required to perform some work. Measurement of nutritional status in emergencies relies mainly upon taking body measurements, monitoring clinic records, or measuring the prevalence of oedema (swelling).