[en] Highlights: • ¹³⁷Cs concentrations in indoor air in 60 buildings near FDNPP were investigated. • ¹³⁷Cs concentrations decreased with decreasing the aerodynamic diameter of aerosol. • ¹³⁷Cs concentrations was inversely related to the square distance from the FDNPP. • ¹³⁷Cs concentrations was higher during dusting than during vacuuming. • Estimated deposition dose in TB and alveolar were 1.9 and 2.8 Bq, respectively. - Abstract: After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, outdoor decontamination was conducted by the Japanese Government, whereas indoor contamination was the responsibility of individual residents. However, no data exist regarding ¹³⁷Cs exposure during indoor cleaning. Therefore, indoor ¹³⁷Cs radioactivity concentrations in indoor air were determined during cleaning in 60 houses within the evacuation area near the FDNPP, Fukushima Prefecture, Japan. Radiocesium activity concentrations per cubic meter of indoor air during dusting (mean ± SD (median)) was 6.8 ± 7.9 (4.7) and 1.6 ± 2.7 (0.78) Bq/m³ for all aerodynamic diameter of aerosol particle and PM2.5, respectively. Radiocesium activity concentrations in indoor air decreased with decreasing the aerodynamic diameter of aerosol particles (mean: 0.099, 0.22, 0.41, 0.92, 2.2, and 2.9 Bq/m³ for the aerodynamic diameter of 6.6 μm, respectively) and was inversely proportional to the square of the distance from the FDNPP. Indoor ¹³⁷Cs radioactivity concentrations were significantly higher during dusting than during vacuuming with either a cyclone handy cleaner or a normal vacuum cleaner. The mean deposited activities in tracheobronchial and alveolar regions was estimated to be 1.9 and 2.8 Bq, respectively, during a 2-h dusting period in the studied houses. For values below the detection limit, half of the detection limit was used for calculating the means and SDs.