Here are some commonly asked questions about the earthquake and nuclear disaster in Japan. Submit your questions using the form below.

1. Japan earthquake
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   2. Another setback in effort to control Japan reactor
   3. Most nuclear plans on track outside Japan, Germany
   4. After Japan quake, victims still await aid
   5. Superheroes cheer children in Japan's disaster zone
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      Four British citizens held in Bali cocaine case
   7. 
      Time-lapse of  aftershocks
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      Images of chaos, destruction

Question: I've read that food and water in Japan has been contaminated by radiation. Should I be concerned that produce or fish from Japan will end up on my dinner table?

Answer: Some produce and meat products from Japan have been found to have radiation levels in excess of the allowable level. But experts say that at current levels, the contamination holds no risk for American consumers and only a minor, manageable risk for people living near the damaged nuclear complex. And fish are less likely to be hit by unsafe levels of radiation than land-dwelling species, they say.

Question: I’ve heard that plutonium has been found outside the Fukushima plant? What is the significance of that?

Answer: The Tokyo Electric Power Co., which operates the plant, said it found three radioactive isotopes of plutonium — plutonium 238, 239 and 240 — in five locations outside the plant in soil tests on March 21-22. It said two samples out of the five may be the direct result of the accident at the plant following the tsunami. It said the detected levels were minute — almost the same as those from the fallout detected in Japan following past nuclear tests by the United States and Russia, which ended in 1980 — and do not pose a major risk to human health

The reason plutonium is such a concern is that it is the most toxic of the isotopes that can be released from a nuclear reactor or explosion. Plutonium does not exist in nature. It can only be created by nuclear fission. Plutonium is fatal to human in very tiny doses. It accumulates in the bones and it has very long half lives. Plutonium 238 has a half-life (half of it will be gone) of 88 years and Plutonium 239 a half-life of 24,100 years.

That's scary stuff, but some experts say that plutonium may not pose as much of a risk to public health as radioactive iodine, which is being produced in far greater quantities. You can read more about that here. And you can learn more about plutonium in this fact sheet from the U.S. Argonne National Laboratory.

Question: What are the health risks of radiation?

Answer: Radiation can kill at high levels. For example, 50 percent of people given whole body exposures of 4 sieverts for an hour would be expected to die within 60 days without medical attention, mostly from infections. Duration of exposure also plays a role in determining the health risks.

Acute Radiation Sickness often appears with exposure to between 0.5 and 1 sievert per hour. Symptoms include fatigue, hair loss, nausea, vomiting and diarrhea as well as skin changes such as swelling, redness, itching and radiation burns.

Exposures of less than 0.5 sieverts typically produce changes in blood chemistry, but no symptoms, according to the U.S. Environmental Protection Agency.

Experts say exposure to lower levels of radiation has the potential to cause various kinds of cancers, with higher levels of radiation seen as more dangerous.

The highest radiation reading recorded so far outside the Fukushima plant was 1,000 millisieverts per hour — or 1 sievert — in contaminated water in an overflow tunnel outside the plant’s Reactor No. 2 on March 27. The highest reading in the air so far was 400 millisieverts, or 0.4 sieverts, per hour on March 14, according to Japanese authorities.

You can learn much more about radiation risks and effects here.

Question: Is fallout from a nuclear bomb the same as fallout from a nuclear plant accident?

Answer: No. Due to differences in nuclear fuels and the resultant reactions that take place when a reactor accident occurs, the radioactive elements of the fallout (called isotopes) from a nuclear power plant accident are not the same as those found in fallout from the detonation of a nuclear device. Read more in this U.S. State Department fact sheet.

Question: What happens if the fuel rods or spent fuel at the Fukushima plant completely melt down?

Answer: That's a matter of some debate in the scientific community, but both sides agree that the worst case would be a powerful explosion that would distribute radiation over a wide area.

Click here to read more about the meltdown scenario, or here for a March 24 article on what's going on at the nuclear complex.

