About the Atomic Crisis in Japan – Background Information and News Sources

Summary:  We might be reading much about radiation during the next few days.  Most of these articles will provide little context.  Here’s some useful background.  A follow-up to News about the earthquake in northeastern Japan.


  1. About radiation:  units and definitions
  2. Effects of radiation
  3. For more information about Japan’s nuclear reactor crisis

(1)  About radiation (from the Encyclopedia Britannica)

(a)  Gray (Gy), a unit of absorbed dose of ionizing radiation. 

One gray is equal approximately to the absorbed dose delivered when the energy per unit mass imparted to matter by ionizing radiation is one joule per kilogram. The gray replaced the rad; one gray equals 100 rads.  The gray was defined in 1975, named after Louis Harold Gray (1905–1965). 

(c)  Sievert (Sv), a unit measuring absorption of radiation by tissue (equivalent doses)

The sievert is the … replacement for the rem, the long-standing special unit for measuring biological absorption of radiation. Like the rem, the sievert takes into account the relative biologic effectiveness (RBE) of ionizing radiation, since each form of such radiation — e.g., X rays, gamma rays, neutrons — has a slightly different effect on living tissue. Accordingly, one sievert is generally defined as the amount of radiation roughly equivalent in biologic effectiveness to one gray (or 100 rads) of gamma radiation. … It is named after Rolf Sievert (1896-1966).

(c)  Units

  • Exposures can be expressed as totals, or as flows (units/time, such as gray/hour). 
  • Common prefixes are milliSievert (one-thousand, mSv) and hecot-gray (hGy), using the The International System of Units (SI; see Wikipedia).

(2)  Effects of radiation

(a)  Standard definitions in grays (from the Encyclopedia Britannica)

  • sublethal dose range:  (1–2 Gy):  sore throat, pallor and diarrhea; no fatalities (except for complications)
  • midlethal dose range:  (2.5–5 Gy):  nausea, vomiting, loss of hair, fever, hemorrhages, and emaciation; death for 50% of the population
  • supralethal dose range:  (6–10 Gy):  diarrhea, vomiting, inflammation of throat, and emaciation; death for 100% of the population

(b)  Exposures in Sv, from the World Nuclear Association (via the BBC)

  • 2 mSv/yr:  Typical background radiation experienced by everyone (average 1.5 mSv in Australia, 3 mSv in North America)
  • 9 mSv/yr :  Exposure by airline crew flying New York-Tokyo polar route
  • 20 mSv/yr:  Current limit (averaged) for nuclear industry employees
  • 50 mSv/yr:  Former routine limit for nuclear industry employees. It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe
  • 100 mSv/yr:  Lowest level at which any increase in cancer is clearly evident
  • 350 mSv/lifetime:  Criterion for relocating people after Chernobyl accident
  • 1,000 mSv single dose:  Causes temporary radiation sickness such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose
  • 5,000 mSv single dose:  Would kill about half those receiving it within a month

 For more detail about exposures in terms of sieverts, see Wikipedia

(c)  “How much radiation is dangerous?“, Reuter’s Factbox article, 15 March 2011 — Excerpt:

  • People are exposed to natural radiation of about 2 mSv a year.
  • Airline crew flying the New York-Tokyo polar route are exposed to 9 mSv a year.
  • Exposure to 100 mSv a year is the lowest level at which any increase in cancer is clearly evident. A cumulative 1,000 mSv would probably cause a fatal cancer many years later in five out of every 100 persons exposed to it.
  • Exposure to 350 mSv was the criterion for relocating people after the Chernobyl accident, according to the World Nuclear Association.
  • A single 1,000 mSv dose causes radiation sickness such as nausea but not death. A single does of 5,000 mSv would kill about half of those exposed to it within a month.

Also see “Chernobyl fallout not as bad as first feared“, The Times,7 September 2005.

(d)  For more information see Acute Radiation Syndrome: A Fact Sheet for Physicians, Centers for Disease Control and Prevention.

(e)  Update:  from the International Atomic Energy Agency (IAEA) Tsunami Update page:

The Japanese authorities have informed the IAEA that the following radiation dose rates have been observed on site at the main gate of the Fukushima Daiichi Nuclear Power Plant.  At 00:00 UTC on 15 March a dose rate of 11.9 millisieverts (mSv) per hour was observed.  Six hours later, at 06:00 UTC on 15 March a dose rate of 0.6 millisieverts (mSv) per hour was observed.  These observations indicate that the level of radioactivity has been decreasing at the site.

 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.

(3)  For more information about Japan’s nuclear reactors

Originally published at Fabius Maximus and reproduced here with permission.