The Chornobyl disaster was a catastrophic nuclear accident that occurred on the 26th April 1986 at the No. 4 nuclear reactor in the Chornobyl Nuclear Power Plant (ChNPP), near the city of Pripyat in the north of Ukraine (110 km from Kyiv), and it was the most serious accident ever to occur in the nuclear power industry. According to the International Nuclear and Radiological Event Scale (INES) the Chornobyl catastrophe was the highest, the 7th, level “major accident,” resulting in widespread health and environmental effects requiring implementation of planned and extended countermeasures (1). To date, there have been two accidents of this kind: the Chornobyl catastrophe (2) and Fukushima Daiichi nuclear disaster, a series of events beginning on 11 March 2011 (3, 4). In any case, radiological impact on people and environment of the Chornobyl catastrophe is around 10 times higher than that following the Fukushima Daiichi accident.

According to the latest estimates (5), ~1.8 EBq ¹³¹I radioactivity was thrown out of the destroyed ChNPP Unit, significantly higher than that following the Fukushima disaster. The same is concerning external doses of exposure: in Chornobyl the averaged dose for clean-up workers (“liquidators,” 600,000 persons) is assessed to be around 100 mSv (2, 6); for evacuees (135,000 persons) 33 mSv, for strict control zone inhabitants (living in the territories with radioactivity deposition >555 kBq·m⁻²) 50 mSv, and for low exposed (5,000,000 persons) 10–20 mSv (2, 6). On the contrary, the highest doses for Fukushima rescue workers were assessed to be 10–50 mSv, for Fukushima population 1–10 mSv, for Japan 0.1–1 mSv per year (3). For comparison, worldwide annual exposure to natural radiation sources would generally be expected to be in the range 1–10 mSv, with 2.4 mSv being the present estimate of the central value (7).

There are very contradictive assessments of the Chornobyl catastrophe health effects. The International, UN-associated, organizations (International Atomic Energy Agency (IAEA), World Health Organization (WHO), and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) recognized only 31 death cases directly related to the accident, 134 verified cases of Acute Radiation Sickness (ARS) and around 6,000 cases of thyroid cancer reported in children and adolescents who were exposed at the time of the accident. They came to the conclusion that there was no sufficient scientific evidence concerning any other cancer and non-cancer Chornobyl health effects, while cataracts, leukemia, and cardiovascular (including cerebrovascular and neurocognitive) diseases are considered to be at the frame of radiation-associated effects in liquidators. Although those most highly exposed individuals are at an increased risk for radiation-associated effects, the great majority of the population is not likely to experience severe health consequences as a result of radiation from the Chornobyl accident. Many other health problems have been noted in the victims/survivors that were not attributed to radiation exposure, but to psychological consequences only. Therefore, in general, the conclusions of those agencies are as follows: the ChNPP accident was a tragic event for its victims, and those most affected suffered major hardship. Some people who dealt with the emergency lost their lives. Although those exposed as children, and the emergency and recovery workers, are at increased risk of radiation-induced effects, the vast majority of the population should not be concerned by serious health consequences. For the most part, individuals were exposed to radiation levels comparable to or just a few times higher than annual levels of natural background, and future exposures continue to slowly diminish as the radionuclides decay. Although lives had been seriously disrupted by the Chornobyl accident, from a radiological point of view, positive prospects for the future health of most individuals should prevail (2, 6, 8–10).

The UN Chornobyl forum (2006) concluded about persistent mental health worsening of the Chornobyl accident survivors as a result of (1) stress-related disorders; (2) effects on the developing brain; (3) organic mental disorders in liquidators, and (4) suicides. The revealed excess of cardiovascular and cerebrovascular disorders in liquidators was recognized as needing further investigation (8, 9).

By contrast, at the same time, ecologically oriented publications (Green Peace-related, (11–14)) argued that a so-called “atomic lobby” underestimates the environmental and health effects of the Chornobyl catastrophe. Green Peace is positive about the dramatic excess of cancer and non-cancer morbidity and mortality of the Chornobyl victims due to radiation exposure (11–14). However, it is likely that the truth seems to be in the middle (15–17).

