A meter records low levels of radiation in Tokyo. The blast raised levels to 11,930 micro sieverts near Japan’s Fukushima Daiichi plant, 150 miles away. (KYODO)

The International Atomic Energy Agency reports that 185,000 people have been evacuated from towns near the plant, and that Japan has distributed 230,000 units of stable iodine to nearby evacuation centers. (Day 5 updates here.)

Here’s a primer on the emergency situation:

What’s the latest on radiation levels in Japan?

Radiation, defined as energy traveling through space, has two types: non-ionizing and ionizing. Ionizing radiation, commonly found in man-made devices, is the type emitted from Japan’s nuclear reactors and support facilities. Radiation levels measured 11,930 micro sieverts per hour after Tuesday’s explosion, several times the amount a human can be exposed to in one year. Radiation shrank to 496 micro sieverts per hour within hours after the explosion — a number government spokesman Yukio Edano called “much higher than the normal level ... but one that causes no harm to human health.”

What is causing the explosions?

During normal conditions, the fuel rods in the a reactor’s core are submerged and cooled by water. A loss of grid power triggered by the earthquake compromised the reactors’s ability to regulate temperature. This graphic explains the circumstances leading up to the explosions, and The Wall Street Journal has an explainer piece here.

Who is being evacuated?

The Japanese government advises that those living within 12.5 miles of the plant, which is 150 miles north of Tokyo — to leave the area. People living within 19 miles have been advised to stay indoors.

How could iodine help?

Iodine has not been administered to residents yet, but a dose of stable iodine can prevent accumulation of radioactive iodine (I-131) in the thyroid gland for up to 24 hours — this would buy each evacuated person a little longer than one day of protected exposure if the situation worsens. This piece from NPR will tell you more about how stable iodine works.

What are the other risks?

Other than radiation, psychological stress is dangerous to people directly affected by the rising nuclear threat, according to Fred Mettler, a University of New Mexico professor who studied Chernobyl for the World Health Organization:

The psychological effects were the biggest health effects of all — by far. In the end, that’s really what affected the most people.

Human inability to detect radiation — it has no odor or color — can pose more of a psychological threat than a physical one. Although radiation is a carcinogen, it doesn’t pose the danger that others do. Studies of more than 80,000 survivors of the Hiroshima and Nagasaki blasts have found that about 9,000 people died of some form of cancer, but only about 500 cases were from heightened radiation exposure.

What is the total death toll from the earthquake and tsunami?

Along the coast, 6,000 are confirmed as either dead or missing, according to police. Officials found about 2,000 bodies along the coast Monday, and the Kyodo News Agency found that about 30,000 people in devastated areas are unaccounted for.

Over the course of the tsunami and earthquake recovery effort, more than 500,000 people have been moved from hard-hit areas and 15,000 have been rescued.

Got a question?

Leave it in the comments below or tweet your question to @washingtonpost using #JapanQuestions. At 2 p.m. EST Tuesday, Adrian Heymer, executive director of Strategic Programs at the Nuclear Energy Institute, will be online to answer them.