ARKHANGELSK, April 13. /TASS/. For the first time, scientists begin to study seasonal and daily fluctuations in the content of radioactive natural gas radon in the Pinega Nature Reserve's caves, said Andrey Puchkov, of the Laverov Federal Research Center for Integrated Arctic Studies' Laboratory of Environmental Radiology (Arkhangelsk) of the Urals Branch of the Russian Academy of Sciences.

"For the first time, we have studied radon distribution in caves of the Pinezga karst speleo mass. We have found that in the winter regime, when the atmospheric temperature is no higher than 2-3 degrees, the air flows from the atmosphere into the cave. In that mode, we observed background values of radon activity in the cave. However, when the air temperature rises and when it passes a certain barrier (the temperature gradient between cave air and atmospheric air), we could see an immediate reverse draft of air from the cave. This has led to an increase in radon activity in the cave air and in the atmospheric air near the cave entrances," he told TASS.

A noticeable reverse air current in one of surveyed caves started at a temperature of plus 7-8 degrees. The researchers compared the content of radioactive gas in the cave, in the area at the entrance and around the cave. The radon activity change depends directly on external conditions. In winter, the cave is ventilated with atmospheric air, thus the radon content in the cave drops significantly. However, in summer and autumn, radon-enriched air moves from the cave depths to the outside, it is pulled out through cracks and fractures, like from a chimney. This explains a slightly increased radon activity in some places during the summer and autumn seasons.

"Periodically, we have recorded exceeded background values. Whether it's dangerous or not, it's too early to say. It is necessary to conduct more detailed studies to identify periods of minimum and maximum levels of radon activity in caves. It is important to mind that normally visitors stay in the cave for a short time, most often for recreational and tourist purposes. This aspect is of bigger importance for accompanying guides, as well as for those who conduct scientific research inside caves," the expert said.

Radon is a naturally occurring radioactive gas that is formed during the decay of uranium, which is present in all rocks and soils. This colorless and odorless gas may be present in high concentrations in indoor air, for example, in residential buildings. It is believed to be one of the causes of lung cancer. Therefore, before starting construction, the building area must be checked for the presence and concentration of radon.

Radon source in caves

There are several thousand caves in Russia, but seasonal or daily radon activities in them have not been studied enough, the scientist said. At the moment, there are only assumptions about the source of this radioactive gas. "Some name crystalline basement as the radon source, others say that the sources are water-mechanical deposits that get accumulated in the cave. Another theory supposes groundwater enriched with natural radionuclides may be the source of radon. As for the Pinega area, this is something we still need to find out," he added.

The studies were conducted during a joint expedition to the Pinega Nature Reserve caves. The expedition featured the Russian Arctic National Park, the Russian Geographical Society, the Institute of Geoecology of the Russian Academy of Sciences, and the Laverov Center of the Urals Branch of the Russian Academy of Sciences. The researchers have explored a number of caves: Kitezh, Golubinsky Gap, Bolshaya Golubinskaya, and others.

The Pinega karst cave system is the biggest in the Russian Arctic Zone. In the Arkhangelsk Region there are about 480 caves, where 140 are located in the Pinega Nature Reserve and its protected area.

The scientists will continue analyzing fluctuations in radon concentrations in caves during 2026. Based on results of the work, they will give recommendations on the best time to visit the Pinega caves.