ICRP Publication 126: Radiological Protection against Radon Exposure

The proportion of all lung cancers linked to radon is estimated to lie between 3% and 14%, depending on the average radon concentration in the country and the method of calculation.

Radon is the second most important cause of lung cancer after smoking in many countries. Radon is much more likely to cause lung cancer in people who smoke, or who have smoked in the past, than in lifelong non-smokers. However, it is the primary cause of lung cancer among people who have never smoked.

There is no known threshold concentration below which radon exposure presents no risk. Even low concentrations of radon can result in a small increase in the risk of lung cancer.

In Publication 115 (ICRP, 2010) on the risk of lung cancer associated with exposure to radon and its progeny, the Commission made a thorough review and analysis of the epidemiology of radon for both workers (underground miners) and the general population. Its main conclusions were as follows.

There is strong evidence from cohort studies of underground miners and from case–control studies of residential radon exposures that radon and its progeny can cause lung cancer. For solid tumours other than lung cancer and leukaemia, there is, to date, no convincing or consistent evidence of any excess risk associated with exposure to radon and its progeny.

Appropriate comparisons of estimates of the risk of lung cancer from miner studies and from indoor studies show good consistency. Three pooled residential case–control studies (in Europe, North America, and China) gave similar results. After correcting for random uncertainties in the radon activity concentration measurements, the European pooled residential case–control study reported an excess relative risk of 16% (95% confidence intervals: 5–32%) per 100 Bq m⁻³ (Darby et al., 2005). This value may be considered as a reasonable estimate for risk management purposes at relatively low and prolonged radon exposures in homes, considering that this risk is linked to an exposure period of at least 25 years.

The cumulative risk of lung cancer up to 75 years of age is estimated for lifelong non-smokers as 0.4%, 0.5%, and 0.7% for radon activity concentrations of 0, 100, and 400 Bq m⁻³, respectively. The cumulative risks of lung cancer by 75 years of age for lifelong smokers are close to 10%, 12%, and 16% for radon activity concentrations of 0, 100, and 400 Bq m⁻³, respectively (Darby et al., 2005, 2006). Cigarette smoking remains the most important cause of lung cancer.

Based upon a review of epidemiological studies of underground miners, including studies with relatively low levels of exposure, a detriment-adjusted nominal risk coefficient of 5 × 10⁻⁴ per WLM (0.14 per J h m⁻³) is adopted for the lung detriment per unit exposure. This value was derived from recent studies considering exposure during adulthood, and is close to twice the value calculated in Publication 65 (ICRP, 1993).

As a result of this review, the Commission recommended, in its Statement on Radon, a detriment-adjusted nominal risk coefficient for a population of all ages (mixed adult population of non-smokers and smokers) of 8 × 10⁻¹⁰ per Bq h m⁻³ for exposure to radon-222 gas in equilibrium with its progeny (ICRP, 2010). The Commission’s findings are consistent with other comprehensive estimates, including that submitted to the United Nations General Assembly by UNSCEAR (2009).

Radon-222 decays to form one atom of non-gaseous polonium-218. In turn, this atom decays into other radionuclides as shown in Fig. 1.1. Short-lived radon decay products (polonium-218, lead-214, and bismuth-214) exist in air as unattached radionuclides, and attached to aerosol particles with the unattached fraction depending on local conditions. They can be removed by deposition on surfaces and ventilation.

As radon is an inert gas, nearly all of the radon that is inhaled is subsequently exhaled. However, a large proportion of inhaled radon progeny deposits in the airways of the lungs. Due to their short half-lives, dose is delivered to the lung tissues before clearance can take place, either by absorption into blood or by particle transport to the alimentary tract. Two of these short-lived progeny, polonium-218 and polonium-214, emit alpha particles, the deposited energy of which dominates the dose to the lung. In contrast, doses to systemic organs and gastrointestinal tract regions are low. As a consequence, the equivalent dose to the lung contributes more than 95% of the effective dose following inhalation of radon progeny. The effective dose from the inhalation of radon gas alone is typically less than 10% of that from inhaled radon progeny.

