|Fig. 1: Alexey Akindinov's painting "Chernobyl Day of Pripyat" depicting details of the clean-up efforts after the Chernobyl disaster (firefighting, tunnel under the reactor building, radioactive rubble) as well as a view of the city of Pripyat with its famous ferris wheel and the power plant in the background. (Source: Wikimedia Commons)|
The explosion of the Unit 4 reactor of the Chernobyl power plant, the worst accident in the nuclear power industry, happened on the 26th April 1986. Due to the breach in containment, caused by the explosion, about 4% of the fuel material was released into the environment and contaminated significant part of Europe.  Main areas of contamination are situated in Ukraine, Belarus and Russian Federation, predominantly to the north and northwest of the reactor. 
Due to the extent of the damage and danger, the disaster had direct impact on unprecedently large number of people. Cardis et al. list several groups involved along with their approximate sizes: 
240000 liquidators (workers, tasked with cleaning up the fallout from 30 km exclusion zone around the reactor). Approximately 600000 liquidators in total were involved in the clean-up (an artist's impression of some of these efforts have been depicted in the paining by Alexey Akindinov, shown in Fig. 1). The quoted number is the fraction from 1986-87 receiving the highest doses of radiation).
116000 evacuees (further 220000 people were relocated later).
Over 5 million inhabitants of the main areas of contamination (270000 of whom lived in the Strict control zones with Cs-137 deposition density above 5.55 × 105 Bq m-2
This sample of the population allowed for large-scale studying of the impact of the radiation on human health. Doses of radiation received varied based on the age, location and activities of an individual at and immediately following the accident or time and nature of evacuation.
Radiation effects immediately following the explosion (before the radioactive cloud has spread) affected only the persons in the vicinity of the plant. The UNSCEAR report from 1988 lists several possible sources of radiation, the most significant of which was the uniform γ- and β-irradiation of extensive skin surface, coming predominantly from iodine and cesium isotopes.  Inhalation and skin deposition of released radioactive particles affected the dose a person received to a smaller degree.
Baverstock et al. identify the iodine isotopes I-133 to I-135 as the most abundant that were released from the reactor.  However, the main sources of irradiation after the cloud has spread are I-131 (half-life of 8 days) to Cs-137 (half-life of 30 years). There are multiple ways for the elements to get in contact with humans. External routes involve ground and skin deposition. Internal routes involve inhalation from the air and ingestion (by consumption of vegetables contaminated by rainfall or milk from cows fed on contaminated pastures). Iodine accumulates predominantly in the thyroid gland and as such is harmful mostly to children. Cesium is deposited throughout the whole body more or less uniformly.
According to the UNSCEAR 1988 report, a total of 203 persons were at the site of the disaster in the early morning of 26th April 1986, who received mostly whole-body γ-ray doses.  On-site medical personnel treated 132 people on site. One person died within the first hour.
115 individuals were later treated for acute radiation sickness in specialized centre in Moscow (13 other patients were treated in Kiev) with bone marrow doses received ranging 0.8-16 Gy. In half of the cases (58 persons) the treatment was complicated by the presence of extensive β-radiation skin injuries.
Excluding one person dying on-site and another after having received first-aid in Prypiat hospital, 28 people died of acute radiation sickness after receiving medical treatment.
Numerous studies of cancer occurrence among the Chernobyl victims have been carried out and a comprehensive report has been published by Cardis et al. on the 20th anniversary of the accident. 
The most significant finding is the dramatic increase in thyroid cancer among the general population, from the original 0.03-0.05 cases to 4 cases per 100000 by 1995. The biggest impact appeared in the population of children and adolescents (aged 17 or less at the time of the accident), where almost 5000 cases of thyroid cancer have been diagnosed in the impacted areas of former Soviet Union between the accident and 2002, 15 of which have been fatal. Thyroid cancer is caused by the radioactive I-131, inhaled or ingested, getting stored in the thyroid gland. This gland is active and has bigger relative size during the childhood and adolescence, therefore the doses of radioactivity received by young people are higher than in adults.
Increased risk of leukemia has been previously found in the A-bomb survivors, patients treated with radiation or people in nuclear industry. However, no conclusive evidence has been found, at this point, relating the Chernobyl accident and increase of leukemia. Some increase has been found among the Russian liquidators. However, uncertainties in the estimation of the radiation dose require further studies.
