Earthquake in Japan – Radiation Exposure Calculations

It seems that there is a lot of confusion about radiation exposure calculations right now. In fact, I am even confused myself when I see all the numbers floating around with no clear indication of what is good or bad. So here’s a quick guide to the numbers. To put it succinctly, the current report about the exposure level in the reactors is at 12 mSv/hr, (EDIT: this was the report around 1200 Tokyo Time GMT +9), which was just a sudden momental spike in the reading. The figure has since dropped to 0.5 mSv/hr as of 1630 (GMT +9)

Since the numbers are all quoted in mSv, I am going to use mSv in all calcuations below. 1 Sv = 1000 mSv. It is not meant to be a conclusive guide and I will of course, update when I get more accurate information. Meanwhile, this will serve as a rough guide to what we are facing.

Some background information:

Natural Background 3.1 mSv/year
Domestic Pilots 2.2 mSv/year
Average US Exposure 6.2 mSv/year
7 Hour Airline Flight 0.02 mSv
Chest x-ray (2 views) 0.10 mSv
Chest CT 7.0 mSv

Radiation sickness is generally associated with acute (a single large) exposure. Nausea and vomiting are usually the main symptoms. The amount of time between exposure to radiation and the onset of the initial symptoms may be an indicator of how much radiation was absorbed, as symptoms appear sooner with higher doses of exposure. The symptoms of radiation sickness become more serious (and the chance of survival decreases) as the dosage of radiation increases. A few symptom-free days may pass between the appearance of the initial symptoms and the onset of symptoms of more severe illness associated with higher doses of radiation.

An average low dosage that will start to lead to radiation poisoning is at about 1’000-2’000 mSv. Nausea and vomiting generally occur within 24–48 hours after exposure to this mild dose due to radiation damage to the intestinal tract lining. The radiation will begin to destroy the cells in the body that divide rapidly. Headache, fatigue, and weakness are also seen with mild exposure.

Moderate radiation poisoning is at around 2’000-3’500 mSv. At these levels, nausea and vomiting beginning within 12–24 hours after exposure. In addition to the symptoms of mild exposure, fever, hair loss, infections, bloody vomit and stools, and poor wound healing are seen with moderate exposure.

Severe radiation poisoning occurs at 3’500-5’000 mSv and very severe radiation poisoning occurs at above that level. In the latter case, nausea and vomiting occurs in less than 1 hour, followed by diarrhea and high fever. In the former case, the time to nausea and vomiting is below 30 minutes followed by by the appearance of dizziness, disorientation, and low blood pressure. Both cases are of course, followed by all the other symptoms typical of lower levels of exposure. Severe exposure is fatal about 50% of the time.

What does it all means?

The simplest way to think about it as a threat is to imagine the distance between you and the reactors. Via the inverse-square law, basically an object twice as far will receive a quarter of the energy. This means that let’s say at 1 metre away from the reactors its 400 mSv/hr, even at 10m away, its only 4 mSv/hr, scarcely more than the average dose a person receive in a year. Of course, this is what you will receive in an HOUR so you got to bear this in mind as well. I am not saying its safe, just that its not as dangerous/bad as what you think unless you would have to be in there for 24 hours in which case I have to say “Good Luck” to you.

I believe the annual industrial worker limit of radiation exposure/year is at 50 mSv. Upon reaching this figure, the worker would be pulled out from the radiation zone and can only return after a year of normal, non-industrial exposure. As for emergency rescue teams etc, the hazmat suits they wear DO NOT protect them from radiation exposure, which can only be shielded by thick materials such as lead. The suits are only used to prevent them from accidentally acquiring radiation particles/fallout in the course of their work. Their threshold of exposure/year is at 100 mSv which reflects on the higher level of risk they might be exposed to.

Last bit of information:

Whole body vs. part of body exposure
In the case of a person who has had only part of their body irradiated then the treatment is easier, as the human body can tolerate very large exposures to the non-vital parts such as hands and feet, without having a global effect on the entire body. For instance, if the hands get a 100 Gy (100’000 mSv) dose which results in the body receiving a dose (averaged over the entire body) of less than 1 Gy (1’000 mSv) then the hands may be lost but radiation poisoning may not occur. The resulting injury would be described as localized radiation burn.

One of the primary dangers of whole-body exposure is immunodeficiency caused by the destruction of bone marrow and consequent shortage of white blood cells. It is treated by maintaining a sterile environment, bone marrow transplants (see hematopoietic stem cell transplantation), and blood transfusions.

At this point in time, I’m inclined to post this from the excellent XKCD comic.

Numbers/facts quoted are from Xrayrisk and Wikipedia.

Satellite photograph from DigitalGlobe

CORRECTION: The readings above (12 mSv/hr) are the readings at the gate of the reactor building, NOT at civilian areas like Tokyo. In metropolitan areas like these, the readings would be far lower. In fact, the readings for these areas are about the same as normal usual background readings, meaning there is ABSOLUTELY nothing to worry about right now.

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EDIT: Excellent letter from a German nuclear scientist at MIT describing the entire situation in depth and about why is there nothing to be worried about. The article is a little long but its DEFINITELY worth a read! This guy should be given a regular column!

EDIT 2: The highest radiation reading recorded in Tokyo today was 0.9 μSv/hr around 1200 (GMT +9), when a plume of radioactive wind was blown towards it. The reading is NOT in milliSievert/hour (mSv/hr) but in microSievert/hour (μSv/hr) as pointed out kindly by Duren in the comments. 0.9 μSv/hr = 0.0009 mSv/hr which means you shouldn’t even be worried at all.

The readings for Tokyo after that are about the same as normal usual background readings, meaning there is ABSOLUTELY nothing to worry about right now. As far as I am aware of, the wind direction is currently out towards the Pacific Ocean.

EDIT 3: I have updated a link to the pdf of the official reading released by Tokyo Electric Power Company (TEPCO) in the first paragraph. The highest level reached so far, was and still is the 12 mSv/hr around 0900 (GMT +9).

EDIT 4: Apparently, there was a one-off reading of 400 mSv/hr happening around 1415 (GMT +9) at the Fukushima nuclear plant. So far after combing through numerous news reports, the main source for this seems to be the International Atomic Energy Agency website.

Dose rates of up to 400 millisievert per hour have been reported at the site.

They followed up with a clarification about an hour ago around 2025 (GMT +9) with some words of caution about the figure.

As reported earlier, a 400 millisieverts (mSv) per hour radiation dose observed at Fukushima Daiichi occurred between units 3 and 4. This is a high dose-level value, but it is a local value at a single location and at a certain point in time.

I would like to stress that like what they mentioned, it was a one-off reading which although high, might not be truly representative of the radiation being emitted. For instance, a 400 mSv/hour reading, if only occuring for 5 minutes, would only give off an effective radiation of (400/12) = 33 mSv. Remember, radiation poisoning usually only occur in a single large blast of at least 1000 mSv as I have highlighted above so while the reading is dangerous, it is by no means critical or even fatal if its a short-term/temporary reading in an isolated incident. Once you have factor in the distance like in my example above, the figure is not as high as it once seems.

As usual, please feel free to comment below if there are any discrepancies noticed in the post.

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