Thoron concentration levels have been measured in the high background radiation area located around Kudankulam nuclear power plant i.e. in the Agastheeswaram, thovalai taluks (sub-district) of the Kanyakumari district using solid-state nuclear track detectors. Measurements have been carried out on quarterly cycles making four measurements at each location throughout a year. Totally 100 dwellings have been selected for the study. Thoron concentration has been found to vary between 4.6 and 143.5 Bqm^-3 with the geometric mean value being 25.1 Bqm^-3. Results have been analyzed for different categories of dwellings with respect to their construction type.