Radioactivity in outdoor air

STUK monitors the levels of radioactive substances outdoors at eight localities in Finland. You can retrieve measurement data from different periods by first selecting the location.

Results of outdoor radioactivity measurements


Most of the regularly detected cesium-137 originates from the Chernobyl nuclear power plant accident that happened in 1986. Cesium-137 released into the atmosphere during the Fukushima nuclear power plant accident in March 2011 travelled to Finland through the air, causing a temporary rise in activity concentrations. In addition to cesium-137, other radioactive substances, such as radioactive iodine (iodine-131) from Fukushima were detected in Finland.

The activity concentrations of artificial radioactive substances have been low and have no health impacts.

In the graphs and table below, microbecquerels per square metre of air (µBq/m3) is used as the unit for activity concentration. Activity concentration of one µBq/m3 means that one radioactive atom on average decays in one cubic metre of air in one million seconds (11.6 days). For example, the average radon activity concentration in the indoor air of Finnish homes is approximately 120 becquerels per cubic metre, which is approximately 100,000,000 times higher compared with typically detected concentrations of artificial radioactive substances outdoors.


Air particles are collected with pumps in samplers to a fibreglass filter, and the filters are analysed in a laboratory. The method can detect very small changes in radiation.

The Helsinki station automatically collects, processes and analyses the sample and reports the data. The sampler absorbs 550 cubic metres of air per hour via a fibreglass filter. The station automatically replaces the filter every day.

The Kotka, Kajaani and Rovaniemi stations are equipped with high-volume samplers with 900 cubic metres passing through the fibreglass filter per hour. The sampling rate at the Imatra, Kuopio, Sodankylä and Ivalo stations is 150 cubic metres per hour.

Normally, the filters of high-volume samplers are changed manually once a week and the filters of smaller samplers twice a week. In a radiation hazard situation, the filters are changed more frequently.

The samplers are equipped with charcoal filters in addition to the fibreglass filters. Charcoal collects gaseous substances, such as iodine. The charcoal cartridges are changed and analysed once a month.

Filters are analysed carefully

The filters are analysed with a gamma-ray spectrometer at STUK.

The gamma-ray spectrometer measures the intensity of gamma radiation from the sample as a function of radiant energy. Radioactive materials emit gamma rays only with certain energies specific to each material, so different materials can be identified based on the gamma spectrum. The concentration of radioactive materials on the filter can be calculated from the number of gamma photons reaching the gamma-ray spectrometer. When the volume of air that has passed through the filter is known, it is possible to calculate the average concentration of the materials in the air. All in all, the time between the commencement of sampling and the completion of the analysis data is between three days and two weeks, depending on the station.

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