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March 15, 2011



Thank you for compiling this Matt, it is reassuring now that I have decided to stay in Tokyo. I hope you are right that we are at no great risk as it stands. Still sleeping with one eye open and the rest of the time all eyes on the news!


Finally some numbers-based facts. I've been slightly panicked at the lack of information, but if what you say is true it seems (once again) to be a lot of media hysteria in search of a nice lead story. Thanks for this.


Great and informative post. I found the following post on the subject fascinating as well from some experts with the International Disaster Response Network (@IDRN_News) that you might find interesting for discussion: "Here is our REAL Risk Assessment of Japan's Nuclear Plants" http://ow.ly/4ewph


Thank you for this compilation, you are helping to inject some common sense into the overall panic.

The situation after quake & tsunami is horrible enough, so it is good to see that there are people who assess the nuclear situation with some common sense.

I have deep trust in the Japanese people and know that they will be able to eventually cope with this catastrophe.

William Van Hecke

Wow, thanks for this. And I wasn't expecting to see it, but thanks for the confirmation that Andy is okay. Hi Andy!

Peter real

Decision if you stay or go is very personal. As much as you want to rationilize it you will not find a simple answer. It depends on your responsibilities in tokyo. There is a certain risk that this situation will get out of control. In my opinion if there are no mandatory reasons to stay in japan right now it is better to temporarily leave.




Excellent Reporting! Thanks! This is very helpful!


Thank you. This is better than all information on tv together. Our thoughts are with you. Karin, Germany

Margaret Jane Nakiyuka

This information is good puts our minds at ease as long as it remains within the factual line. In situations like these for various reasons it is hard to find facts. Understably facts may cause panic and it is also very important to manage information at this delicate time in a way that does not upset the already upset. Looking at all the facts on the table it is probably safer and reasonable to get the truth however bitter. it does give one direction of what to do next
Uncertainty causes panic too.


It may be helpful to use the banana scale, since people can't relate to CT scans:


The current level is about 1 banana per day.

Andy Szymanski

Some more relevant data I have gathered:

-Fire broke out at 9:38 AM at the #4 reactor. This timeline matches perfectly with the spikes that we've seen at the gate of the facility (Tepgo graph) and the resultant spike from the Hino monitoring station (given a window of 2-3 hours for winds to bring it down to Tokyo)

-NHK has announced that Setagaya and Shinjuku monitoring posts read 0.89 (or 0.809; seems to be some typos) this morning, meaning that the Hino monitoring station is tracking correctly. I believe this means we have an accurate source.


Thanks very much for compiling this info, Matt!

I’ve been following you closely on Twitter, as well as many others who (I believe) are in authority to make solid and unsensational comments. Yet, it’s still difficult to piece the overall puzzle together.

I hope that those, who are currently in Tokyo, or in the region of Fukushima, who are not able to digest the countless messages in the local tongue, can take some level of comfort in the information that Matt has provided since the beginning of this natural disaster, possibly the worst that I’ve seen in my lifetime.


Awesome, Matt. I'm sending this to my parents.

cassandra Harada

I would like to send you a hug from Fuchu-Shi. Thank you for this, you made it much easier to understand!


Matt, you got the sievert explanation wrong- you can get a chronic or acute dose. Acute means a high dose in a short period, while chronic means many little doses over a long period. Sievert is a measure of total (accumulated) dose. The dose rate is measures in Sieverts/time, e.g mSv/hr; Sv/day; etc.


Hey Matt - this report says there was a 400,000 microseivert blast (http://www.jiji.com/jc/c?g=soc_30&k=2011031500861&j4). How does that factor into all this?


Thank you for this summary. My own measurement of radiation in Yokohama roughly correlates with the data shown in Hino. Just keep in mind that this particular cloud must have been material released earlier, not from the fire this morning. At wind speeds today, it would take 10-15 hours to travel from Fukushima to Tokyo/Yokohama. Fortunately, the wind has turned and until Wednesday evening, further exposure is unlikely. Now if we could find more ways way to silence the self-important panic-mongers ...

Chris O.

Excellent article and counterpoint to all of the media hysteria and anti-nuclear Luddites who are using this incident to further their agenda. Thank you for posting!

