Ionizing Radiation levels

Health and environmental consequences of exposure to ionizing radiation

    1. Distinguish annual dose rates from acute exposure:
            Annual rates are important in setting working conditions, environmental standards, and public health standards.
                The same dose delivered over a longer period has much less biological effect, because the body is able to repair itself.
            Acute exposure rates are important in assessing consequences of nuclear accidents,
                    such as TMI (1979), Chernobyl (1986), Fukushima (2011).
                    Acute rates are highly influenced by distance of exposure, type of radiation, shielding, and other factors.

    2. The SI unit of radiation dose is 1 mSv (1 millisievert)
                Actual dose rates to any particular person depend on location, occupation, diagnostic & medical exposure, airplane travel, etc.
                1mSv is equivalent to 100 mrem (100 millirem) in historical units [note that the multiplier is 100:1]
                see [http://www.mun.ca/biology/scarr/Radiation_definitions.html

**US NRC (Nuclear Regulatory Commission) and EPA (Environmental Protection Agency) dose estimates and action levels**
Level (mSv)	Duration	Conditions of ionizing radiation exposure
0.001 - 0.01	Hourly	Cosmic ray dose on high-altitude flight, depends on position and solar sunspot phase
0.01	Annual	USA dose from nuclear fuel and nuclear power plants
0.01	Daily	Natural background radiation, including radon
0.1	Annual	Average USA dose from consumer products
0.15	Annual	EPA cleanup standard
0.27	Annual	Dose from natural cosmic radiation (0.16 coastal plain, 0.63 eastern Rocky Mountains)
0.28	Annual	Dose from natural terrestrial sources
0.46	Acute	Estimated largest off-site dose possible from Three Mile Island accident (1979)
0.48	Day	NRC public area exposure limit
0.66	Annual	Average USA dose from human-made sources
1	Annual	Limit of dose from all DOE facilities to a member of the public who is not a radiation worker
2	Annual	USA average medical and natural background Human internal radiation due to radon, varies with radon levels
2.2	Acute	Average dose from upper gastrointestinal diagnostic X-ray series
3	Annual	Average dose from all natural sources
5	Annual	NRC occupational limit for minors (10% of adult limit) NRC limit for visitors
5	Pregnancy	NRC occupational limit for pregnant women
10 ~ 50	Acute	EPA nuclear accident emergency action level
50	Annual	NRC occupational limit
100	Acute	EPA acute dose level estimated for increased cancer risk of 0.8%
150	Annual	NRC occupational eye lens exposure limit
250	Acute	EPA voluntary maximum dose for emergency non-life-saving work
500	Annual	NRC occupational whole skin, limb skin, or single organ exposure limit
750	Acute	EPA voluntary maximum dose for emergency life-saving work
500 ~ 1,000	Acute	hematopoietic radiation sickness from short-term exposure
500 ~ 1,000	Detonation	Hiroshima & Nagasaki nuclear bomb victims
~3,000	Acute	gastrointestinal syndrome fatal without treatment
4,500 ~ 5,000	Acute	LD_50/30 in humans, even with medical treatment: = Lethal dose for 50% of the exposed population in 30 days

Notes:
    1 mSv is taken as a reference annual dose, which is modified by other acute and continuous exposures. For example, 100 hours annual commercial flying time adds as much as 1 mSv of exposure. The TMI accident (the largest in US history) added about 50% of the annual levels in the immediate vicinity of the plant.

    NRC (Nuclear Regulatory Commission) levels are set with a view to establishing safe working conditions for radiation workers. Note that such workers are allowed annual exposure of 50 mSv. Most lab workers do not approach this level. [As a student intern in an NRC lab, I worked with a shielded source, and never registered exposure above background. As a grad student and faculty researcher working with radioactively-labelled DNA nucleotides, the principal concern is eye exposure at close range. Isotope sources are easily shielded with plexiglass]. Note that rescue workers involved in life-saving work (as at Fukushima) may voluntarily expose themselves to acute exposure sufficient to cause radiation sickness.

    EPA (Environmental Protection Agency) levels are set with a view to protecting the the general public from environmental disasters. This distinguishes voluntary exposure by a small workforce from equivalent involuntary exposure to a much larger public. For example, the annual exposure of a radiation worker to 10mSv is within limits, whereas the same acute dose to the general public would trigger emergency action (evacuation, prophylactic medical treatment, etc.).

    [The principal radiation exposure from a nuclear reactor accident is from radioactive Cesium ¹³⁷Cs and Iodine ¹³¹I. Prophylactic treatment against ¹³¹I is large-scale distribution of iodine tablets, to saturate the thyroid and prevent uptake of the radioactive form. ¹³¹I has a half-life of 8 days, and is not a long-term environmental hazard. ¹³⁷Cs in contrast has a half-life of more than 30 years, and creates a long-term environmental exposure risk].

    Long- & short-term medical problems become of significant concern when doses exceed 100X the reference 1 mSv dose. The widely-used LNT (Linear No Threshold) dose-response curve predicts that each added 1 mSv exposure increases any one individual's lifetime risk of death due to cancer by ~0.008%. For a civil population of 1,000,000, the same exposure would be expected to result in ~80 added cancer deaths. [Given an added risk X of 0.008% per mSv per individual, and a civil population of 10⁶ exposed to the same dose, X = 0.008% x 10⁶ = 8 x 10^-3 x 10^-2 x 10⁶ = 8 x 10¹ = 80].

    Hematopoietic radiation sickness occurs with an acute exposure to 500 mSv. Major symptoms are loss of white-cells and T-lymphocytes, which compromise the body's ability to fight infection. Acute radiation sickness can be successfully treated; evidence indicates that survivors have significantly increased long-term cancer rates.

    Gastrointestinal radiation sickness occurs with acute doses > 3,000 mSv. Symptoms are vomiting immediately upon exposure, disorientation, general organ failure, and the hematopoietic symptoms. Death may be rapid, even with treatment.

     Estimated radiation exposure doses to nuclear bomb survivors from Hiroshima and Nagasaki generally fall in the middle of this range, because people receiving higher doses were killed by blast. Acute radiation sickness killed many of the initial blast survivors, because medical systems were disrupted and treatment regimes were not understood. First-generation survivors had massively increased rates of leukemia. Second-generation children of the survivors in general did not display increased medical problems, nor is there evidence for increased mutation rates. This is due in part to the increased radio-sensitivity of fetuses, such that most fetuses exposed to radiation in utero spontaneously aborted.