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. Accidents like Three Mile Island and Chernobyl released radiation and forced thousands nearby to evacuate, resulting in over 16,000 deaths from Chernobyl. Radiation must be handled carefully to avoid serious health problems.