Today is the one year anniversary of the terrible tragedies of Tōhoku earthquake and tsunami and the subsequent Fukushima Daiichi nuclear disaster. There have been recent fears brought on by high levels of radiation found in fish around the reactor, but how safe is Tokyo? I am visiting Tokyo for a few days and brought my geiger counter along, so read on to find out.
The first order of business was to secure a 9V battery for the device, which has sat unused in its box since my wife bought it last year. This was more challenging than it should have been due to my rusty 20-year-old Japanese, but led to a nice walking tour of Shinjuku around my hotel.
Battery secured, I fired up the device. It is easy to forget that the consumer electronics industry benefits from fierce competition to provide simple and intuitive user interfaces to its products. The geiger counter industry apparently does not. While the device is pretty simple, consisting of just three switches, three buttons, and a display, it was not obvious to me which settings to use and how to interpret the results. The product manual wasn’t much help either.
When I turned on the Audio switch, the device clicked just like in the movies. Every click made my heart jump a little. It was clicking once or twice every second, but wasn’t going nuts. I crossed my fingers and hoped that things were safe.
I put the device on the floor and set the mode to “mR/hr”, which measures milli-Roentgens per hour. It started at zero and just kept going up, up, and up. 0.001, 0.006, 0.009, 0.010, 0.013 … all the way to 0.026 at its highest. I moved the counter from the floor to the table and the readings went down, fluctuating between 0.009 and 0.013. Was this good? Bad? I scratched my head and consulted the Internet.
The most useful site I found was the Health Physics Society’s answer to the question “How many mR/hr (alpha,beta,gamma,or x ray) could be considered unsafe for continuous exposure?” There are two different answers, one for radiation workers and one for the general public. At first that seemed really strange to me, but it’s because the general public contains people of all ages and levels of health. Duh.
Right, right, get to the point already. I know. Here’s what they say:
The limit for radiation delivered in addition to the natural background radiation to individuals of the general public is 500 mrem in any one year and 100 mrem/yr on a prolonged basis. If we interpret continuous exposure as 8766 hours per year (24 hrs/day x 365.25 days/yr), 100 mrem/yr translates to 0.011 mrem/hr or 11 microrem/hr. This would be in addition to the natural background radiation which is quite variable but which averages about 300 mrem/yr or 34 microrem/hr
I believe that the device is measuring the absolute radiation, in which case the readings are well below the limit of .046 mR/hr, and is even below the “natural background radiation which averages .034 mR/hr. Hooray! I am safe being here for a few days, and so are the people of Tokyo who are exposed to this continuously.
I am curious to test fish here myself, but don’t want to freak anybody out. Perhaps I’ll take some sushi takeout tomorrow and see how it looks.