Doses depend mainly on the concentration of radon progeny, the duration of exposure, the breathing rate, and the aerosol properties, including the activity size distribution of the radon progeny aerosol and the unattached fraction. If the exposure is characterised by radon gas measurements, a value for the equilibrium factor, F, is required to estimate the concentration of radon progeny in air. For radiological protection purposes, most of the parameters in the dosimetric models, such as breathing rate, correspond to values for Reference Worker or Reference Person. For the dosimetric model considered by the Commission (ICRP, 2014b), two occupational exposure situations have been considered: a mine and a generic indoor workplace. The calculated dosimetric coefficients for these two situations do not distinguish between smokers and non-smokers. This approach is considered appropriate for radiological protection purposes.

For dwellings, the dose coefficient has been calculated to be 13 mSv per WLM (Marsh and Bailey, 2013). With this dose coefficient and exposure parameters of F = 0.4 and occupancy of 7000 h year⁻¹, an annual average radon concentration of 300 Bq m⁻³ corresponds to a dose within, but towards the upper end of, the 1–20-mSv range of reference levels for existing exposure situations. For comparison, a radon concentration of 300 Bq m⁻³ in homes corresponds to an annual dose of approximately 10 mSv using the epidemiologically derived dose conversion convention (see Para. 27), applying the revised nominal risk coefficient in Publication 115 (ICRP, 2010) and Marsh et al. (2010). A dose coefficient of 11 mSv per WLM has been obtained for exposures in mines using the dosimetric approach, essentially the same as obtained by the dose conversion convention.

According to WHO, indoor radon exposure poses a significant public health hazard (WHO, 2009) due to the estimated radon-attributable lung cancer death rates in comparison with other cancers. People spend much of their time indoors, primarily at home. From a public health perspective, as domestic exposure to radon is the most important, a radon protection strategy has to focus primarily on exposure in dwellings rather than in public spaces and workplaces, where premises are under formal management and regulation is more appropriate.

Although there are no epidemiological studies of domestic exposure of children to radon, they are generally assumed to be more sensitive to radiation than adults. However, in line with its integrated approach, and given that risk accumulates throughout life, the Commission does not recommend the use of specific indicators and advice for children. Nevertheless, the significant presence of children in a building should be an argument for strengthening awareness and implementing both preventive and mitigating actions as a priority.

From a public health perspective, radon reduction is a long-term objective. Prevention of radon exposure is most relevant in new buildings. The implementation of preventive measures in new and renovated buildings provides a good partial solution, with cost-effectiveness increasing with experience and application of building codes (STUK, 2008). This also helps to develop awareness amongst professionals involved in the construction sector.

Remediation in existing buildings is also often appropriate in buildings with high radon concentrations. In such situations, there may be a primary source of radon ingress, and radon levels can often be reduced by a factor exceeding 10.

There is a large distribution of individual radon exposures, and the evidence of risk of lung cancer exists at levels of long-term average radon concentration below 200 Bq m⁻³ (ICRP, 2010). As a consequence, the aim should be to reduce both the overall risk for the population and, for the sake of equity, the highest individual exposures to levels that are as low as reasonably achievable. However, the total elimination of radon exposure is not feasible.

Radon exposure is not the only source of risk for the population. The radon protection strategy should be properly scaled, with other health hazards and priorities identified in the country taken into account. Furthermore, comparison and integration between the radon protection strategy and other public health policies, such as non-smoking and indoor air quality policies, should be sought in order to avoid inconsistencies and achieve better effectiveness.

Considering the ubiquity of radon exposure, and the multiplicity and diversity of situations and decision makers, a straightforward, realistic, and integrated radon protection strategy, addressing most situations with the same approach, is appropriate. It must be supported and implemented on a long-term, potentially permanent basis, and involve all the relevant stakeholders.

