The ‘20 millisieverts per year’ exhibition title refers to the maximum dose of ionising radiation originating from a nuclear power plant to which citizens of Fukushima  can now be exposed in a year. (A millisievert is the unit in which ionising radiation doses are measured).
In March 2011 the catastrophic accident at the Fukushima Daiichi nuclear power station in Japan resulted in radioactive fallout contaminating at least 8% of the land surface of Japan with levels >10,000 Bq/m² and the evacuation of 164,865 people.
After six years the multifaceted catastrophe continues to unfold, despite extensive and costly remediation efforts. As well as the unresolved crisis at the Daiichi nuclear plant itself, urban and rural areas remain highly contaminated and numerous psychological, emotional, physical, social, political and economic consequences continue to unfold for many people.
Following the 2011 disaster, the permitted dose of non-occupational ionising radiation from a nuclear power plant was raised to 20 millisieverts per year for citizens of Fukushima .
In the rest of Japan and the rest of the world the maximum permitted dose to a citizen is 1 millisievert per year, as recommended by the International Commission on Radiological Protection (ICRP).
Furthermore, in the UK the maximum annual dose to a member of the public from any single nuclear facility is 0.5 millisieverts per year. In other words, 1 millisievert is the maximum from two or more nuclear facilities combined. This dose includes ionising radiation from materials outside of your body and/or from radioactive materials which you have inhaled, eaten in your food or drunk or absorbed through your skin.
But it doesn’t include ‘background radiation’ such as from NORM (naturally occurring radioactive materials) such as radon, which is emitted from the ground, or from radioactive matter dispersed planet-wide as the result of above-ground nuclear weapons tests. In the UK the average annual dose from background radiation is 2.7 millisieverts, but in some areas such as Cornwall, where there is a lot of radon-emitting granite, annual background radiation can be as as high as 7.8 millisieverts per year. In Japan the average annual dose from background radiation is 0.43 – 1.26 millisieverts per year . As it is, background radiation poses a risk of harm to the health of those who are exposed to it. Hence, for example, the importance placed on minimising the presence of radon in homes and workplaces.
Women and children more vulnerable to ionising radiation
Younger people are much more at risk from harm due to exposure to ionising radiation, more so if they are female, so the maximum annual occupational dose for a nuclear worker aged 16 – 18 years is 6 millisieverts per year.
And for a nuclear worker 18 years of age and above, the limit is 20 millisieverts per year, averaged over five years, with a maximum limit of 50 millisieverts in any one year. For nuclear workers and others who are exposed to ionising radiation at work, such as aeroplane pilots and crew, the maximum lifetime exposure is set at 400 millisieverts . This is because the harm caused by exposure to ionising radiation is cumulative .
Consensual vs non-consensual exposures
However workers doing these kinds of work have chosen to do so, ie they have chosen to endure these exposures of up to 20 millisieverts per year, unlike the citizens of Fukushima  which was contaminated by the radioactivity from the Fukushima Daiichi nuclear power plant.
From March 2017 financial support to 6,531 ‘voluntarily’ evacuated households living within or outside of Fukushima, was terminated, even if their homes are in areas which are contaminated with radioactivity from which they could receive doses of radiation of up to 20 millisieverts per year, regardless of age, gender or state of health.
This is very different to the action taken to protect citizens in Ukraine after the Chernobyl nuclear accident of 1986:
This raising of the annual threshold to 20 millisieverts per year puts enormous pressure on many of those who evacuated, compulsorily as well as ‘voluntarily’, to return to live in areas which are still contaminated, as the financial support for them to live elsewhere was terminated in March 2017.
Some of those who were compulsorily evacuated are relieved to be returning to their former homes, despite the concerns they may have about the increased risk to their health from living in areas with such levels of radiation.
Breach of human rights
But many others feel outraged and betrayed by the impending loss of financial support and the consequent pressure to return. Many people in Japan and elsewhere consider this to be a serious breach of human rights.
These people may have already suffered multiple losses of any combination of the following as the result of the nuclear catastrophe: the death(s) of family members and/or friends; support from family and/or friends; physical/mental health; marriage; home; livelihood and trust -in the government, the media and the scientific and medical establishments. While living in evacuation they may have ‘put down roots’ in their new communities. If they are children they may have settled into school and made new friends: six years is a long time, particularly for a child.
A selection of memoirs, statements and letters by people from Fukushima, about how they’ve been affected by the catastrophe, can be found here:
 The Post 3/11 (“three-eleven”) Anti-Nuke Movement, Citizens Nuclear Information Center, Japan: http://www.cnic.jp/english/?p=3661
The cost of cleaning up Fukushima, Japan Times editorial, 12 December 2016 http://www.japantimes.co.jp/opinion/2016/12/23/editorials/cost-cleaning-fukushima/#.WH8zrDKcYo8
 A table with radiation exposure limits is on page 7 of this 2011 IAEA review: https://www.iaea.org/OurWork/ST/NE/NEFW/documents/IDN/ANL%20Course/Day_5/RiskOverview_revised.pdf
 All UK figures are those cited by Steve Fisher of the UK NDA (Nuclear Decommissioning Authority) in his 2015 presentation: http://nuclearrc.sa.gov.au/videos/effects-and-threats-of-radiation-151215-630pm/
 Figures cited by P. Andrew Karam, and S. M. Javad Mortazavi, p497: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/34/086/34086353.pdf
 Human consequences of radiation: A gender factor in atomic harm: Civil Society Engagement in Disarmament Processes – The Case for a Nuclear Weapons Ban, Mary Olson April 2017: https://www.researchgate.net/publication/317154002_Human_consequences_of_radiation_A_gender_factor_in_atomic_harm_Civil_Society_Engagement_in_Disarmament_Processes_-_The_Case_for_a_Nuclear_Weapons_Ban
 Fukushima Catastrophe at 6: Normalizing Radiation Exposure Demeans Women and Kids and Risks Their Health, Cindy Folkers, 6 March 2017 :https://www.counterpunch.org/2017/03/06/fukushima-catastrophe-at-6-normalizing-radiation-exposure-demeans-women-and-kids-and-risks-their-health/
 A further table with more radiation exposure limits, as set by Euratom, the ICRP, IAEA and Germany is here: http://www.nucleonica.net/wiki/index.php/Radiological_limits
 If you add together all the other common ‘consensual’, non-occupational exposures to ionising radiation during a person’s lifetime – such as from dental and medical X-rays, CT scans, radiotherapy treatments, airport X-ray scanners, air travel, sunbathing, and smoking the cumulative risk of harm to the body increases significantly. This risk increases further still with consumption of food and drink which is contaminated with legally permitted levels of radioactivity.
Grappling with nuclear catastrophe in Japan: Peggy McInerny, on a talk by Norma Field: http://www.international.ucla.edu/japan/article/165856
 Dr Ian Fairlie writes:
“New emerging evidence from Fukushima shows that nuclear disasters and their aftermaths can kill thousands of people due to necessary evacuations. Between 2011 and 2014, about 2,000 Japanese people, including many old people, died from ill-heath and suicides connected with the evacuations.
Some nuclear advocates, including government officials, have said these deaths are the fault of the evacuations, as if they were unconnected with the nuclear disaster. This is incorrect: the evacuations were necessary to avoid large radiation exposures from the radioactive fallout due to the plumes from the Fukushima explosions and meltdowns.
In future, such deaths should be included in assessments of the fatalities from nuclear disasters.”
Dr Fairlie’s fuller report on the health effects from Fukushima is available here: