Radon Transport, Accumulation Patterns, and Mitigation Techniques Applied to Closed Spaces

Citation: Sicilia, I.; Aparicio, S.; González, M.; Anaya, J.J.; Frutos, B. Radon Transport, Accumulation Patterns, and Mitigation Techniques Applied to Closed Spaces. Atmosphere 2022,13, 1692. https://doi.org/ 10.3390/atmos13101692 Academic Editors: Federica Leonardi, Giorgia Cinelli and Daniel Rabago Received: 25 August 2022 Accepted: 10 October 2022 Published: 16 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). atmosphere Article Radon Transport, Accumulation Patterns, and Mitigation Techniques Applied to Closed Spaces Isabel Sicilia 1,2,* , Sofía Aparicio 3, Margarita González 3, JoséJavier Anaya 3and Borja Frutos 1 1Department of Construction, Eduardo Torroja Institute for Construction Science, Spanish National Research Council, 28033 Madrid, Spain 2Faculty of Sciences, University of Cantabria, 39005 Santander, Spain 3Department of Acoustics and Non-Destructive Evaluation, Institute for Physical and Information Technologies “Leonardo Torres Quevedo”, Spanish National Research Council, 28006 Madrid, Spain *Correspondence: [email protected]; Tel.: +34-913020440 Abstract: In this study, different techniques for the mitigation of radon gas in indoor spaces were investigated. For this purpose, two different scenarios of a public building were analyzed: two symmetrical facility galleries and a reverberation chamber. Although most workplaces in this building have low radon levels, the complex structure houses spaces have very high radon concentrations. The study also included the surrounding areas of these spaces. The radon concentration and differential pressures were measured, and different mitigation techniques were applied: sealing, balanced ventilation, pressurization with the introduction of fresh air, and depressurization over each space. The pressurization solution was proven to be the most effective way to reduce radon concentration in both scenarios. The introduction of fresh air diluted the radon concentration, and the slight increase in the pressure reduced the entry of gas by the advection mechanism. On the other hand, the depressurization technique was the least effective mitigation technique, as it generated a negative pressure gradient that facilitated a higher radon flux from the source. Therefore, before applying any mitigation technique, it is necessary not only to study the space to be remediated but also the possible impact on neighboring spaces. Keywords: radon; differential pressure; mitigation; indoor spaces; singular building 1. Introduction Radon ( 222 Rn) is a naturally occurring radioactive gas. Radon decays into radioactive elements called radon progeny. These elements can be inhaled into the lungs, which is the main cause of lung cancer after smoking [1] and the first in non-smokers [2]. Due to the high presence of radon in various soils, the gas seeps into indoor spaces [3,4] . According to [ 5 ], there are two basic radon entry mechanisms: diffusion due to the radon concentration gradient in the environment and advection caused by the pressure difference between the building envelope and the outside atmosphere [ 6 , 7 ]. Radon is approximately 7.5 times heavier than air. It concentrates in basements or ground floors and can be transmitted to upper levels by transport processes and human activity [ 8 ]. The concentration of radon indoors can vary according to the climate, the season, and the presence of water in soil [ 9 – 11 ], and it is important to carry out a study over time in order to cover different situations. Common radon mitigation methods include sealing and radon membranes, the ventilation of indoor spaces, and sump depressurization systems or underfloor ventilation [ 12 ]. These have been widely studied in houses and small spaces [ 13 , 14 ]. Furthermore, the monitoring results obtained from real models with controlled conditions can be extrapolated to real buildings. Radon mitigation techniques must be customized for each particular building, becoming more complicated when applied to singular buildings, such as old buildings, where Atmosphere 2022,13, 1692. https://doi.org/10.3390/atmos13101692 https://www.mdpi.com/journal/atmosphere Atmosphere 2022,13, 1692 2 of 18 plans and/or technical information are not available. Buildings with poor ventilation that remain closed after working time used to have higher levels than residential places [ 15 ]. Better construction and improved ventilation methods reduced radon levels in workplaces [ 16 ]. At the same time, the direct application of radon mitigation systems can be difficult due to their large dimensions. According to a pilot study performed in Spain [ 17 ], around 23.5% of public administration buildings studied had radon levels greater than 300 Bq/m 3 . These levels depended mainly on the materials and construction techniques and the state of preservation of the building [ 18 ]. The presence of cracks and damage in the building envelope increases the exchange of radon between the spaces. Another case is the study of radon levels in underground spaces. Indoor air quality has been studied in settings such as tunnels [ 19 ], caves [ 20 , 21 ], and underground galleries and shelters [ 22 ]. In these cases, the main source of radon was the soil and underground walls. A lack of ventilation leads to higher radon concentrations. At the same time, old buildings lack effective isolation from the ground. Although the radon diffusion coefficient of bricks is about one-tenth of the radon diffusion coefficient of soils [ 23 ], traditional masonry walls exhibit small cracks and construction flaws, which can increase radon entries [24]. The measurement and control of air quality today have growing relevance for public and heritage buildings, even if the application of remediation techniques is not always easy [ 24 – 27 ]. In this study, we analyzed the radon levels and accumulation patterns in different places of a public research center, the Institute for Physical and Information Technologies “Leonardo Torres Quevedo” in Madrid, Spain. The main objective was to analyze the effectiveness of different methods to reduce radon levels in selected closed spaces: two symmetrical underground galleries and a reverberation chamber situated aboveground, which is very airtight. Although most of the workplaces in this building have radon levels below 300 Bq/m 3 , the complex structure houses spaces with very high concentrations of radon. Additionally, the study included the surroundings of these spaces: an office and an underground corridor neighboring the two galleries and another office and spaces surrounding the reverberation chamber. The radon concentrations and differential pressures were measured while applying different mitigation techniques: sealing, ventilation, pressurization, and depressurization on each space. 2. Materials and Methods 2.1. Construction under Study In this work, the measurements were made in a historical building built in 1944, the Institute for Physical and Information Technologies Torres Quevedo (ITEFI). This building has always been a public research center and currently belongs to the Spanish National Research Council (CSIC). The building, located in Madrid, Spain, is situated on a plot of coarse-grained clayey sand and a brownish-reddish clayey soil [ 28 ]. The soil was classified as having a medium potential risk of radon, according to [29]. The building is made up of different wings with laboratories and offices. The spaces studied are in the main wing and the acoustic wing (Figure 1). In the main wing, the mitigation system acts on a gallery located in the semi-basement. The influence of the mitigation actuation in the symmetrical gallery, a corridor that provides access to both galleries, and an office on the ground floor, placed above one of the galleries, was studied. In the acoustic wing, the mitigation system acts on a reverberation chamber. The influence of the mitigation actuation was studied in an office near the chamber and the corridor that connects both spaces. The reference for the differential pressure measurements was placed in a neighboring shed. The characteristics of each space can be seen in Table 1. Atmosphere 2022,13, 1692 3 of 18 Atmosphere 2022, 13, x FOR PEER REVIEW 3 of 18 Figure 1. Placement of the studied spaces in the building. The courtyard garden level was taken as the reference height (+0.00) for all plans. Table 1. Dimensions and characteristics of the analyzed spaces. ？ Volume Outdoor Contact Indoor Contact Level (Courtyard Reference) m3 m 2 m 2 m MAIN WING Gallery A / 0.8 −0.42/−1.66 Gallery B / 0.8 −0.42/−1.66 Corridor 361 contact with ventilated spaces with several spaces −3.08 Office 62 2.9 window 0.87 ACOUSTIC WING Reverberation chamber 210 / 5.8 0 Office—acoustic 45.6 1.95 window 1.7 0 Corridor—acoustic 77.9 / with several spaces 0 Shed—acoustic 0 2.1.1. Main Wing: Galleries and Adjacent Spaces The galleries are in the main wing, behind the west façade. They are two symmetrical areas, with an axis of symmetry in the middle of the main entrance (Figure 2). Gallery A is in the north position, and Gallery B is in the south. Both spaces are T-shaped. The two galleries are physically independent. Figure 2. Plan of the basement with positions of spaces and sensors. Galleries A and B. The galleries are slightly below the street level. The corridor is around 1.30 m below street level (Figure 3). Both galleries were built as facilities and equipment spaces, each with a small door leading to the basement corridor. The enclosure of the galleries is solid Figure 1. Placement of the studied spaces in the building. The courtyard garden level was taken as the reference height (+0.00) for all plans. Table 1. Dimensions and characteristics of the analyzed spaces. ? Volume Outdoor Contact Indoor Contact Level (Courtyard Reference) m3m2m2m MAIN WING Gallery A / 0.8 −0.42/−1.66 Gallery B / 0.8 −0.42/−1.66 Corridor 361 contact with ventilated spaces with several spaces −3.08 Office 62 2.9 window 0.87 ACOUSTIC WING Reverberation chamber 210 / 5.8 0 Office—acoustic 45.6 1.95 window 1.7 0 Corridor—acoustic 77.9 / with several spaces 0 Shed—acoustic 0 2.1.1. Main Wing: Galleries and Adjacent Spaces The galleries are in the main wing, behind the west façade. They are two symmetrical areas, with an axis of symmetry in the middle of the main entrance (Figure 2). Gallery A is in the north position, and Gallery B is in the south. Both spaces are T-shaped. The two galleries are physically independent. Atmosphere 2022, 13, x FOR PEER REVIEW 3 of 18 Figure 1. Placement of the studied spaces in the building. The courtyard garden level was taken as the reference height (+0.00) for all plans. Table 1. Dimensions and characteristics of the analyzed spaces. ？ Volume Outdoor Contact Indoor Contact Level (Courtyard Reference) m3 m 2 m 2 m MAIN WING Gallery A / 0.8 −0.42/−1.66 Gallery B / 0.8 −0.42/−1.66 Corridor 361 contact with ventilated spaces with several spaces −3.08 Office 62 2.9 window 0.87 ACOUSTIC WING Reverberation chamber 210 / 5.8 0 Office—acoustic 45.6 1.95 window 1.7 0 Corridor—acoustic 77.9 / with several spaces 0 Shed—acoustic 0 2.1.1. Main Wing: Galleries and Adjacent Spaces The galleries are in the main wing, behind the west façade. They are two symmetrical areas, with an axis of symmetry in the middle of the main entrance (Figure 2). Gallery A is in the north position, and Gallery B is in the south. Both spaces are T-shaped. The two galleries are physically independent. Figure 2. Plan of the basement with positions of spaces and sensors. Galleries A and B. The galleries are slightly below the street level. The corridor is around 1.30 m below street level (Figure 3). Both galleries were built as facilities and equipment spaces, each with a small door leading to the basement corridor. The enclosure of the galleries is solid Figure 2. Plan of the basement with positions of spaces and sensors. Galleries A and B. Atmosphere 2022,13, 1692 4 of 18 The galleries are slightly below the street level. The corridor is around 1.30 m below street level (Figure 3). Both galleries were built as facilities and equipment spaces, each with a small door leading to the basement corridor. The enclosure of the galleries is solid brick (Figure 4). The unknown state of conservation of the galleries prevents entry for an in-depth assessment. Atmosphere 2022, 13, x FOR PEER REVIEW 4 of 18 brick (Figure 4). The unknown state of conservation of the galleries prevents entry for an in-depth assessment. Figure 3. Positions of spaces and sensors. (a) Office plan and section A-A′, with Gallery A and corridor. Section B-B′: Gallery A, passage section, and neighbor spaces. Radon sensors (R) are highlighted in red. Pressure sensors (P) are highlighted in blue. (a) (b) (c) Figure 4. (a) Gallery A, inner space observed from the entry door. (b) Gallery A, back. Inner space observed from Hole 1. (c) Gallery A, corridor, and door with the extraction system. The doors of both galleries and the holes placed in the first-floor office were completely sealed with a flexible radon-proof polyolefin sheet (with a diffusion coefficient of less than 1 × 10−11 m2/s). (a) (b) Figure 3. Positions of spaces and sensors. ( a ) Office plan and section A-A 0 , with Gallery A and corridor. ( b ) Section B-B 0 : Gallery A, passage section, and neighbor spaces. Radon sensors (R) are highlighted in red. Pressure sensors (P) are highlighted in blue. Atmosphere 2022, 13, x FOR PEER REVIEW 4 of 18 brick (Figure 4). The unknown state of conservation of the galleries prevents entry for an in-depth assessment. Figure 3. Positions of spaces and sensors. (a) Office plan and section A-A′, with Gallery A and corridor. Section B-B′: Gallery A, passage section, and neighbor spaces. Radon sensors (R) are highlighted in red. Pressure sensors (P) are highlighted in blue. (a) (b) (c) Figure 4. (a) Gallery A, inner space observed from the entry door. (b) Gallery A, back. Inner space observed from Hole 1. (c) Gallery A, corridor, and door with the extraction system. The doors of both galleries and the holes placed in the first-floor office were completely sealed with a flexible radon-proof polyolefin sheet (with a diffusion coefficient of less than 1 × 10−11 m2/s). Figure 4. ( a ) Gallery A, inner space observed from the entry door. ( b ) Gallery A, back. Inner space observed from Hole 1. (c) Gallery A, corridor, and door with the extraction system. The doors of both galleries and the holes placed in the first-floor office were completely sealed with a flexible radon-proof polyolefin sheet (with a diffusion coefficient of less than 1×10−11 m2/s). Atmosphere 2022,13, 1692 5 of 18 2.1.2. Acoustic Wing: Reverberation Chamber and Adjacent Spaces This area is a single story. The reverberation chamber is an isolated space with a rhomboid shape (Figure 5). The floor is built over a concrete slab. As an acoustic chamber, the space is completely isolated and highly airtight. Atmosphere 2022, 13, x FOR PEER REVIEW 5 of 18 2.1.2. Acoustic Wing: Reverberation Chamber and Adjacent Spaces This area is a single story. The reverberation chamber is an isolated space with a rhomboid shape (Figure 5). The floor is built over a concrete slab. As an acoustic chamber, the space is completely isolated and highly airtight. Figure 5. Positions of spaces and sensors. Acoustic wing and reverberation chamber section. Radon sensors (R) are highlighted in red. Pressure sensors (P) are highlighted in blue. The radon levels were checked in the reverberation chamber and the office. The differential pressure levels were checked in the reverberation chamber, the office, and the corridor that connects both spaces. 2.2. Sensors and Data Acquisition Systems In this section, the sensors and data acquisition systems used in the experiments are presented. The radon levels, differential pressure, temperature, and relative humidity were monitored in different areas. For this purpose, 3 different systems were used: a radon measuring system, a radon extraction system, and a differential pressure system. Some of these systems were developed by the authors. 2.2.1. Radon Measuring System Several commercial radon systems from the FTLab Company, based on the pulsed ionization chamber method, were used to measure the radon values at different places. These included two Radon Eye smart radon detectors connected to Wi-Fi and two Radon Eye detectors connected via Bluetooth with readings every hour. The Radon Eye + detectors also include a temperature and humidity sensor. 2.2.2. Radon Extraction System A wireless system was designed and developed by the authors to extract radon gas from the interior of buildings. This system is composed of 2 subsystems: the system in charge of monitoring the radon data (radon measuring system) and the system for activating or deactivating a fan to extract the gas (actuation system). An IoT protocol called MQTT is used to communicate between both systems. Radon measuring system The designed monitoring system (Figure 6) is based on a calibrated RD200M radon sensor from the FTLab Company connected to a Raspberry Pi 3 B+. The Raspberry is in charge of measuring the radon value every 10 min, saving it in a file, and publishing the value in a topic using MQTT. In case the radon value is greater than the threshold, the Raspberry also publishes an alarm in a different topic to be read by the fan system. Figure 5. Positions of spaces and sensors. Acoustic wing and reverberation chamber section. Radon sensors (R) are highlighted in red. Pressure sensors (P) are highlighted in blue. The radon levels were checked in the reverberation chamber and the office. The differential pressure levels were checked in the reverberation chamber, the office, and the corridor that connects both spaces. 