As radon exposure is mainly a domestic issue, the success of a radon protection strategy depends, to a large extent, on the decisions taken by individuals to reduce the risk in their home, when relevant (self-help protection). Clear awareness among the general population about the risk associated with radon is required, particularly in radon-prone areas, to help individuals in taking their responsibilities. It has to be recognised that, at present, apart from some countries with established radon policies, this awareness is often poor and should be increased. Improvements can be achieved through the development of action plans that describe the risks of radon and the actions that are needed to address it. The provision of a good infrastructure and support for information, measurement, and remediation are prerequisites.

The level of enforcement of actions that are warranted is closely related to the degree of legal responsibility for the situation. The owner of a house may have such responsibilities if the house is rented or sold. An employer has a legal responsibility for the health and safety of the employees. The manager of a school (or the local authority) may have a legal responsibility for the health of the pupils as well as the staff. The same consideration may apply to other public buildings and workplaces. The requirements related to such responsibilities should be commensurate with the wider public health policy in the country.

The issue of responsibility clearly shows the need for a graded approach in defining and implementing a radon protection strategy. Such a graded approach should be based on both ambition and realism.

Radon exposure situations have the characteristics of existing exposure situations as the source is unmodified concentrations of ubiquitous natural activity in the earth’s crust. Human activities can create or modify pathways, increasing indoor radon concentrations compared with outdoor background levels. These pathways can be modified by preventive and mitigating actions. The source itself, however, cannot be modified and already exists when a decision of control has to be taken. Radon in dwellings and workplaces are given as examples of existing exposure situations in Publication 103 (ICRP, 2007, Para. 284).

Exposure to radon in uranium mining is often managed in the same way as a planned exposure situation, because uranium mining is part of the nuclear fuel cycle and also because workers are occupationally exposed to other radiation sources as well as radon, such as external exposure to gamma radiation, and inhalation or ingestion of dust. It is for national authorities to decide which other radon exposure situations involving workers are to be regarded from the outset as planned exposure situations.

Radon is not likely to give rise to an emergency exposure situation despite the fact that the discovery of very high concentrations in a place can require the prompt implementation of protective actions, particularly when the exposure affects other occupants for whom the decision maker for a property has a duty of care.

The philosophy of Publication 103 (ICRP, 2007) compared with Publication 60 (ICRP, 1991) is to recommend a consistent approach for the management of all types of exposure situations. This approach is based on application of the optimisation process below appropriate dose restrictions (i.e. dose constraints or reference levels).

Occupational exposure is radiation exposure of workers incurred as a result of their work. However, because of the ubiquity of radiation, the direct application of this definition to radiation would mean that all workers should be subject to a regime of radiological protection. The Commission therefore limits its use of ‘occupational exposures’ to radiation exposures incurred at work as a result of situations that can reasonably be regarded as being the responsibility of the operating management (ICRP, 2007, para. 178). In most workplaces, radon exposures are adventitious (i.e. they are not caused by, or associated with, the nature of the work undertaken, but arise simply through workers and others being present in the employer’s premises).

Publication 65 (ICRP, 1993, Para. 86) indicates that ‘workers who are not regarded as being occupationally exposed to radiation are usually treated in the same way as members of the public’. This is still valid, considering that the health and safety of the workers continue to be the responsibility of their employer. In other words, in general workplaces where radon exposure is adventitious, radon is not managed by controlling individual exposures, but by controlling the building or location in order to ensure the overall protection of its occupants.

In cases where radon exposure is concomitant with exposure in a planned exposure situation (e.g. radon exposure in a nuclear facility or in a hospital radiology department), the Commission recommends a pragmatic approach. Radon exposures of workers should only be part of their overall occupational exposure if this is necessary within the specific graded approach for workplaces, as described in Section 3.3.5.

