The Chernobyl disaster, considered the worst nuclear disaster in history, occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in the Ukrainian Soviet Socialist Republic, then part of the Soviet Union, now in Ukraine. From 1986 onward, the total death toll of the disaster has lacked consensus; as peer-reviewed medical journal The Lancet and other sources have noted, it remains contested. There is consensus that a total of approximately 30 people died from immediate blast trauma and acute radiation syndrome (ARS) in the seconds to months after the disaster respectively, with 60 in total in the decades since, inclusive of later radiation induced cancer. However, there is considerable debate concerning the accurate number of projected deaths that have yet to occur due to the disaster's long-term health effects; long-term death estimates range from 4,000 up to 9,000 (per the 2005 and updated 2006 conclusions of a joint consortium of the United Nations) for the most exposed people of Ukraine, Belarus, and Russia, to 16,000 cases in total for all those exposed on the entire continent of Europe, with figures as high as 60,000 when including the relatively minor effects around the globe. Such numbers are based on the heavily contested linear no-threshold model.
This no-threshold epidemiology problem is not unique to Chernobyl, and similarly hinders attempts to estimate low level radon pollution, air pollution and natural sunlight exposures. Determining the elevated risk or total number of deaths from very low doses is completely subjective, and while much higher values would be detectable, lower values are outside the statistically significant reach of empirical science and are expected to remain unknowable.
From model-based epidemiological studies, the incidence of thyroid cancer cases due to the accident by 2065 compared with other cancer-inducing sources (diet etc.) across Europe, is roughly 1 in 10,000 as a probable worst-case scenario. Thyroid cancer is relatively amenable to treatment for several decades. Attributing a 1% mortality rate by Tuttle et al. to the 16,000 cases across Europe as predicted by Cardis et al. results in a likely final total death toll from radiation-induced thyroid cancer of around 160.
There have been no validated increases in solid cancer reported from the liquidator cohorts, and observed increases in leukemia have been statistically insignificant. The liquidators were adult at exposure and the average external dose was 117 mSv.
A paper in Science has stated that there have been no transgenerational effects of radiation exposure in children born of those working as liquidators. This study used whole genome sequencing in a cohort of parent and child blood samples.
Differing direct, short-term death toll counts Initially, the Soviet Union's toll of deaths directly caused by the Chernobyl disaster included only the two Chernobyl Nuclear Power Plant workers killed in the immediate aftermath of the explosion of the plant's reactor. However, by late 1986, Soviet officials updated the official count to 30, reflecting the deaths of 28 additional plant workers and first responders in the months after the accident. In the decades since the accident, many former Soviet officials and some Western sources had also determined a total of 30 direct casualties.
For their part, some surviving evacuees of regions now included in the Chernobyl Exclusion Zone and the Polesie State Radioecological Reserve argue that the official toll of the accident's direct casualties excludes trauma and ARS deaths that they themselves claim to have witnessed in the weeks and months after the reactor explosion. In response, constituent agencies of the United Nations—including the United Nations' International Atomic Energy Agency (IAEA) and the Chernobyl Forum—discount such evacuee claims as misinformation, "urban legends", or radiophobia.
2005 and 2006 UN reports debate At the first international conference on the Chernobyl disaster in August 1986, the IAEA established but did not make official a figure of 4,000 deaths as the total number of projected deaths caused by the accident over the long term. In 2005 and 2006, a joint group of the United Nations and the governments of Ukraine, Belarus, and Russia—acknowledging the ongoing scientific, medical, social scientific, and public questioning of the accident's death toll that had emerged over the then-20 years since the disaster—worked to establish international consensus on the effects of the accident via a series of reports that collated 20 years of research to make official previous UN, IAEA, and World Health Organization (WHO) estimates of a total 4,000 deaths due to disaster-related illnesses in "the higher-exposed Chernobyl populations".
However—as an April 2006 special report in the peer-reviewed, scientific journal Nature detailed in response—the accuracy and precision of this United Nations-led joint group's projected death toll of 4,000 were immediately contested, with several of the very scientists, physicians, and biomedical consortia whose work the joint group had cited alleging publicly that the joint group had either misrepresented their work or interpreted it out of context.
Others have also found fault with the United Nations-led joint group's findings in the years since their initial publication, arguing that the 4,000 figure is too low—including the Union of Concerned Scientists; surviving Chernobyl liquidators; evacuees of Chernobyl, Pripyat, and other areas now included in the Chernobyl Exclusion Zone and the Polesie State Radioecological Reserve; environmental groups like Greenpeace; and several of the Ukrainian and Belarusian scientists and physicians who have studied and treated relocated evacuees and liquidators over the decades since the accident.