Andy Szymanski

Additional relevant data (most courtesy of NHK):

-Radiation readings in Iwaki City, Fukushima (approx. 40 kilometers from the reactor) peaked at 23.72 microseiverts/h early this morning, but soon dropped to 3.94 microseiverts/h 2 hours later. Higher than usual for certain, but not substantial (it would take approx 1,725 hours exposure at this level to equal one CT scan).

-Radation levels in Tokai Village, approximately 110 kilometers from the reactor, reported levels of 5 microseiverts/h in the morning that soon dropped to 2 microseiverts/h by noon. Once again, higher than usual but still very low (almost 3500 hours of exposure to equal one CT scan).

The above, coupled with the readings that we know of in Tokyo proper (0.89 microseiverts/h at around 12:30) indicate with facts what the experts have assured us all along - radation exposure drops off at an exponential rate to distance from source.

Numbers don't lie, my friends.


matt of 400,000: that was inside the plant. As reported by Matt, the reading in Tokio had a peak of 1 microS because of that blast and the readings at the plant's main entrance are now back to around 2 miliS.
So, there was a peak of high release but that was controlled with the end of the fire in reactor 4.

Simon Allen

This is really great, thankyou.


Well, the geiger level chart I saw was not as optimistic as the one you have posted:


Andy Szymanski

The peak releas at the plant of 400 milliseiverts (400,000 microseverts) was recorded in a single instant right next to the reactor.
The theory (corroborated by government sopkesperson Edano in his press conference) is that this reading came from a portion of the concrete outer building of the reactor that blew out from the explosion - it likely absorbed a much larger amount than anywhere else.

Fact remains that at MAX peak (approx 9:30 AM) the reading at the gate of the plant was 12 milliseverts (12,000 microseverts) at a location just tens of meters away from the reactor. These levels soon decreased (see Tepco graph).

This relationship seems important as it indicates that the 400 milliseivert reading was a very local phenomenon (within yards).


That chart (at tokyogeiger.info) is cut off around noon, at the peak of the radiation level.

Thanks for this collected info - very useful to pass on to family and friends. Also great updates by Roy at http://www.mutantfrog.com/2011/03/15/radiation-safety-update/


Thanks for the corrections, Mandy and others. Keep them coming and let me know if I implemented them correctly.


Thanks for the article. I recommend being more specific here:
"That's something like three ten thousandths of a millisievert."
I recommend saying this instead:
"That's 0.0003 millisievert, or 1/3333 of a millisievert."


Paco, your link is a "snapshot" taken at noon. The one I link to is live, and shows a rapid drop-off to normal levels.

Andy Szymanski

Mandy is correct - the explanation is a bit confusing.

Matt, I would edit the explanation of "seiverts" to reflect that the unit is in itself only a measure of exposure - but it is important to note that all numbers that are reported (even the ones on NHK that don't have "pre hour" tacked on the end) are PER HOUR.

All radation numbers reported herein are in microseiverts/h or milliseiverts/h, not acute "instant" exposure numbers.


Thank you so much for this. The panic is worse than the problem itself, and the misinformation is flying fast and furious. I'll be sure to link this to anyone worried about the reactors and the radioactive material.

Andy Szymanski

New info just coming in:


This link shows the readings at the front gate of the plant, tens of meters from the reactor.
As we can see, the reading has dropped to 489.8 microseiverts/h (0.4898 milliseiverts/h) by 4:30 PM.

Even at these readings, you would have to stand in front of the plant for 14 hours to absorb enough radiation to equal 1 chest CT scan.

All of this information available on Tepco's site - facts are your friend.

Andy Szymanski

CoCreatr, thank you for your information on the wind speeds. I had no idea how long it would actually take to reach Tokyo.

The graphs from the Hino site seemed to match the Tepco gate readings so well that I assumed they were from the same source - thank you for correcting this.

It seems that there were two major release events this morning - the one that happened very early this morning (which I believe was at the #2 reactor) and the fire at reactor #4 around 9:30 AM.


There is also this live broadcast ''Geiger counter in Chiba'', 200km south of Fukushima Nuclear Plant :



Updated to reflect comment suggestions.