The Commission’s approach to the management of radon exposure is also directly related to the type of location. Publication 65 (ICRP, 1993) makes a distinction between the approach to protection in dwellings and the approach to protection in workplaces. Considering that a given individual typically moves from place to place, in dwellings, workplaces, and mixed-use buildings, in the same area, the Commission now recommends an integrated and graded approach to protection in all buildings using the requirements for public exposure. In addition, the Commission considers that it is appropriate to apply its occupational exposure requirements in some workplaces that are identified on the basis of either the reference level, as a quantitative criterion, or a list of activities or facilities, as qualitative criteria (see Section 3.3.5).

Due to this new approach, the Commission no longer uses the term ‘entry point’, introduced in Publication 103 (ICRP, 2007, Para. 298), to describe the concentration above which occupational protection requirements apply to radon exposure in workplaces.

In Publication 103 (ICRP, 2007), the Commission abandoned the concept of an action level, and instead introduced the concept of a reference level in conjunction with the optimisation principle. The reference level represents, in emergency and existing controllable exposure situations, the level below which the goal is to reduce all doses to a level that is as low as reasonably achievable, taking economic and societal factors into account. The reference level is also the level above which it is judged to be inappropriate to allow exposures to occur.

The consequence of using the concept of reference level instead of the concept of action level is that optimisation should be applied as appropriate above and below the reference level, and not only above. It must be kept in mind that reference levels do not represent a demarcation between ‘safe’ and ‘dangerous’, or reflect a qualitative change in the associated health risks for individuals.

According to Publication 103 (ICRP, 2007), the chosen value for a reference level depends upon the prevailing circumstances of the exposure situation under consideration (ICRP, 2007, Para. 234). In order to provide guidance for selecting appropriate values, the Commission has defined a dose scale (ICRP, 2007, Table 5) reflecting the fact that, within a continuum of risk (linear non-threshold assumption), the risk that someone is willing to accept depends on the exposure circumstances. This scale is divided into three bands reflecting greater or lesser need for action, which is dependent upon the characteristics of the exposure situation: controllability of the source; individual or societal benefit from the situation; and requirements with regard to information, training, and dosimetric or medical surveillance. Numerically speaking, the three bands for acute or annual doses are: <1 mSv, 1–20 mSv, and 20–100 mSv.

According to Publication 103 (ICRP, 2007), reference levels for existing exposure situations should typically be set in the 1–20-mSv band. This applies when individuals receive direct benefits from the exposure situation, and when exposures may be controlled at source or, alternatively, by action in the exposure pathways, so that general information should be, where possible, made available to enable individuals to reduce their doses. Radon exposure cannot normally be controlled at the source (apart from a few exceptions), but can be controlled through many pathways by preventive and mitigating actions, which are not disproportionately disruptive. People generally receive an obvious direct benefit from being indoors. Thus, there is a benefit from continuing to use or live in a building rather than moving to another building or even another area.

In Publication 103 (ICRP, 2007), the Commission retained the upper value of 10 mSv of effective dose, adopted in Publication 65 (ICRP, 1993), for the individual reference level. This value, which is in the middle of the 1–20-mSv band, is consistent with the rationale provided in Publication 103 (ICRP, 2007, Table 5). For the sake of continuity and practicality, the Commission continues to recommend a reference level of the order of 10 mSv year⁻¹. The Commission also recommends the use of a derived reference level given in terms of an indoor radon concentration (in Bq m⁻³), which is a measurable quantity.

