Discusses use of tracer gases for the measurement of natural ventilation rates States advantages of using radio isotopes are increased speed and sensitivity. Gives expressions for calculating air change rates using radio isotopes from thedecrease in signal. Suggests use of krypton 85 or Xenon 133 as tracers. Discusses errors in the method. Reports study of air quality in a naturally ventilated building in Yakutsk. Air change rates, temperatures and concentrations of carbon monoxide were measured in kitchens with gas stoves. Finds concentrations of CO exceeding permitted levels and attributesthis to low natural ventilation.