There are few well-designed systematic epidemiological studies of mental health following the Chornobyl catastrophe. There is a consensus about long-term mental health deterioration, mainly at subclinical levels, attributed to stress and other non-radiation factors in residents of radioactively contaminated territories (18, 19), and cleanup workers, such as depression, anxiety and posttraumatic stress disorder (PTSD), suicidal ideation or attempted, or completed suicides (20, 21), alcohol abuse, poorer perceptions regarding personal physical and mental health (22). However, all these studies had no dosimetric support, with the exception of self-report exposure estimation (22). Therefore, the assessment of the radiation risk analysis for neuropsychiatric disorders in Chornobyl liquidators was impossible, which is why the attribution of all of these disorders to stress alone has no scientific base.

Interestingly, similar data on long-term mental health deterioration were reported after the A-bombing in Hiroshima and Nagasaki, Japan, 1945 (23, 24), Three Mile Island crisis, USA, 1979 (25, 26), and the Fukushima Daiichi accident, Japan, 2011 (27, 28).

Although a considerable worsening of mental health in the Chornobyl catastrophe survivors has been documented, however, the role of ionizing radiation in mental health deterioration is still unclear. Therefore, the purpose of our study was to assess the possible role of ionizing radiation in the pathophysiology of neuropsychiatric disorders amongst cleanup workers of the Chornobyl catastrophe (liquidators).

According to the register's data (SRU), the incidence of organic, including symptomatic, mental disorders (organic psychosis) (ICD-9: 293.0–294.9: ICD-10: F00-F05; F06.0, F06.2) was 101.99 per 10,000 persons, and it showed the highest statistically significant (Pv < 0.001) relative risk (RR = 3.15; 95% CI: 2.6; 3.7) (Table 2).

Non-psychotic organic disorders (ICD-9: 310.0–310.9; ICD-10: ICD-10: F06.32, F06.4-F06.7F07.0, F07.8, F07.9) showed a RR=1.99 (Pv <0.001; 95% CI: 1.6; 2.5). These disorders included organic depressive, emotionally labile [asthenic], mild cognitive, anxiety, dissociative, as well as organic personality disorders.

Acute cerebrovascular disorders (strokes) had a RR = 1.40 (Pv < 0.001; 95% CI: 1.3; 1.5), and consequences/chronic cerebrovascular pathology, including cerebral atherosclerosis, hypertensive encephalopathy, and chronic cerebral ischemia, a RR = 1.23 (Pv = 0.05; 95% CI: 1.0; 1.5).

According to these previous findings, an expert assessment of neuropsychiatric disorders was carried out (Table 3). In comparison with the unexposed control group, liquidators showed significantly more cognitive disorders, in particular, mild cognitive impairment and dementia, and an increased percentage of affective disorders, mainly severe depression. The frequency of affective and stress disorders was significantly increased in clean-up workers exposed to doses ≥50 mSv, as compared to those exposed to lower doses, with a tendency toward a greater frequency of cognitive impairment (Table 4).

According to the survival analysis, we considered the time of the onset of any neuropsychiatric pathology after the Chornobyl accident measured in years as the event in which the pathology arose after the catastrophe. As shown in Figure 1, neuropsychiatric disorders in clean-up workers appeared much earlier, that is to say, between 3 and 5 years after the disaster (Log-Rank Test = 4.96, P = 0.000). By contrast, only 30 years later, these disorders occurred approximately at same rate both in the clean-up workers and the unexposed control group. In addition, in the clean-up workers who were also evacuated from the Chornobyl exclusion zone, neuropsychiatric disorders began to emerge significantly later than 7–10 years after the disaster than in those not evacuated (Log-Rank Test = −3.13, P = 0.002). During the first 15 post-accidental years, the dependence of the occurrence of neuropsychiatric disorders upon the irradiation dose was detected at doses >300 mSv. The onset of these conditions occurred quite early, almost immediately after the disaster; at doses of 50–300 mSv 2 years later, and at doses lower than 50 mSv after ten 10 years (χ² = 8.74 P = 0.01), this dependence disappears.

