On Sunday April 26, 1986, at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine, reactor # 4 exploded. For the 25 years from 1986 to 2011, this incident has been referred to as the world's worst nuclear power plant accident.

Until the Nuclear Industrial Safety Agency announced on April 12, 2011 that at Japan's Fukushima Daiichi nuclear power plant it was raising the crisis level to 7, Chernobyl was the only incident ranked a 7 on the International Nuclear and Radiological Event Scale produced by the International Atomic Energy Agency.

The Chernobyl accident is the most serious accident in the history of the nuclear industry. Indeed, the explosion that occurred on 26 April 1986 in reactor # 4 of the nuclear power plant, and the consequent fires that lasted for 10 days, led to huge amounts of radioactive materials being released into the environment.

The cloud from the burning reactor spread many types of radioactive materials, especially iodine-131 and caesium-137, over much of Europe. Because radioactive iodine disintegrates rapidly, it largely disappeared within the first few weeks of the accident. Radioactive caesium however is still measurable in soils and some foodstuffs in many parts of Europe. The greatest concentrations of contamination occurred over Ukraine, Belarus and Russia.

Since the accident, some 600 000 people have been involved in emergency, containment, cleaning, and recovery operations, although only few of them have been exposed to dangerous levels of radiation. Those who received the highest doses of radiation were the emergency workers and personnel that were on-site during the first days of the accident (approximately 1000 people).

At present, more than five million people live in areas that are considered to be ‘contaminated’ with radioactive materials from the Chernobyl accident. The area closest to the reactor site was most heavily contaminated and the 116 000 people who lived there were evacuated soon after the accident.

How has human health been affected by the Chernobyl accident?

After the accident, people were exposed to radiation both directly from the radioactive cloud and the radioactive materials deposited on the ground, and through consuming contaminated food or breathing contaminated air.

Doses of radiation received during and immediately after the accident were high for some emergency workers, but much lower for later recovery-operation workers and people living in the contaminated areas.

At present, 100 000 people living in contaminated areas still receive a higher dose of radiation than the limit recommended for the general public.

 It is difficult to tell precisely the number of deaths – past and future – attributable to the Chernobyl accident, because people who have been exposed to low levels of radiation often die from the same causes as unexposed people.

Confusion about the impact of the accident has given rise to highly exaggerated claims that tens or even hundreds of thousands of people have died as a result of the accident. In fact, a much smaller death toll can be directly attributable to Chernobyl radiation.

In the general population of the contaminated regions, there is so far no convincing evidence that Chernobyl has increased the number of cases of leukaemia or solid cancers, except for childhood thyroid cancer.

How has the environment been affected by the Chernobyl accident?

Some areas of Europe were substantially contaminated, particularly in Ukraine, Belarus and Russia, and by the large quantities of radioactive materials released from the damaged reactor. Most of these materials have since transformed into stable, non-radioactive materials but some will remain radioactive for a long time.

The urban areas near the reactor were heavily contaminated and rapidly evacuated. Since the accident, surface contamination has decreased and levels of radiation measured in the air are now the same as before the accident in most of these areas.

Regarding agriculture, the contamination of crops, meat, and milk with short-lived radioactive iodine was a major concern in the early months after the accident. Now and for decades to come, contamination with longer-lived radioactive caesium is the main concern in some rural areas.

Forest food products such as berries, mushrooms, and game contain particularly high levels of long-lived radioactive caesium and this contamination is expected to remain high for several decades.

As a result of the accident, water bodies and fish became contaminated with radioactive materials. The contamination soon decreased as a result of dilution and decay but some of the materials remained trapped in the soils around contaminated rivers and lakes. Today, most water bodies and fish have low radioactivity levels, although the levels in some closed lakes remain high.

The accident immediately affected many plants and animals living within 30 km of the site. There was an increase in mortality and a decrease in reproduction and some genetic anomalies in plants and animals are still reported today. Over the years, as the radioactivity levels decreased, the biological populations started to recover and the area has become a unique sanctuary for biodiversity.

What are the social and economic costs of the Chernobyl accident?

The Chernobyl accident and the measures taken to deal with its consequences have cost just to Ukraine over 15 billion of USD. Today, social benefits are paid to the millions people who are considered to have been affected in some way by the Chernobyl accident. This spending is a huge burden on national budget and is unsustainable.

After the accident, more than 330 000 people were resettled outside the most severely contaminated areas. This relocation reduced their exposure to radiation but was a deeply traumatic experience for many.