Ladyartemis 112

I just wanted to say thank you for giving us these great facts. It really has helped me to deal with my decision to stay in Tokyo. I will send this to my family and I hope that they will calm down as well. Thanks again!


Arr, so you are the source of the edited version I received by email, then condensed to a paragraph and posted my friends. Well done, at last a person with a scientific mind. These reporters cause a lot of worry with their latest 'big stories'.


Interesting compilation of what is known. But that's not the main point in assessing a risk. The question is: What can you gain if you stay, and what can you loose.

I think it's a simple answer.


If you have the opportunity, leave the country as quickly as
possible. If the authorities tell the truth, it's to late.

Andy Szymanski

Good news to report:

The radation levels at the two locations I mentioned in an earlier post have continued dropping.

As of 2 PM today, the reading in Iwaki City (40 kilometers from the reactor) has dropped to 1.5 microseiverts/h. It had reached a peak of 23.7 microseiverts/h at 4 AM.

Similarly, as of 5 PM today the reading at Tokai Village in Ibaraki (110 kilometers from the reactor) has dropped to 1.3 microseiverts/h. It had reached a peak of 5.8 microseiverts/h at 7:46 AM.

All good news.
Tokyo has returned to normal background levels as of 1:30 PM, but this shows a definite and steady drop in areas much closer to the reactors.


This wasn't written advocating anyone either stay or go, but rather to give people the information they need to make a decision based on the facts as they are currently known. If you use them to make either decision, more power to you.

Personally? I am staying. So my wife and her family. So is Andy. But everyone's situation is different.


I think the key thing to watch is another explosion in #2. I understand its unclear what ignited explosion in #2 (and for that matter #1 and #3). Its supposed to be controlled environment and these explosions are not supposed to happen. Because the pressure shelter (where suppression chamber is located) is to protect in event of big meltdown, the outer containment layer for #2 is a huge line of defense. . If that goes, (or the same explosion that happened in pressure chamber in #2 happens in #1 or #3), then I think its real problematic. Good news is that things in #1 and #3 seem to be stabilizing. My fingers are crossed big time for #2.

Aria Murasaka

Brilliant explanation! Thanks for taking the time to research the facts and analyze them (something that many journalists seem unable to do, even though it should really be their job!)

I would hope that this gets translated in Japanese, or that someone talking the language would write something similar because from all the tweets I can read commenting the general nhk broadcast at least, there seems to be utter confusion, disbelieving and misunderstanding about where things stand right now

Bill J.

I don't recall who posted this pic, but it's a handy reference for exposure levels.


Andy Szymanski

Additional coverage from NHK news.

Dr Nakagawa, professor from the Department of Medicine at Tokyo University (Japan's most prestigious public institution) has the follwing comments regarding the (extremely) low amounts of radiation detected within Tokyo city limits (0.809 microseiverts/h):

"I believe that this did indeed result from the escape of radiation at the Fukushima plant, and it is not surprising that it was detectable even here in Tokyo. That being said, the levels are extremely low and pose no risk to human health whatsoever. It is very difficut to imagine a scenario in which radiation [in Tokyo] will reach levels that could cause any health effects. I urge the public to be at ease."
(My translation)

I would like to point out that all of this information is ready available. The above quote and the levels measured in various locations quoted in my previous post are from this article:


These facts (the descending levels and the medial advice) have been broadcast every hour om NHK live TV.
Perhaps this is why Japanese seem to be more calm than many foreign residents.

Andy Szymanski

Edit typos in above post:

"...pose NO risk to human health whatsover."

These facts (the descending levels and the MEDICAL advice) have been broadcast every hour on NHK live TV.

Apologies; typing quickly.


most people can not choose, there are no true facts, they are all veiled. therefore, they are like 'Frogs in Water':

"if you put a frog in boiling hot water, it would jump out. But put it in cold water, and heat it up gradually, it would slowly boil to death…


I advise you to follow as well what the ASN (French Nuclear Authority). Criticality level rising to level 6.

See press release http://globz.org/tmp/asn.pdf


Hey. I am in almost the same situation :)
I am leaving the 24th to japan from sweden.
You can read on my blog too.


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