The concentration of radon in an underground research facility (URF) was measured by setting up 12 sampling points in the
URF and with 3 different measurement methods. All the methods were calibrated in the radon laboratory of the No. 6 Institute
of Nuclear Industry. The accumulation of radon in the URF was observed before a ventilation system was applied. The reduction
of radon concentration in the URF by 1-hour ventilation was also observed. Experimental result indicates that the concentration
of radon in the URF increased from 15 to 50 Bq·m−3 in 5 days without ventilation, and decreased to less than 10 Bq·m−3 with 1-hour ventilation. Applying the average working time of 4 hours per day of the workers in the URF, the additional effective
dose is 0.75 msv·y−1 when 1 hour ventilation is applied before entering the URF and 13 mSv·y−1 without ventilation. These figures strongly suggest that for the health of the workers, ventilation in such underground research
facilities is needed.