In order to map the thoron prone areas of the coastal region of Kollam district, a well known HBRA of south India, comparative study of radon and thoron exhalation rate was conducted. The in situ measurement of radon and thoron exhalation has been taken. These studies were correlated with the gamma radiation level. The average value of thoron exhalation is found to 5.55 ± 1.35 Bq m^-2 s^-1 along the coastal areas and the radon exhalation rate is found to 107.6 ± 32 Bq m^-2 h^-1. The value of thoron exhalation was found 12 times greater than the global values in Neendakara and Chavara region and about 6 times greater in the Alappad region.