Question: Why not entomb the plant, as the Russians did with Chernobyl?

Answer: Authorities in Japan aren't yet thinking about a permanent entombment. But they are looking into the idea of covering up the "hot" fuel rods being stored at the site with piles of sand and soil, laced with lead and neutron-absorbing boron. You can read more about this option here.

Question: What is the risk of the released radiation to the U.S.?

Traces of radiation from the damaged Fukushima plant have reached the United States and traveled

as far east as Massachusetts

, but

pose no health threat

, public health experts say.  

Question: Then why are people snapping up potassium iodide tablets?

Answer: May people are frightened and are acting out of an overabundance of concern. Potassium iodide is used to "block" the thyroid's intake of iodine-131, a radioactive isotope released during nuclear fission that can cause thyroid cancer. Experts and public health authorities are not recommending that Americans take any precautions at this juncture.

Question: How do we know how much radiation is reaching the U.S.?

Answer: The U.S. cannot monitor sparse concentrations of radiation from space. However, the Departments of  Defense and Energy and the Environmental Protection Agency and the Pentagon are capable of monitoring radiation in the air. Click here to learn more about U.S. radiation monitoring.

Question: I've heard Americans are being evacuated from Japan. What are the details?

Answer: The State Department in mid-March authorized the voluntary departure of up to 600 family members and diplomats who wish to leave Japan. The department also said it would assist Americans wishing to leave the country. Here are some details on that. It also set up an e-mail address — japanemergencyusc@state.gov — for Americans seeking help leaving the country. The Pentagon also has signed the order authorizing a voluntary departure for eligible families members on the entire island of Honshu. It is unknown how many people would be eligible, but a spokesman confirmed that the number is in the "thousands."

Question: I’ve been planning a trip to Tokyo, but the earthquake, tsunami and nuclear crisis has me worried. Should I go?

Answer: The U.S. State Department is advising Americans to defer travel to Japan if possible. The government also is offering to evacuate Americans in Japan.

Question: Are any Americans confirmed to have died in the quake or tsunami?

Answer: A single death of a U.S. citizen — 24-year-old teacher Taylor Anderson of Richmond, Va. — has been confirmed.  State Department officials say "several" other Americans remain unaccounted for. The department has a web page with more information.

Question: Could a similar nuclear disaster happen here?

Answer: The Nuclear Regulatory Commission says the odds of an earthquake causing catastrophic failure at a U.S. nuclear plant are astronomical — assessing the risk on average as a 1 in 74,176 chance that a plant's nuclear core could be damaged by an earthquake, potentially exposing the public to radiation. The NRC says that "operating nuclear power plants are safe," but it also recently revised its risk assessments based on new seismic data and acknowledges that, for some plants, the margin of safety has been reduced. You can read more about the risk assessments here.

Question: Are coastal U.S. plants also safe if they are swamped by a tsunami?

Answer: Operators of California's two nuclear plants say they are well prepared for a possible tsunami strike. Some experts, however, want new risk assessments in light of the catastrophe in Japan. Learn more here.

Question: How much will the twin disaster in Japan cost?

Answer: It’s early in the assessment process, so any estimate is just that. But the Japanese government is forecasting the damage at $309 billion, which would make it the costliest natural disaster ever. For purposes of comparison, Hurricane Katrina left $125 billion in damage in its wake. Read more about estimating the damage.

Question: Will some products be difficult to find as a result of the disruption to the Japanese economy?

Answer: Experts say the auto industry will be affected by the shutdown of manufacturing centers in Japan, with the subsequent parts shortage also hurting foreign automakers. Some market watchers see price hikes coming as the impact of the disruption to supply chains is felt. Prices and availability of electronic goods also may be affected, with Toshiba, Sony, Panasonic, Fujitsu and Mitsubishi all reporting disruptions to their production lines.

Question: What can I do to help?

Answer: Click here for a list of relief organizations assisting in Japan.