In its Handbook on Indoor Radon, WHO stated that a national reference level for dwellings of 100 Bq m⁻³ is justified from a public health perspective, but recognised that this level cannot be implemented in many countries (WHO, 2009). The value of 300 Bq m⁻³ is already introduced in standards such as the International Basic Safety Standards (IAEA, FAO, ILO, OECD/NEA, PAHO, UNEP, WHO, 2011) and the European Basic Safety Standards (EURATOM, 2014) that were revised recently. As stated in Publication 103 (ICRP, 2007, Para. 295), it is the responsibility of the appropriate national authorities, as with any other controllable radiation source, to establish their own national derived reference levels, taking the prevailing economic and societal circumstances into account, and to apply the process of optimisation of protection in their country. The Commission strongly encourages these authorities to set a national derived reference level that is as low as reasonably achievable in the range of 100–300 Bq m⁻³. It is important to note that derived reference levels relate to the annual mean concentration of radon in a building or location. Moreover, although the absolute risk of lung cancer arising from radon exposure is significantly greater in smokers than non-smokers, the Commission’s recommendations for protection against radon do not distinguish between smokers and non-smokers.

As individuals move from place to place in the same area, a radon protection strategy should be developed by national authorities, and implemented in a consistent and integrated manner in the various locations. As a consequence, the Commission recommends, a priori, use of the same derived reference level in dwellings and in mixed-use buildings (e.g. schools, hospitals, shops, cinemas) that have access for both members of the public and workers, and, by extension, in workplaces without public access when radon exposure is not considered as occupational (e.g. office buildings, typical workshops).

In relation to optimisation, the Commission considers that distinction should be made between prevention, which aims to maintain the expected exposure at a level that is as low as reasonably achievable under the prevailing circumstances, and mitigation, which aims to reduce the residual dose to a level that is as low as reasonably achievable (see Fig. 3.1). For radon exposure, prevention focuses on the implementation of building codes in order to avoid high indoor radon concentrations in new buildings, while mitigation focuses on the reduction of high radon concentrations in existing buildings through the use of techniques such as managed ventilation.

The objective of the optimisation process is to reduce the overall risk to the general population and, for the sake of equity, the individual risk to the most exposed individuals, i.e. those in the upper-end of the distribution of individual exposures (see Fig. 3.2). In both cases, the process includes the management of buildings or location, and should result in radon concentrations in ambient indoor air that are as low as reasonably achievable below the national derived reference level. In a given building, after successful implementation of management actions, the only ongoing requirements would be to perform routine inspection and maintenance of the mitigation system, and periodic monitoring to ensure that radon levels remain low.

The aim to significantly reduce the risks of radon to the general population has a time frame of several decades rather than several years.

Optimisation of protection from radon exposures in buildings and locations can be determined using standard optimisation techniques, including cost–benefit analysis and multi-attribute techniques. All relevant attributes, such as technical, economic, societal, and ethical, must be taken into account, with due consideration to the issue of equity, particularly for very high ongoing exposures. Thus, comparisons can be made between the financial costs associated with the estimated number of cases of lung cancer that are likely attributable to radon at different levels of exposure, the selection of preventive and protective actions for a given population, and the costs of those actions to reduce radon exposures (HPA, 2009; WHO, 2009). Such analyses can be used to inform decisions on the cost-effectiveness of measures to reduce radon levels in existing properties and new buildings.

The first step is to characterise the exposure situation of individuals and the general population in the country, as well as other relevant economic and societal criteria, and the practicability of mitigating or preventing the exposure. The appropriate value for the derived reference level may then be established by a process of generic optimisation that considers national or regional attributes and preferences, together, where appropriate, with consideration of international guidance and good practice elsewhere. Many factors such as mean radon concentration and radon distribution, number of existing homes with high radon levels, etc. should be taken into consideration.

When a national derived reference level has been established, preventive and mitigating actions should be undertaken to help achieve a substantial reduction in radon exposures, as required. It is not sufficient to adopt marginal improvements with the aim of reducing radon concentrations to a value just below the national derived reference level. The national derived reference level should also be applied at the design stage of any new building, whatever its purpose.

The value of the national derived reference level for radon exposure should be reviewed periodically to ensure that it remains appropriate.

