Risks and Sources of Exposure in the Event of a Serious Accident
Editorial review 2026
The main sources of exposure following an accident and a major release of radioactive material are irradiation from the radioactive cloud, inhalation of radioactivity, contamination of the skin and clothing, radioactive deposits on the ground, and the long-term ingestion of contaminated food and water.
Irradiation from the radioactive cloud is an external irradiation occurring at a distance and caused by gamma rays. When radioactive materials are released following an explosion, the cloud formed from the initial plume is carried away by the winds.
Inventory of Chernobyl releases
Short-lived volatile elements contributed most to the initial activity dispersed during the Chernobyl accident, mainly xenon-133 and iodine-131, as shown in this inventory. Xenon-133, a noble gas responsible for external exposure, is less dangerous than radioactive iodine isotopes, which accumulate in the thyroid through the food chain (especially milk). During the first weeks and months, activity decreased considerably with the disappearance of short-lived elements until longer-lived elements such as cesium became predominant.
© IN2P3 (Source: Chernobyl Atlas)
It is particularly dangerous in the early stages because of the predominance of highly radioactive elements, after which radioactive decay over time and atmospheric dilution reduce its importance. At Chernobyl, shortly after the explosion, xenon-133 was the most active element, followed by iodine-131 (although iodine-131 was predominant, shorter-lived iodine isotopes such as iodine-132 and 133 should not be overlooked). Xenon is a noble gas, only slightly toxic because it is not assimilated by living organisms. It disperses rapidly.
Although air provides poor shielding, irradiation from the cloud decreases with distance. In the case of gamma rays with a characteristic energy of 1 MeV, the number of gamma rays not stopped by air is divided by 1000 every 900 meters.
Inhalation of radioactivity can potentially be more dangerous. It is caused first by radioactive aerosols from the cloud as it passes overhead, and later by dust particles resuspended into the air.
Contamination of the skin and clothing occurs mainly during initial response operations and affects people staying in highly contaminated areas. Like inhalation, it occurs during the passage of the cloud and later through the resuspension of radioactive dust or the handling of contaminated materials.
Deposits of radioactive isotopes on the ground are a source of external gamma irradiation. Less intense than irradiation from the cloud, it occurs at closer range and lasts longer. During the first weeks, it is mainly caused by short-lived radioactive isotopes such as iodine-131. Over several years, cesium-137 becomes dominant because of its radioactive half-life of 30 years.
Contamination of the food chain
The contamination of goat milk in the Corsican mountains in May 1986 is an example of radioactive elements entering the food chain through deposits on vegetation. Some Mediterranean plants, such as thyme or dried fruits, may contain more radioactive elements than others. Goats feed on these plants. Contamination levels of iodine-131 in their milk reached 15,000 becquerels per liter. However, this milk was not consumed by very young children, who are the most sensitive population group. This contamination decreased rapidly, being divided by 2 every 8 days.
© IRSN/drawing: Martine Beugin
Contamination of the food chain. The ingestion of contaminated food and water leads to internal exposure. Contamination initially comes mainly from iodine-131 during the first weeks and later from cesium-137 as time passes. These two isotopes, absorbed by plants, enter the food chain.
This information is based on scientific reference reviews.