Effort has been made to study the increase in other kinds of solid cancers, however, no conclusive evidence has been found so far. This can be attributed to various limitations of the studies - too few subjects, no information about the radiation dose, wrong methodology etc. Some increase has been found in the appearance of breast cancer in Ukraine and Belarus, however, this has been attributed to improvement in diagnosis and registration. Cardis et al. note that the latency period of cancers other than thyroid and leukemia are likely to be long (10-15 years at least) and the risk of cancer developing remains increased over the whole life time of a person, therefore it may be too early to fully evaluate the impact of the Chernobyl accident on cancer. 
Transparency of the eye lens is to some degree affected by the ionizing radiation. This condition is referred to as radiation cataract. It has been previously defined by the International Commission on Radiological Protection as an effect with a threshold on dose. Above this threshold, the severity increases with the dose. The threshold has been assumed to be 0.5-2.0 Gy for acute or 5 Gy for extended irradiation, respectively. Worgul et al. studied a sample of 8607 liquidators for cataract development 12 and 14 years after their exposure.  They found cataracts with characteristics of radiation exposure in 25% of the cases, suggesting the need for lowering the 5 Gy threshold for protracted doses by about an order of magnitude. This finding is in agreement with previous findings among the atomic bomb survivors by Minamoto et al.  Continuation of the study is required to reveal further temporal development of the vision conditions.
The modification of the DNA by ionizing radiation has long been assumed as the main health-damaging effect (to the point of becoming a part of pop-culture, where many super heroes start their careers thanks to a genetic modification via some radiation accident, e.g. Spider Man). Baverstock et al. has pointed out difficulties and an ongoing discussion about the relation between the radiation dose and its effects. 
Germline mutation refers to the mutation of germ cells. These are cells that contain only half of the genetic information and can unite with a cell of the opposite sex to create an offspring. These are therefore mutations that propagate between generations. Such mutations can be monitored by studying minisatellite mutations, or mutations of repeated sections of DNA with the length of 10-100 base pairs. By studying the population around a region in Belarus contaminated by the Chernobyl accident, Dubrova et al. found for the first time an experimental evidence of an increase of germline mutations by ionizing radiation.  Given, that no such increase has been found in the children of the atomic bomb survivors from Japan, a claim is made that a long-term, instead of acute, exposure is required to observe these effects.
Mental health and psychological issues of the population affected by the Chernobyl disaster cannot be omitted when evaluating the event's health impact. Two different possible sources of mental issues can be considered - the direct effect of radiation on an individual's brain and the effects stemming from the experienced trauma, such as forceful relocation, fear based on the lack of knowledge of radiation effects or stigmatizing by the society after the resettlement.
A review of the scientific efforts 25 years after the accident has been published by Bromet et al.  The authors point out suggestive evidence of the impact of radiation on psychological issues, citing several studies of highly-exposed liquidators that developed issues such as schizophrenia or "accelerated aging" with elevated rates. In a similar fashion, the psychological health of children in utero at the time of the accident has been studied. However, the conclusions of different studies were often inconsistent and the research methods unclear, which makes drawing strong conclusions impossible. It is important to point out that, in general, such an effect is real and has been observed in the elevated rate of mental issues of pre-natal survivors of the A-bomb. 
Indirect psychological consequences of the Chernobyl accident are easier to investigate. Increased rates of suicide, post-traumatic stress disorder (PTSD) and depression has been measured in groups of liquidators. Additionally, increased worry about health were found in women from the contaminated regions (no such trend was observed in men), also connected with higher rates of depression, PTSD and subjective feelings of poor mental health, compared to the test groups.
The vast impact of the Chernobyl disaster allows us to study the impact of small to moderate doses of radiation on health in significantly big groups of human population. The main death toll seems to come from the radiation-related thyroid cancers in the fraction of population in their teens at the time of the event, which is related to the consumption of Iodine-contaminated milk. No increase has been found in any other kind of cancer, however, at this time, almost 30 years after the explosion, it may still be too early to draw the final conclusions. Details of the individuals' life-style (e.g. smoking) impact and bias the results as well.
Large-scale impact of the catastrophe has dramatically increased the available data on the radiation impact on health, which have in the past been mostly studied on the atomic bomb survivors. The present-day results (e.g. the data on radiation-induced cataracts) help with refining of the dosage limits for the people working in the nuclear industry.
© Ondrej Urban. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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