The radon protection strategy should include a programme of actions including provision of general information on radon behaviour and risk, campaigns aiming to increase awareness among the targeted public, campaigns of concentration measurements, and organisation of technical or financial support for measurements and remediation actions (see Section 4). These actions may be implemented preferentially in certain areas, such as radon-prone areas, and in buildings that are heavily occupied. This might include buildings that are occupied by many people, or those that have occupants who are present, often residentially, for a high proportion of the time.

In situations involving legal responsibilities (e.g. employer/employee, landlord/tenant, builder/buyer, seller/buyer, public building with high occupancy), some mandatory provisions may be required. Mandatory provisions, commensurate with the degree and type of responsibility, may be more appropriate than incentive-based provisions if an assessment indicates that they would be more effective under the prevailing circumstances. Such provisions could be to ensure good traceability, record keeping, and compliance with the derived reference level.

A radon protection strategy should ensure that requirements are commensurate with the means available to the responsible persons or organisations, and that the benefit in terms of risk reduction outweighs the disadvantages. For example, requirements should not deter people from initial radon measurements, result in reduced property value, or involve excessive procedures. Where a building has high radon concentrations, the response should include the involvement of, and communication with, relevant stakeholders, such as the building users. In cases of non-compliance with the reference level, the consequences should also be adapted to the situation. For example, the consequences for those responsible for the dwellings could be to provide the result of the measurement (e.g. to an authority or the buyer) or the obligation to undertake remediation.

In most workplaces, radon exposures of workers are adventitious and are more related to the location than the work activity. Such exposures are not considered as occupational, as defined by the Commission. These considerations are without prejudice to the legal responsibility of the employer towards its employees. Workplaces in this category include most mixed-use buildings, such as schools, hospitals, post offices, jails, shops, cinemas, office buildings, and general workshops.

In all workplaces where radon exposure is regarded as being adventitious and not considered as occupational, the first step of the graded approach consists of managing the working location using the national derived reference level (300 Bq m⁻³ or less) and implementing the optimisation process. The legal responsibility of the employer towards its employees is exercised by applying the regulatory requirements, consensus standards, or other standards that may be laid down for the control of radon exposure in buildings as part of implementation of the national radon strategy.

The relationship between a measured radon concentration and effective dose depends upon several parameters including the equilibrium factor, the time of exposure, etc. that can vary greatly between locations. Therefore, if the derived reference level is exceeded in a workplace, this does not necessarily mean that the dose reference level, representing annual doses of approximately 10 mSv, would be exceeded.

Consequently, in workplaces, if difficulties are encountered in keeping indoor radon concentrations below the derived reference level, the radon protection strategy should provide, as a second step of the graded approach, the possibility to undertake further investigation using a more realistic approach. This means making an assessment of radon exposure, taking the actual parameters of the exposure situation into account (e.g. actual time of occupancy, measurements of radon progeny). Doses calculated in this way should be compared with the dose reference level of 10 mSv year⁻¹ to decide on the need for, and type of, further action. At this stage, the aim is still to ensure the overall protection of the users of the building, rather than to control doses to specific individuals.

In workplaces where, despite all reasonable efforts to reduce radon exposure, individual doses persist above 10 mSv year⁻¹, the workers should be considered as occupationally exposed and their exposure should be managed using the relevant radiological protection requirements established for occupational exposure: identification of the exposed workers, information, training, dose monitoring (in doses or potential alpha energy concentration) and recording, and health surveillance. In any case, the individual doses should not exceed the upper value of the 1–20-mSv band for the setting of reference levels in existing exposure situations. This is the third step of the graded approach.

In some specific types of workplaces, national authorities may decide that exposure of workers to radon should be considered as occupational regardless of being above or below a reference level, because the workers are inevitably and substantially exposed to radon, and their exposure to radon is more intimately or obviously related to their work activities. A list of such workplaces or work activities (e.g. mines and other underground workplaces, thermal spas) should be established nationally.

