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Basic considerations1. Legal base fixing the limiting value for radon in residential and recreational premises.(Radiological Protection Ordinance of 22 June 1994)2. Engagement in a campaign measuring the radon concentrations, finding the areas with radon prone buildings, and mapping those areas.3. Collaboration with the homeowners living at a high radon concentration to define a project how to reduce it.

An Austrian farmhouse has been mitigated; it was built around the turn of the century in theconventional farmhouse style of that time.

Radon surveys started in Estonia in 1989. The Department Building Physics at the Estonian BuildingResearch Institute measured radon levels in dwellings, in building materials produced in the country,and in the soil. The indoor radon concentrations in more than 400 houses were measured under grabsamplingtechniques using Lucas cells. The highest measured radon level was 6700 Bqm-3. The resultsof these measurements made during 1989-91 showed that the main source of indoor radon is the soilunderneath buildings.

In Denmark, a new survey of indoor radon-222 has been carried out. One-year alpha trackmeasurements (CR-39) have been done in 3019 single-family houses. There is from 3 to 23 housemeasurements in each of the 275 municipalities. Within each municipality, houses have been selectedrandomly. One important outcome of the survey is the prediction of the fraction of houses in eachmunicipality with an annual average radon concentration above 200 Bqm-3.

Numerical modelling is a powerful tool for studies of soil gas and radon-222 entry into houses. It isthe purpose of this paper to review some main techniques and results. In the past, modelling hasfocused on Darcy flow of soil gas (driven by indoor-outdoor pressure differences) and combineddiffusive and advective transport of radon. Models of different complexity have been used. Thesimpler ones are finite-difference models with one or two spatial dimensions. The more complexmodels allow for full 3D and time dependency.

Radon, radon progeny and unattached radon progeny were measured in two chambers with differentcharacteristics at the Jenolan Caves, New South Wales, Australia, in 1996. Meteorological parametersand condensation nucleus concentrations were measured in order to understand the processesgoverning the radon concentration and degree of disequilibrium with the progeny. One chamber waspoorly ventilated, and rainfall proved to be the most important influence on radon concentration.

In Poland as in many other countries regulations limiting permissible level of radon concentrations indoors are in force. It implies a need for a methodology of searching for houses with elevated radon concentration. Since it was discovered that the bedrock was the most powerful source of radon indoors it has been observed by many researchers that homes with enhanced radon concentrations occur more frequently in particular areas.

A nation-wide survey of radon concentrations in Norwegian dwellings was undertaken in the period 1987 - 89. In this survey, radon measurements were made by CR-39 etched track detectors (six months integration time) in the main bedroom of approximately 7500 randomly selected dwellings built before 1980. The annual average radon concentration in Norwegian bedrooms was calculated to 51 Bq/m3, and 3.7 % of the results exceeded 200 Bq/m3.

The International Commission on Radiological Protection (ICRP) and The Council of the European Union has recommended the Member States to take action against radon in homes and at workplaces. In the ERRICCA project the Group on Legal and Building Code Impact was designated to study the current radon legislation and to give advice on future conduct of laws and recommendations. On behalf of the Group a questionnaire on radon-legislation have been send out to all European States and a selection of non-European States.

The effect of indoor gamma dose rate, permeability of soil, substructure, and ventilation habits of inhabitants were studied using data of 84 low rise residential houses collected in an area of enhanced indoor radon concentration. The radon concentrations varied from 30 to more than 5000 Bqm-3. Cross-tabulation, comparisons of means and multiplicative models were used to test the significance of the effects. In this study a quite high percentage of explained variation R2 - 68% - was found. It was found that the most important factors were the substructure and the permeability of the soil.