Long term performance of different radon remedial methods in Sweden

The object of this project was to investigate the long time effectiveness of different radon remedial methods. The ten years project started 1991. From start the investigation comprised of 105 dwellings (91 single-family houses and 14 flats in multi-family buildings). In all of the dwellings remedial measures were carried out in the eighties. Before and immediately after the reduction the local measured the radon concentrations. New measurements of the radon concentrations have been made every third year; in 1991, 1994, 1997 and in 2000. Twelve different radon remedial methods and method combinations were used. The radon sources were building materials as well as sub-soils. In all of the dwellings the radon concentrations were measured by nuclear track films during 3 months (January-March) measurements and in half of them the air change rates by passive tracer gas methods. The results of the 2000 and the 1991 (within brackets) studies showed that the radon concentration was up to 200 Bq/m sup 3 in 54 (54) single-family houses and 7 (7) flats, between 210 Bq/m sup 3 and 400 Bq/m sup 3 in 23 (18) single-family houses and 5 (6) flats, and higher than 400 Bq/m sup 3 in 12 (18) single-family houses and 2 (1) flats. The study 1991 showed also that in about 40 % of the cases the radon concentration had increased by more than 30 % only a few years after reduction actions had been taken. In 19 dwellings the radon concentration was at least doubled. In no fewer than 38 dwellings the radon level has been over 400 Bq/m sup 3 in at least one of the four measuring occasions. The change in radon concentrations was not specific to any given method but seemed to be evenly distributed over all of them. The investigation results showed the necessity for repeated measuring where counter measures have been taken. The causes for increasing radon levels have been made clear in all except 2-3 cases