CNN's Chief Medical Correspondent Dr. Sanjay Gupta offers an eyewitness look at Japan's natural and nuclear disasters, "Sanjay Gupta, M.D." 7:30 a.m. ET Saturday-Sunday.
Akita City, Japan (CNN) -- Just a few hours ago, we made the decision to travel from Sendai to an area in northwestern Japan called the Akita prefecture. Collectively, we did this to be careful, not fearful. To understand why, it is worth explaining what has happened and what the impact is on the human body.
Since the tsunami struck last Friday, there have been concerns about the nuclear power plants in Fukushima. At first, we heard the necessary cooling systems were not functioning well. There were several instances where gases, mainly hydrogen mixed with low levels of radioactive particles, were vented from the facilities to prevent a pressure overload. There were three separate explosions, and Tuesday, a fire burned at one of the six reactors.
Japanese government officials and the nuclear agency assume the high possibility of at least a partial meltdown in three of the reactors -- but they can't get close enough to confirm that. As far as the worst case scenario -- a complete meltdown? Officials tell us the safety features of the plant -- the containment layers of the core of the reactor -- should prevent the radioactive material from escaping. As my colleague Stan Grant told me, though, that is a lot of trust to put in steel and concrete when all else seems to have failed.
Early Tuesday morning, Chief Cabinet Secretary Yukio Edano announced the radiation levels were high enough to endanger human health. We also learned the USS George Washington, docked 175 miles away from the Fukushima plant had detected low levels of radioactivity. At the time, we were around 40 miles away from the plant, and all of that informed our decision to travel farther away to Akita.
Along with Stan Grant, I have been piecing together the various bits of information that have slowly come out, and contextualizing it from a medical perspective. First of all, the way the radiation is measured is important. For example, a gray is used to measure the amount of radiation absorbed. If you want to really get an idea of the impact on the body, a sievert is the best measure, because it takes into account not only the amount of radiation absorbed, but also the type of radiation.
That brings up another important piece of the puzzle. Just how much radiation was released. According to readings made on Tuesday, levels close to one of the plants registered as high as 400 millisieverts (mSv). Those were readings inside the gates of the plant, and it is fair to say, the levels would be lower as one moves farther away, and the radioactive components decay.
Here is a little context: If you live in a country like the United States, you are being exposed to around 3 mSv a year from background radiation. Get a chest X-ray, and on average, you will receive around .02 mSv, according to the Food and Drug Administration. A CT scan of the chest, and you are getting a few hundred times that: a one-time dose of about 7 mSv, the FDA says.
To get an idea if someone will develop radiation sickness, you need to know the sieverts and also the length of exposure. For example, exposure to a full sievert at one time would be considered a mild exposure. Within six hours, one would start to feel nauseated. In the case of a larger exposure, between 2 and 6 sieverts, nausea would come on in about two hours, as well as significant bleeding from the intestines. Above 6 sieverts, and mortality starts to approach 90%.
Again, the highest official reading we have heard is 400 mSv -- less than a half of a full sievert. It is important to note that long-term exposure can be a problem as well, even at lower doses. In this case, radiation can cause mutations in the genetic structure of cells.
Based on the information we have heard from officials, a sophisticated and thorough cleanup of the area will be necessary to prevent long-term radiation exposure to the residents in the area around Fukushima. It is also likely many of these nuclear reactors have been rendered useless. And, it is still possible that higher and more dangerous levels of radiation could be released. As things stand now though, it is tough to conceive that anyone outside the 20-kilometer evacuation zone would have any long-term health problems.
It would be impossible to provide a complete summation of all the specifics regarding radiation exposure and associated health problems. But, after reading this, are you more or less concerned about what is happening in Japan?
Reviewed by nuclear radiation expert Daniel Polanski --Georgia Department of Public Health