Radiation can be useful but also dangerous to humans. The unit of rem is used to measure radiation dosage based on the harm it causes people. Exposure to radiation can cause nausea, vomiting, headaches and loss of white blood cells. Many people killed by the Hiroshima and Nagasaki atomic bombs actually died from radiation effects, not the explosions. The Three Mile Island accident occurred due to maintenance worker errors, forcing 140000 people to evacuate near the plant. The Chernobyl disaster released radiation equivalent to ten Hiroshimas and killed over 16000 people.