The present study suffers from some limitations that should be acknowledged. First, there exist different approaches in diagnosing mental disorders between the Western and Eastern countries, that is why we used ICD 9 and 10 criteria. Second, the dramatically social changes in post-soviet societies might have led to uncertainties in individual radiation doses estimation. Third, we did not control for possible confounding non-radiation factors. However, in spite of these limitations, we are of the opinion that our data are important in highlighting an evident brain damage following radiation exposure.

The main findings of our study represent a clear evidence of radiation-induced detrimental cerebral effect, as shown by the organic brain damage, or acute and chronic cerebrovascular pathology, depressive, and stress-related disorders amongst liquidators. The overall risk of neuropsychiatric pathology increases significantly with the irradiation dose and decreases with the attained age.

Our data are consistent with the excess and significant radiation risks for morbidity and mortality due to non-cancer radiation effects, mainly cardiovascular (including cerebrovascular) disorders in A-bomb- survivors (50–52), military radiochemical complex “Mayak” with the dramatic radiation accidents in the 50th of the XX Century (USSR, Russian Federation) (53, 54), other nuclear workers cohorts (55–58), and Chornobyl liquidators (59–62).

Therefore, the role of exposure to ionizing radiation in provoking brain effects, as a result of the Chornobyl accident is significant, especially at doses more 0.2–0.25 Sv of external irradiation, cannot be denied any longer.

Again, previous data also detected a radiation dose-dependent deterioration of the actual (current) cognitive functioning in comparison with premorbid (pre-radiation exposure) IQ due to the verbal IQ decrement was also revealed following the ARS as a result of the Chornobyl catastrophe (42), still in the liquidators as well, exposed to lower radiation doses (43), suggestive of a radiation-associated mild cognitive impairment (MCI). In addition, some characteristic post-radiation electroencephalographic (EEG) pattern (63) and neurophysiological radiation biomarkers according to quantitative EEG (qEEG) were found at exposure to radiation doses >1 Gy/Sv following ARS, so that the completely innovative neurophysiological (qEEG-based) biodosimeter has been proposed (64). Neurocognitive, neuropsychological, neurophysiological, neuroimaging, and neurovascular dose-related disturbances were revealed after exposure to ionizing radiation at doses >0.3 Gy/Sv (65–70). The changes in the amplitude-time parameters of the cognitive auditory evoked potentials, which dominate in the left fronto-temporal area, namely the Wernicke's cortical zone (71, 72), were detected in the Chernobyl clean-up workers. Based on the event-related potentials (ERP) research, the extreme radiosensitivity of the human brain was confirmed following irradiation even at the low doses, especially at the level of the cortical-limbic system in the dominant hemisphere and the Wernicke's area as well. The new dose-dependent effects for some brain changes in humans were estimated to occur at doses >0.05 Sv (71–74).

Although the pathophysiology of the brain effects of ionizing radiation is still unclear, some possible explanations have been proposed. They include, amongst others, inhibition of neurogenesis, mainly in the hippocampus, telomere length and gene expression changes, apoptosis, neuroinflammation, autoimmune processes, and glial mechanisms (75–81).

The present study indicates that liquidators suffered from an excess of cognitive, affective and stress-related disorders. The risk of disease rises with dose of exposure. Their radiation risks for organic psychoses, non-psychotic organic brain damage, acute, and chronic cerebrovascular pathology were similarly high. It is disappointing that clinical and epidemiological studies with international expertise on the assessment of the neuropsychiatric effect of the Chornobyl disaster together with dosimetric support still need to be done. We are of the opinion that liquidators should be followed along their lifespan together with their offspring, in order to deepen our knowledge on radiation health effects and to improve radiation protection and safety for the next generations.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

The studies involving human participants were reviewed and approved by Institutional Research Body, State Institution “National Research Center for Radiation Medicine of the National Academy of Medical Sciences of Ukraine”, Kyiv, Ukraine. The patients/participants provided their written informed consent to participate in this study.

All authors: design, planning, analyses, writing, references search, editing, and revision.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.