The effects of the Chernobyl accident in Finland

The accident at Chernobyl nuclear power plant in April 1986 will expose Finns to a total radiation dose of two millisieverts during 50 years. We receive a similar dose each year from radon. Half of the total dose from Chernobyl came during the first ten years after the accident.

Radioactive fallout fell down with rain

In the accident, high amounts of radioactive materials were released and propelled more than a kilo-metre up into the air by the force of the explosion and fire. At this altitude, winds blew initially towards Finland, Sweden and Norway.

Airflows carried the radioactive substances released into the air during the accident over Finland in about 24 hours. However, these air masses were so high that they could not be detected on ground level.

Only about two days after the accident, on 27 April, did the radioactive substances fall to such an extent that they were detected in the surface air close to the ground in Finland. The first observations were made in Nurmijärvi at the Finnish Meteorological Institute’s metering station and then at the Defence Forces’ metering station in Kajaani. The information reached STUK on 28 April at 10 a.m. STUK immediately started to investigate.

Over Sweden, the air masses fell slightly earlier than in Finland and the elevated levels of radioactivity were detected more quickly. The first thought in Sweden was that there was a radioactive leak at the Forsmark nuclear power plant. This conjecture turned out to be wrong.

Later, as the direction of winds changed, radioactive substances were also carried to other areas, in particular Central Europe. In Finland, the concentrations of radioactive materials were at their highest in the evening of 28 April. More than 30 radioactive substances were identified in air. After two or three days, the air was almost completely clean again. A couple of weeks after the accident, Finland received another plume of radioactive materials from Chernobyl. However, the concentrations were lower compared with the first plume.

Most of the radioactive materials were washed down with rain. The amount and intensity of the rain varied in different parts of Finland, which also meant that the amount of radioactive substances that fell to the ground varied considerably.

The fallout carried a lot of short-lived radioactive materials that disappeared from nature in just a few days or months. In the long term, with regard to radiation doses, the most important substances are the radioactive cesium isotopes, cesium-137 and cesium-134. Their half-lives are 30 and approximately two years, respectively.


An estimate of cesium-137 fallout in Finland in 2019.
An estimate of cesium-137 fallout in Finland in 2019.

Cesium in soil causes an additional external radiation dose

The external radiation dose received by humans results from fallout, that is, radioactive materials fall-en to the ground. At first, the highest measured dose rate resulting from the Chernobyl fallout was five microsieverts per hour. It is 30 to 50 times higher than the dose rate of normal natural background radiation. However, it was not high enough to necessitate human protective measures. Protective measures shall be taken if the radiation level exceeds 100 microsieverts per hour. For children, the threshold is ten microsieverts per hour. Protective measures for food production are already initiated at dose rates of one microsievert per hour.  

The unit of measurement of external radiation is microsieverts per hour (μSv/h)

The unit of measurement of human dose of radiation is millisievert (mSv)

Short-lived radioactive materials in the fallout decayed rapidly. As a result, the external radiation also decreased. At the end of May 1986, the dose rate caused by radioactive substances was less than one tenth of the value measured immediately after the fallout.

The dose from internal irradiation caused by the Chernobyl fallout comes from natural produce

Our internal radiation dose is almost completely caused by radioactive substances received in food. Milk, meat and other farm produce were the main source of cesium in the first years after the accident. However, their cesium concentrations decreased rapidly. Since 1988, most cesium has been received from inland fish, game, wild mushrooms and wild berries. STUK monitors radioactivity in food.

The amount of cesium in humans can be measured through whole body counting, i.e. by measuring the whole person with purpose-built equipment. The cesium amounts in Finns peaked in the summer of 1987. In the region with the heaviest fallout, the average level was 4,000 becquerels. The average of all Finns measured was 2,000 becquerels of cesium-137. In people who eat a lot of natural produce the doses were five- to tenfold compared with others. At present (year 2020), Finns have on average about 100 becquerels of cesium mainly from Chernobyl. For comparison: We all have a few thousand becquerels of naturally-occurring radioactive potassium-40 in our systems.

In 1987, Finns received a radiation dose of approximately 0.13 millisieverts from radioactive cesium in food. By 1996, the internal radiation dose had decreased to a fifth, that is, 0.02 millisieverts per year. Today, the extra dose is entirely caused by cesium-137 and is now less than a hundredth of the total annual dose. The accident does not have a visible impact on Finland’s cancer statistics

With regard to radiation effects, the 1986 Chernobyl nuclear accident is still clearly the worst of its kind. For example, the general population’s radiation exposure caused by the Fukushima accident was only about a tenth of the radiation exposure of the local residents in and near Chernobyl.

According to a study conducted by STUK, the Cancer Registry and the National Institute for Health and Welfare (THL), the fallout caused by the Chernobyl nuclear accident has not noticeably increased the number of cancers in Finland.

Chernobyl fallout and cancer incidence in Finland 1988–2007 (

 The purpose of the study was to investigate whether the total number of cancers in Finland has changed as a result of the radiation exposure following the Chernobyl accident in 1986. According to the study, the incidence of cancer in areas with the heaviest fallout did not increase more than in other areas of the country during the decade after the accident and later.

The study included approximately two million people who had lived permanently in the same low-rise residential buildings for at least a year after the accident.  The analysis was based on a country-wide map grid (250 metres x 250 metres) and all cancer cases found in the population, excluding breast, prostate and lung cancer, whose regional differences are largely dependent on screening activity or smoking. The country was also divided into four radiation exposure zones based on external radiation measurements carried out at STUK. The incidence of cancer in the exposure areas was compared be-fore and after the Chernobyl accident.

The study was able to take advantage of high-quality Finnish registers, so the analysis was based on a very large study population.

Previously it has been established in Finland that the most radiation-sensitive cancers, children’s leu-kaemia and thyroid cancer, have not increased as a result of the Chernobyl accident.

However, a Swedish study has reported that in the areas in Sweden that received the highest levels of the Chernobyl fallout, the number of cancer cases in the decade following the accident has been higher than expected. However, subsequent studies have not confirmed these findings.

The situation was different in areas near Chernobyl. By far the most important health effect caused by the accident is a significant increase in the incidence of thyroid cancer in those who were exposed as a child to radioiodine released from the reactor. Children received radioactive iodine in milk.

By 2005, more than 3,500 thyroid cancers were reported in Ukraine, Belarus and southwest Russia related to the radiation exposure caused by the accident. Other cancers have not been found to have increased as indisputably, which is related to the fact that due to radioiodine, the thyroid gland was exposed to a much higher radiation dose than other organs. A child’s thyroid gland accumulates more radioactive iodine than that of an adult, because the child’s thyroid gland functions more actively.

Then there are the over half a million men from different parts of the then Soviet Union who were sent to clean up the area after the accident. They stayed and worked in the vicinity of the reactor for 3–4 months in most cases. So far as they are concerned, there are indications that radiation may have in-creased the risk of leukaemia and possibly also the prevalence of circulatory diseases.