Liquidator mortality
The uncertain and contested mortality rate of the Chernobyl liquidators is a major factor in the lack of consensus on the Chernobyl disaster's accurate death toll. Following the disaster itself, the Soviet Union organized an effort to stabilize and seal off the reactor area, still awash in radiation, using the efforts of an estimated 600,000According to the World Health Organization, the Soviet Union issued 600,000 certificates to Chernobyl liquidators, making 600,000 the most oft-cited tally of their total numbers. However, other tallies—including published figures of 240,000, 350,000, 500,000, 750,000, and 800,000 total liquidators—often appear in secondary-source accounts of the post-disaster cleanup campaign. "liquidators" recruited or conscripted from all over the Soviet Union.
Since the 1990s—when the declassification of selected liquidator records prompted some direct participants to speak publicly—some with direct involvement in the liquidators' cleanup efforts have asserted that several thousand liquidators died as a result of the cleanup. Other organizations claim that total liquidator deaths as a result of the cleanup operation may number at least 6,000..
The National Commission for Radiation Protection of Ukraine disputed the 6,000 estimate as much too high, maintaining that a Chernobyl-cleanup-related death toll of 6,000 would outstrip confirmed liquidator deaths from all other causes—including old age and car crashes—during the period in question. In contrast, representatives of Kyiv's National Research Centre for Radiation Medicine, the Union of Chernobyl Liquidators, and the WHO's Radiation Protection Programme argue that both the perilous conditions in which the liquidators worked and the secrecy with which the Soviet Union shrouded the highly classified disaster cleanup efforts not only preclude dismissing a liquidator death toll of 6,000, but also indicate that the 6,000 estimate might be too low.
For their part, some surviving Chernobyl liquidators have argued publicly since the declassification of additional records in the early 2000s that official records and bureaucratic assessments do not reflect the full scope of liquidators' claims of disaster-related deaths. Examples of such claims include the comments of surviving liquidators in the Prix Italia-winning 2006 documentary, The Battle of Chernobyl, as well as Valeriy Starodumov's comments in the 2011 Ukrainian documentary Chornobyl.3828, which chronicles Starodumov's, and other liquidators' work and posits its long-term effects on their lives and health.
A 26-year study on a cohort of 4,810 liquidators from Estonia indicated no definite health effects attributable to radiation.
Long-latency diseases Issues related to identifying and tracking long-latency diseases have presented another stumbling block to reaching consensus on deaths beyond the immediate fatalities directly attributable to the initial reactor explosion and subsequent ARS. In the years since the accident, delayed, post-disaster deaths due to solid cancers, leukemia, and other long-latency diseases that might be attributable to the accident's release of radioactive debris have remained an ongoing concern. However, the streamlined standards, methods, and sustained research efforts needed to pinpoint, track, and tally such long-latency disease deaths have remained lacking—resulting in gaps in data and divergent estimates. There is consensus for only one form of long-term physiological effect: thyroid cancer in those who consumed radioactive iodine as children. This is because doses of radioiodine to the child's thyroid were much higher than other isotopes and the child's thyroid is still growing. Of those in the exposed cohort who have developed thyroid cancers, the proportion of cancers attributable to the Chernobyl incident is estimated to be between 7% and 50%. As there is no bio-marker for radiation induced thyroid cancer, the exact number of cases seen in the population cannot be determined.
Addressing long-latency diseases in a widely cited 2008 report, the IAEA reaffirmed its August 1986 conclusion—initially reached at the first international conference on the accident (an event closed to the press and citizen observers) and made official in 2005 and 2006—of a projected 4,000 premature deaths as a result of the disaster. The IAEA based this 4,000 figure on its estimate of a 3% increase in cancers in the regions surrounding the plant, first adopting it at the 1986 conference after rejecting the finding of 40,000 projected deaths that Valery Legasov—inorganic chemist and a lead investigator of the Soviet Union's official Chernobyl disaster commission—had estimated based upon his team's research.
In compensation and payout legal terms, by 2005, the Ukrainian government was providing survivors' benefits to 19,000 families "owing to the loss of a breadwinner whose death was deemed to possibly related to the Chernobyl accident;" by 2019, this figure had risen to 35,000 families. By 2016, some Ukrainian and Belarusian physicians charged with treating large numbers of former liquidators in the decades since the accident were calling for more comprehensive studies and urging that the IAEA's estimated toll of disaster-related deaths from long-latency diseases be revised upwards, claiming that their own data indicates a former liquidator death rate of several thousand per year as a result of diseases related to the disaster.
Greenpeace projected up to a million excess, cancer-related deaths from the Chernobyl disaster. The Chernobyl Forum, the WHO, and other international agencies do not accept this number.
Methodological debates