Extreme reverse seasonal variations of indoor radon concentration: a case study

Indoor radon concentration follows usually a seasonal cycle with higher levels in cooler months and lower levels in warmer months. This cycle depends mainly on temperature and weather conditions of the area (i.e. changes in the atmospheric parameters reflecting of chimney effect and wind effect), with higher radon concentration variations between cooler and warmer months in areas where temperature in winter months is lower. However, the average effect is modified by individual parameters such as living habits (i.e. changes in the house’s ventilation rate), occupancy pattern, and building characteristics, thus resulting in different seasonal variations among dwellings in the same area, including some cases where radon concentration in warm months result to be higher than radon concentration in cold months. These situations are usually referred to as situations with inverse seasonal variations of indoor radon concentration. Reverse seasonal variations can give rise to significant underestimation of the actual annual average of radon concentration when a measurement shorter than one year is performed, e.g. in winter season, and an average seasonal correction factor is applied for estimating annual average. In addition to the inverse seasonal variations described above, some extreme inverse seasonal variation can occour in specific situations. Such situations have been identified in the framework of a study on long-term variations carried out in some tenths of dweellings in Rome and surrounding small towns. In particular two dwellings with high reverse seasonal variability were identified in a small town near Rome, on the basis of six-month measurements carried out for several years by passive devices containing solid-state nuclear detectors. In one of these dwellings radon concentration was monitored also by a continuous radon monitors, showing extreme and very rapid increases of radon concentration during warm months. In this presentation, results of both passive and active measurements of radon concentration will be presented and compared with other situations with extreme inverse seasonal variations found elsewhere.