1. Introduction
Radon (
222Rn) is a colorless and odorless radioactive gas that has been strongly linked to deleterious human health outcomes, specifically lung cancer [
1,
2,
3,
4]. It is the most important source of ionizing radiation among those that are of natural origin, as it constitutes the second cause of death by lung cancer after tobacco [
5]. While the outdoor radon rarely reaches epidemiologically significant levels due to atmospheric dispersal and dilution, in enclosed environments such as residences, the level of radon can accumulate at levels as much as two orders of magnitude higher than outdoors in inadequately ventilated spaces [
6]. Radon mainly infiltrates indoors from the soil adjacent to the building foundation and construction materials [
7,
8]. The World Health Organization (WHO) [
5] recommends maintaining the level of indoor radon at an annual average concentration limit lower than 100 Bq/m
3 in order to avoid the increase in prevalence of lung cancer [
9,
10,
11,
12]. In regions where the natural emission is too high to reach this target, a value of 300 Bq/m
3 should not be exceeded [
5]. The US Environmental Protection Agency (EPA) also recommends remediation actions for radon concentrations higher than 4 pCi/L (equivalent to 148 Bq/m
3) [
13]. The EU Council as well as the Swiss Federal Office of Public Health (FOPH) adopted the reference value of 300 Bq/m
3 [
14,
15]. Identifying the causes of residential radon accumulation at levels higher than these limits is therefore of high priority, as to develop effective interventions for radon level control.
Extensive international indoor radon investigations contributed to the worldwide indoor radon map [
5,
16,
17,
18,
19,
20,
21,
22,
23], and revealed strong associations between indoor radon concentration and outdoor radon emissions as well as dwelling characteristics. Demoury et al. [
24] evaluated the statistically significant association between indoor radon and geogenic radon potential, building materials and age, and foundation type in French residences. Collignan et al. [
25] reported that dwellings in radon-prone parts of France, which are equipped with mechanical ventilation systems, had significantly lower radon concentrations than naturally ventilated ones. They also found that the construction materials were the most influencing factors, followed by the type of foundation. A similar relationship between indoor radon concentrations and aforementioned dwelling characteristics was observed in Denmark [
26], England [
27], Germany [
28], Italy [
29], and in Switzerland [
30,
31].
Radon in energy-efficient buildings is another area of increased public interest and concern [
32,
33]. The requirement for airtightness in energy-efficient buildings can lead to extremely low air infiltration, which can lead to build-up of radon concentrations if not sufficiently diluted by intentional ventilation. Thermal retrofitting, an effective approach to achieve energy-efficient dwellings via reducing air infiltration and increasing thermal insulation of building envelop, has been associated with elevated indoor radon concentrations. A recent study by Meyer [
34] reported two times higher radon concentrations in retrofitted houses than in passive homes in Germany. A significant increase in indoor radon concentrations owing to energy retrofits was also observed in dwellings in the USA [
35] and Lithuania [
36]. In the case study by Jiránek and Kačmaříková [
37], addition of an exterior thermal insulation in homes and retrofitting windows led to 3.4 times higher radon concentration. Based on the UK national radon database, Symonds et al. [
38] found a significant increase of indoor radon levels in houses with double glazed windows, attic and wall insulation. In summary, radon alteration caused by thermal retrofitting could be a critical issue in energy-efficient dwellings.
Switzerland introduced the Energy Strategy 2050 policy to reduce energy-related environmental impact [
39]. Key efforts include construction of energy-efficient buildings and nation-scale building energy renovation program (Programme Bâtiment) [
40]. A building certification scheme, named Minergie, was also established to attest the high-energy efficiency of dwellings and occupants’ comfort [
41]. However, since Switzerland is predominantly situated in radon-prone area of Europe, energy-efficient measures of dwellings could lead to a build-up of indoor radon because of suspected lack of ventilation. Though a national indoor radon level database has been launched in Switzerland [
42] which provides an informative Swiss radon map, limited emphasis is put on understanding the radon levels in energy-efficient dwellings. Pampuri et al. [
43] found a significant increase in radon concentration after thermal retrofitting based on radon survey in 154 dwellings in southern Switzerland. Nevertheless, the study was restricted to only one Swiss canton and it did not take into account Minergie-certified dwellings.
To bridge this knowledge gap, we conducted indoor radon investigation in 650 energy-efficient dwellings in western Switzerland from 2013 to 2015. The objectives of this study were (1) to determine the indoor radon levels in Swiss energy-efficient homes and to compare them between green-certified (Minergie) and energy-renovated dwellings; (2) to probe the associations between radon and dwelling characteristics; and (3) to investigate the influence of thermal retrofitting on indoor radon level. Passive samplers were applied for the radon measurements, and questionnaire surveys were used to collect information about dwelling characteristics and thermal retrofitting. The results of this study could be used to better understand the radon levels in energy-efficient dwellings and, potentially, to interpret the associated health risks. The study is also useful for improving the accuracy of exposure assessment of indoor radon, and for developing improved energy renovation strategies in terms of radon control.
4. Conclusions
This study investigated the radon level in 650 energy efficient dwellings in western Switzerland. We examined the influences of building characteristics and thermal retrofit in new (NM) and renovated (RM) green-certified Minergie dwellings and in energy-renovated noncertified dwellings (R). We observed 40% lower radon levels in Minergie-certified dwellings, but there was no statistically significant difference between renovated Minergie (RM) and energy-renovated (R) dwellings. Indoor radon concentration was higher in older houses, especially in those built with masonry or mixed structures, and natural ground floors. Dwellings situated in high radon risk regions were prone to elevated radon risks. Installation of mechanical ventilation and completely excavated basement contributed to reduced radon concentrations in the living spaces. Thermal retrofitting of windows, roofs, floors, and external walls increased indoor radon concentrations, likely owing to reduced air exchange through air leakage.
Our results indicate that energy renovation measures without attention to indoor environment can adversely influence the level of indoor radon. Alongside the aggressive energy efficiency initiatives in Swiss buildings, these efforts should be accompanied with measures to minimize radon infiltration indoors and to secure adequate ventilation. Radon prevention constructions should take place in specific conditions, particularly for dwellings located in radon-prone areas like Switzerland. Alongside minimizing radon penetration from the ground, the ventilation design should take into account provision of a sufficient amount of outdoor air to dilute indoor radon either by mechanical means or by controlled natural ventilation. Occupants should be informed of the importance of indoor radon control, including renovating their ground floors and ventilating more often, especially in winter seasons. The recommendations should become part of the Swiss building renovation strategies and green-certification programs.