The Radon Working Group, a part of the Global Open Air Quality Standards initiative, convened to discuss and establish recommendations for indoor radon levels. This session built upon previous discussions focusing on other indoor air pollutants such as particulate matter, carbon dioxide, carbon monoxide, formaldehyde, and nitrogen dioxide. The objective of this working group is to develop globally applicable standards for radon.

Review of Existing International Radon Standards

An overview of existing international radon standards and guidelines revealed significant variations worldwide. Many standards lack specific details regarding ceiling limits or whether their values are based on averages. The minimum value identified across various standards was 100 Becquerel per cubic meter (Bq/m³) in most cases, with a maximum reaching up to 400 Bq/m³.

Debate on the Proposed Radon Limit

Based on the observed range and in accordance with World Health Organization (WHO) guidelines, the working group initially recommended adopting a radon limit of 100 Bq/m³. There was general agreement among the participants and no explicit disagreements with the 100 Bq/m³ proposal were voiced during the session.

Consensus on the Absence of a Safe Radon Level

The working group addressed the question of whether a safe level of radon exists, and all participants agreed that there is no safe limit.

Reasons for Varying National Recommendations

The discussion touched upon the reasons behind the wide range of radon recommendations and action levels across different countries. Example, such as in Greece, highlighted that sometimes responsible bodies might adopt the minimum permissible limit set by international entities without necessarily reflecting the actual risk. In the case of Greece, they immediately adopted the European Union’s minimum limit of 300 Bq/m³ by 2018.

In contrast, some northern European countries and Canada, recognizing the significant risk posed by radon, tend to have more stringent approaches. The influence of local constraints and specific radon-prone areas was also discussed, using Portugal as an example where previous legislation mandated radon testing only in certain regions with granitic soils. Portugal has since developed a national risk map, indicating that radon-prone areas do not necessarily align with administrative regions. The implementation of stricter limits in such areas can be challenging, leading to potential “margins of tolerance” which are not well-received by professionals in the field.

The timing of implementing the EU directive on radon limits was also noted as an issue in Portugal, with delays in producing the national risk map.

Importance and Duration of Radon Testing

The duration of radon testing was a key point of discussion. In many cases the thresholds are based on an annual average. The necessity of distinguishing between screening phases and long-term monitoring was highlighted, particularly in light of the EU directive mandating radon screening in every building. If initial screening levels exceed the threshold, a long-term monitoring campaign is required.

The standard long-term monitoring period of 90 days is predominant for ensuring reliable radon measurements. The reasoning behind this 90-day period in Canada is related to the average life expectancy, making it a seemingly short-term test in comparison. However, some studies suggest that shorter monitoring periods, such as one week, might also be representative of long-term concentrations, though further validation is needed. The use of both passive samplers (reference methods) and continuous active real-time monitors was also discussed, with the latter offering the potential for continuous monitoring and integration with other indoor air quality parameters.

The Role of Ventilation in Radon Remediation

The effectiveness of general ventilation systems (HVAC) in remediating high radon levels was questioned. Experience with continuous monitoring devices indicates that ventilation alone might not be sufficient to manage significant radon ingress into buildings, even those with balanced ventilation systems. In such cases, mitigation strategies like sub-slab depressurization might still be necessary. While heat recovery ventilators (HRVs) can generally reduce radon levels (potentially by around half, according to some research), they are not a complete solution and their effectiveness can vary.

Issues with the US Standard and Potential Resistance to Change

The United States has an established radon action level of 150 Becquerel per cubic meter (4 picocuries per liter), around which an entire industry for mitigation has developed. Due to the substantial investment and infrastructure built around this level, significant changes to the US standard are considered unlikely. The building practices in the US, such as using wood in construction and implementing ventilation systems specifically for radon remediation, differ significantly from other regions like Europe. Despite the WHO recommending a lower level, the existing regulatory framework and the volume of existing literature referencing the 150 Bq/m³ level pose considerable barriers to adopting a stricter standard in the US. The shorter 48-hour testing period common in US real estate transactions was also noted as potentially less reliable compared to the longer durations used in other countries like Canada.

Comparison of Measurement Units

The standard unit of measurement used by the WHO and the International Standard Organization is Becquerel per cubic meter (Bq/m³). However, the United States and some other smaller countries use picocuries per liter (pCi/L). While the conversion between these units is straightforward, the difference in numerical values can lead to public misinterpretations. Becquerel values tend to be lower, which some believe is better for conveying the severity of higher readings to the public. Despite the preference for becquerels among academics and internationally, the working group acknowledged the need to potentially use both units to effectively communicate with stakeholders in countries like the US.

Preliminary Conclusions

In summary, the Radon Working Group’s preliminary conclusions include:

A proposed radon limit of 100 Bq/m³, aligning with WHO recommendations. This proposal will be discussed with contacts in the US to understand potential challenges given their existing 150 Bq/m³ standard.

Acknowledgement of the absence of a safe level of radon.

The primary unit of communication will be Becquerel per cubic meter (Bq/m³), with the possibility of including picocuries per liter for clarity with certain regions like the US.

The importance of considering both long-term (around 90 days) and short-term radon testing methodologies and the emergence of continuous monitoring technologies.

Recognition that ventilation alone may not be sufficient to mitigate high radon levels, often requiring specific remediation techniques.

The working group aims to finalize their recommendations and update their white paper to contribute to the development of global indoor air quality standards.

• Recording: Radon WG – 2025 April 03