In workplaces where radon exposure of workers is considered to be occupational, the Commission recommends that the relevant locations should be identified. This might be parts of a building, individual buildings, or parts of a site. Within these locations, the employer should meet the relevant occupational exposure requirements and apply the optimisation principle. If the national authorities decide that a radon exposure situation should be regarded as a planned exposure situation, the dose restriction is likely to be ensured through an occupational dose limit (see Section 3.4).

In the view of the Commission, the decision regarding whether or not exposure of workers to radon is considered as occupational should be made by the national authorities.

Local radon maps and appropriate data should be made available for relevant local, regional, and national authorities; building professionals; home builders; and the general population to help them in planning, constructing, and renovating buildings.

Land planning might be mandatory, for which a radon map remains a useful tool, but it cannot be considered to give definitive results. It is not possible to predict the radon concentration in a given building before construction. Further investigation, such as measurements in soil, might be useful. However, as the radon concentration in a building is dependent on many factors, only a measurement in the finished and occupied building is able to provide a relevant result. A radon map should not result in areas where buildings are forbidden because of radon concentrations, except in very exceptional cases.

Ensuring compliance with building regulations and codes is important. Quality assurance programmes should be implemented at the level of professionals or at a regulatory level, as appropriate. It is important to note that these building regulations and codes alone cannot guarantee that radon levels in new buildings will be below the derived reference level. Therefore, building owners and managers should be made aware that the only way of knowing the radon situation of the building is through measurement. If necessary, postconstruction radon mitigation approaches should be considered.

Ideally, long-term measurements over a whole year, and therefore covering all seasons, are preferred over short-term estimates. However, shorter periods of a few weeks to months are often chosen as difficulties may arise when the period is too long, as detectors tend to be moved or forgotten. Reliable measurements should be representative of the average annual concentration, and seasonal adjustments may be used. The measurements should be accomplished at low to modest costs. Measurement devices should be easily available with clear instructions about their use. After mitigation, a measurement is needed in the same conditions as the initial measurement to test the effectiveness of the mitigation system. Measurements should be repeated periodically to ensure that the situation does not deteriorate.

When using radon-222 progeny measurements in dwellings and general workplaces, conversion to the radon concentration is implemented by assuming a default generic equilibrium factor of 0.4 between indoor radon gas and its progeny, unless evidence shows otherwise.

The primary radon mitigation techniques aim to reduce convection and diffusion radon intake from the soil under the building, and focus on the following actions:

re-inforce the air tightness of the shell of the building (e.g. sealing radon entry routes); and

reverse the air pressure differences between the indoor occupied space and the outdoor soil through different soil depressurisation techniques (e.g. reducing the pressure in the soil beneath the building, installing a radon sump system, applying an overpressure in the cellar, etc.).

Reduction of the indoor radon concentration by dilution with outside air is another mitigation technique used in dwellings. Mitigation is achievable by passive or active means that manage the ventilation of occupied spaces. In heating or cooling an indoor environment, balanced ventilation may be used. Balanced exhaust ventilation neither pressurises nor depressurises the indoor air condition in relation to the pressure of air in soil and outdoors. This ventilation technique dilutes radon after it has entered the building. Fan-powered ventilation can dilute indoor radon after it enters the building, and can reduce pressure differences between the soil and the occupied space. Some of these solutions are not suitable for all types of building, nor are they suitable for all levels of radon. In several cases, a combination of these techniques provides the greatest reduction in radon concentration.

For buildings where an artesian borehole serves as the water supply source, this water may be a potential source of radon. When water degasses radon into the room atmosphere (especially during water spraying), significant short-term exposures may occur. Techniques for mitigating the entry of radon into ambient air from water principally involve degassing the water prior to its use, or water filtration on beds of active charcoal.

Detailed guides presenting the different mitigation techniques, developed by national or international bodies, are available (WHO, 2009).