2.2. Sensors and Data Acquisition Systems In this section, the sensors and data acquisition systems used in the experiments are presented. The radon levels, differential pressure, temperature, and relative humidity were monitored in different areas. For this purpose, 3 different systems were used: a radon measuring system, a radon extraction system, and a differential pressure system. Some of these systems were developed by the authors. 2.2.1. Radon Measuring System Several commercial radon systems from the FTLab Company, based on the pulsed ionization chamber method, were used to measure the radon values at different places. These included two Radon Eye smart radon detectors connected to Wi-Fi and two Radon Eye detectors connected via Bluetooth with readings every hour. The Radon Eye + detectors also include a temperature and humidity sensor. 2.2.2. Radon Extraction System A wireless system was designed and developed by the authors to extract radon gas from the interior of buildings. This system is composed of 2 subsystems: the system in charge of monitoring the radon data (radon measuring system) and the system for activating or deactivating a fan to extract the gas (actuation system). An IoT protocol called MQTT is used to communicate between both systems. Radon measuring system The designed monitoring system (Figure 6) is based on a calibrated RD200M radon sensor from the FTLab Company connected to a Raspberry Pi 3 B+. The Raspberry is in charge of measuring the radon value every 10 min, saving it in a file, and publishing the value in a topic using MQTT. In case the radon value is greater than the threshold, the Raspberry also publishes an alarm in a different topic to be read by the fan system. Atmosphere 2022,13, 1692 6 of 18 Atmosphere 2022, 13, x FOR PEER REVIEW 6 of 18 Figure 6. The radon monitoring system developed by the authors. Actuation system The actuation system is composed of an MKR1000 Arduino connected to a two directions centrifugal air extractor Siber AXC 150A (Figure 6). The Wi-Fi connection of the MKR1000 module is configured for the first time automatically by the Raspberry. After that, the system waits for an alarm to activate the fan. For this purpose, Arduino software is perpetually listening for the alarm using the MQTT protocol. The 70 W extractor has a maximum flow for ventilation of 195 m3/h. In pressurization and depressurization tests, the ideal flow tends to be 0. The extractor produces a theoretical maximum positive or negative pressurization of 343 Pa (zero flow). The air extractor was connected to a flexible air duct. The connection was completely sealed to prevent flow leakages. The exhaust fan forced indoor air to the outside for the depressurization tests and drew in fresh outside air for the pressurization and ventilation tests. The extraction system is connected to Gallery A through a hole made in the door. The ventilation test was carried out by opening the hole that connects the gallery and the office and by opening the office window. In the test carried out in the reverberation chamber, the door was replaced by a cardboard surface suitable for connecting the extraction system. This surface was completely closed and sealed. The pressurization and depressurization tests were carried out towards a single hole, while for the ventilation test, an additional hole was made to provide fresh air inside. 2.2.3. Differential pressure system A multi-sensor system (Figure 7) designed and developed by the authors to measure differential pressures in radon gas transport studies was also used in these experiments. This system is composed of 10 pressure sensors, as described in [30]. The data are collected every 10 min. (a) (b) Figure 7. PressureNet system developed by the authors [30]. (a) The multiplexer card and (b) the modules for pressure sensors. Figure 6. The radon monitoring system developed by the authors. Actuation system The actuation system is composed of an MKR1000 Arduino connected to a two directions centrifugal air extractor Siber AXC 150A (Figure 6). The Wi-Fi connection of the MKR1000 module is configured for the first time automatically by the Raspberry. After that, the system waits for an alarm to activate the fan. For this purpose, Arduino software is perpetually listening for the alarm using the MQTT protocol. The 70 W