A preliminary study of radon concentration in the air was carried out in a tunnel under construction over three different sampling periods from June 2001 to August 2002. The concentration was measured with LR-115 Type II nuclear track etch detectors and E-PERM detectors. The average radon concentrations for each sampling period were : 1082 Bq m-3, 1550 Bq m-3and 414 Bq m-3. Possible formation of convection cells inside the tunnel can explain the variation of radon concentration. A dose assessment calculation was carried out to estimate the effective doses of the construction workers. The maximum effective dose was estimated at 13.5 msv y-1for an eight-hour shift.