Radon and your health

Radon is an odorless, invisible, radioactive gas produced from the decay of naturally occurring uranium in the ground. Radon can seep into homes and buildings through small cracks or holes in the subsurface, and its gaseous form can have significant negative health effects.

The geology of Switzerland means that the risk for radon pollution is relatively high throughout the country, and especially in the more mountainous regions (see map below) - Radon ought therefore to be a major concern for Swiss homeowners, homebuyers, landlords, and building managers. It is estimated that Radon is the second leading cause of lung cancer in Switzerland, second only to smoking, and responsible for 200-300 deaths a year^[1].

The World Health Organization (WHO) recommends that its member countries have a national annual average residential radon concentration reference level of 100 Bq/m³ (Becquerel per cubic metre). According to WHO, the risk of lung cancer increases by about 16% per 100 Bq/m³ increase. ^[3] The Swiss reference level is three times higher than the WHO recommendation at 300 bq/m³. The risk map (above) and the risk analysis is based on the relatively high Swiss 300 bq/m³ reference level. The maximal indoor radon concentration set down in Swiss law is 1,000 bq/m³. ^[4]

Our forefathers appear to have also worried about radon, as tradition holds that builders once observed ants when choosing where to build. According to these accounts, anthills were moved to a planned building site, and locations where ants remained were avoided. Modern research offers a possible explanation for this practice, as European red wood ants are known to prefer nesting along geological fault zones, where soil gases such as radon are more likely to rise to the surface. ^[5]

Geoda’s radon risk evaluation is built in layers. First, we start with the radon risk probability from the Federal Office of Public Health (FOPH) radon map—i.e., the probability (in %) that buildings in the area exceed the Swiss reference level of 300 Bq/m³.

Second, we adjust that baseline slightly using the building’s characteristics shown in the report: building age and building type. In practice, buildings built before 1980 receive a small increase in risk, while newer buildings (2000+) receive a small decrease; multifamily buildings receive a small decrease. The construction-year adjustment reflects that building practices and requirements have gradually improved: SIA 180 (first published in 1989 and revised multiple times, e.g. 1999, 2006, 2014) increasingly incorporated guidance on sealing the building envelope against soil gases (including radon), and in 2018 Switzerland updated the Radiological Protection Ordinance, lowering the reference level from 1,000 Bq/m³ to 300 Bq/m³, in line with newer international recommendations (e.g., from WHO).^[6]

Thirdly, we include the building type. Multifamily buildings receive a small downward adjustment because radon risk is most closely tied to rooms with floors in direct contact with the ground (typically basements and ground-floor rooms in buildings with slab-on-grade construction). In apartment buildings, a smaller share of dwellings are directly exposed to ground-contact pathways, so the expected indoor risk for residents is typically lower—this is why Geoda includes building type (including whether the property is a multi-storey/multifamily building) as part of the evaluation.

The adjusted score is then grouped into a simple green / yellow / red indicator (as in the example report, where a 2% baseline probability results in a green indicator, and the building’s type and construction year are explicitly treated as key factors).

This methodology is approximate and does not replace on-site radon measurements. It is designed to be broadly consistent with the logic behind the official FOPH radon risk evaluation.^[7]