extractor has a maximum flow for ventilation of 195 m 3 /h. In pressurization and depressurization tests, the ideal flow tends to be 0. The extractor produces a theoretical maximum positive or negative pressurization of 343 Pa (zero flow). The air extractor was connected to a flexible air duct. The connection was completely sealed to prevent flow leakages. The exhaust fan forced indoor air to the outside for the depressurization tests and drew in fresh outside air for the pressurization and ventilation tests. The extraction system is connected to Gallery A through a hole made in the door. The ventilation test was carried out by opening the hole that connects the gallery and the office and by opening the office window. In the test carried out in the reverberation chamber, the door was replaced by a cardboard surface suitable for connecting the extraction system. This surface was completely closed and sealed. The pressurization and depressurization tests were carried out towards a single hole, while for the ventilation test, an additional hole was made to provide fresh air inside. 2.2.3. Differential Pressure system A multi-sensor system (Figure 7) designed and developed by the authors to measure differential pressures in radon gas transport studies was also used in these experiments. This system is composed of 10 pressure sensors, as described in [ 30 ]. The data are collected every 10 min. The position of each sensor is described in Table 2and Figures 3and 5. Atmosphere 2022,13, 1692 7 of 18 Atmosphere 2022, 13, x FOR PEER REVIEW 6 of 18 Figure 6. The radon monitoring system developed by the authors. Actuation system The actuation system is composed of an MKR1000 Arduino connected to a two directions centrifugal air extractor Siber AXC 150A (Figure 6). The Wi-Fi connection of the MKR1000 module is configured for the first time automatically by the Raspberry. After that, the system waits for an alarm to activate the fan. For this purpose, Arduino software is perpetually listening for the alarm using the MQTT protocol. The 70 W extractor has a maximum flow for ventilation of 195 m3/h. In pressurization and depressurization tests, the ideal flow tends to be 0. The extractor produces a theoretical maximum positive or negative pressurization of 343 Pa (zero flow). The air extractor was connected to a flexible air duct. The connection was completely sealed to prevent flow leakages. The exhaust fan forced indoor air to the outside for the depressurization tests and drew in fresh outside air for the pressurization and ventilation tests. The extraction system is connected to Gallery A through a hole made in the door. The ventilation test was carried out by opening the hole that connects the gallery and the office and by opening the office window. In the test carried out in the reverberation chamber, the door was replaced by a cardboard surface suitable for connecting the extraction system. This surface was completely closed and sealed. The pressurization and depressurization tests were carried out towards a single hole, while for the ventilation test, an additional hole was made to provide fresh air inside. 2.2.3. Differential pressure system A multi-sensor system (Figure 7) designed and developed by the authors to measure differential pressures in radon gas transport studies was also used in these experiments. This system is composed of 10 pressure sensors, as described in [30]. The data are collected every 10 min. (a) (b) Figure 7. PressureNet system developed by the authors [ 30 ]. ( a ) The multiplexer card and ( b ) the modules for pressure sensors. Table 2. Types of sensors and their placement. Measure Points Radon Pressure Observations MAIN WING Gallery A, door Radon + Raspberry (R1) 1 sensor (P1) Gallery A, back (Hole 1) Radon Eye RD200 + (R2) 2 sensors, at different heights: 1 and 0.5 m from above (P2, P3) Gallery B, door Radon Eye RD200 + (R3) Corridor Radon Eye RD200 (R4) 1 sensor (P4) Office Radon Eye RD200 (R5) 2 sensors (table, P5 + outside, P8) Referential pressure in floor Office (Hole 2) 1 sensor (P6) Office (Hole 3) 1 sensor (P7) ACOUSTIC WING Reverberation chamber Radon Eye RD200 (R6) 2 sensors (inner, P9 + door, P10) Office—acoustic Radon Eye RD200 (R7) 2 sensors (window, P11 + floor, P12) Corridor—acoustic 1 sensor (P13) Shed—acoustic Referential pressure in floor 3. Experiments Different experiments were performed in the galleries of the main wing and the reverberation