  The concentration of radon in the air has been measured in the 700 m-deep
Yangyang underground laboratory between October 2004 and May 2022. The average
concentrations in two experimental areas, called A6 and A5, were measured to be
53.4$\pm$0.2 Bq/m3 and 33.5$\pm$0.1 Bq/m3, respectively. The lower value in the
A5 area reflects the presence of better temperature control and ventilation.
The radon concentrations sampled within the two A5 experimental rooms' air are
found to be correlated to the local surface temperature outside of the rooms,
with correlation coefficients r = 0.22 and r = 0.70. Therefore, the radon
concentrations display a seasonal variation, because the local temperature
driven by the overground season influences air ventilation in the experimental
areas. A fit on the annual residual concentrations finds that the amplitude
occurs each year on August, 31$\pm$6 days.
