Answers to some frequently asked questions about radiation poisoning, drawn from the Science Media Centre of Japan, the World Health Organization in Geneva, the Centers for Disease Control and Prevention in Atlanta, and the Environmental Protection Agency in Washington.

Q: Should people in Tokyo avoid open air?

A: There is no need because Tokyo is far enough away.

Q: What is the worst-case scenario?

A: It depends on how much radiation leaks and the prevailing weather conditions. Radioactive iodine, or I-131, is heavier than air and won’t spread far in mild wind. Iodine 131 has a half-life of eight days.

Q: Is there a risk of secondary radiation exposure from contaminated food?

A: Based on reported radiation levels, there should be no cause for concern.

Q: How do radioactive materials contaminate food?

A: Atomic bomb tests in Nevada during the 1950s and 60s released I-131 into the atmosphere, which was blown thousands of miles away. Animals grazing on I-131 contaminated pastures had the radioactive material in their milk, which poisoned some children who drank it. People exposed to I-131 may have an increased risk of thyroid cancer.

Q: What is ionizing radiation?

A: Ionizing radiation is the energy or particles produced by unstable atoms of radioactive materials. Humans are exposed to low levels of radiation naturally from the earth and the sun.

Q: What are the health consequences of radiation?

A: Exposure to high levels of radiation can cause acute radiation syndrome, or radiation poisoning, resulting in substantial damage to human body tissues, premature aging, and possibly death. Prolonged exposure to lower levels is also associated with increased risk of ill-health.

Q: What are the symptoms of radiation poisoning?

A: The first symptoms of acute radiation syndrome are typically nausea, vomiting and diarrhea. These symptoms start within minutes to days after exposure, and last for minutes to days. A person with acute radiation syndrome may look and feel healthy for a short time, then become sick again with loss of appetite, fatigue, fever, and possibly seizures and coma. This stage may last a few hours or several months. Radiation poisoning also typically causes skin damage.

Q: What level of radiation is dangerous to human health?

A: Radiation is measured in sieverts. Exposure to one sievert of radiation can cause hemorrhaging, four sieverts can cause death within two months, and 2,000 sieverts can cause loss of consciousness within minutes and death within hours.

Q: How is radiation poisoning treated?

A: Potassium iodide can be used to block radioactive iodine from being taken into the thyroid gland, protecting it from injury. It cannot protect other parts of the body, or reverse damage to the thyroid once it has occurred. Prussian blue, a dye used by artists and manufacturers since 1704, can also be used to remove certain radioactive materials from the body.