chamber of the acoustic wing. 3.1. Tests in Galleries Tests in the galleries were carried out to determine the radon concentration levels and the influence of the remediation practice on adjoining living spaces. The holes created for the first radon inspection were sealed. The doors were reinforced with a radon-proof sheet. For this purpose, different tests were carried out to determine the influence of pressurization, depressurization, and ventilation actions on the air quality in Gallery A. During the test, the radon concentration and differential pressure data at different points were recorded (Figure 3and Table 2). Each experiment lasted for at least a week. 1. Pressurization test : The fan drove fresh air into Gallery A, increasing the indoor pressure. 2. Forced ventilation test: The fan extracted the air from Gallery A, forcing the ventilation of the space through the office window. 3. Depressurization test: The fan extracted exhaust air from Gallery A, reducing the internal pressure. 4. Natural one-sided ventilation: Gallery A was connected to the outside through a flexible duct that allowed for the exchange of naturally driven air with the outside. Atmosphere 2022,13, 1692 8 of 18 5. Natural state (reference state): The radon concentrations and pressure levels were measured with the gallery completely closed and sealed. The mitigation systems were disabled. This state is considered to be the reference state for all the experiments. 3.2. Tests in Reverberation Chamber Tests in the reverberation chamber and the nearest office were conducted to find the influence of natural ventilation on reducing radon levels. At the same time, the reverberation chamber was used to analyze the behavior of radon levels in a completely closed space. The indoor radon levels were outside the healthy limits, so the work focuses on establishing remedial actions to achieve a reduction in the radon concentration. The positions of the sensors can be seen in Figure 5. The experiments carried out in these areas included the following: •Initial data: The radon data were recorded in the office and reverberation chamber using two Radon Eye RD200 detectors. The differential pressure data were recorded with pressure sensor equipment. The reverberation chamber maintained normal work activity. •Steady test: The evolution of the radon levels in a completely closed space over a long period of time was determined to establish the maximum radon levels reached in the reverberation chamber. •Reverberation chamber remediation test : Different remediation tests were carried out to determine the healthy levels of radon concentration in the reverberation chamber. (0) The natural state before remedial actions was measured. The levels were recovered between each test to establish the initial radon levels of around 600 Bq/m 3 . (1) The pressurization test introduced fresh air into the acoustic chamber. (2) A forced ventilation test was carried out and repeated in (4) to evaluate a rare increase in indoor radon levels. (3) The depressurization test extracted the exhaust air from the interior. 4. Results and Discussion The two independent experiments (the galleries and the reverberation chamber) covered a whole range of radon mitigation systems applied to closed spaces. The first one was developed in the main wing. The galleries of this space were influenced by external factors due to the indeterminate permeability of the brick walls and the unknown connection to the other urban galleries. These factors give special and unique characteristics to each gallery. The second experiment was developed in the reverberation chamber. Being closed, this space is completely isolated from external influences. On the other hand, the isolation and absence of noise required to work in the reverberation chamber is a handicap for applying mitigation actions to this space. 4.1. Main Wing—Results of Facility Galleries and Nearest Living Places: Natural State The radon levels were tested in two symmetrical T-shaped facility galleries (A and B) and the living spaces surrounding one of the galleries—an office located above Gallery A and a corridor adjacent to both galleries. The two galleries reported high levels of radon. Although both galleries have a similar shape, building forms, and floor, Gallery B had significantly lower radon levels than Gallery A. A cyclical air flow was detected with a period of a few seconds. The source of this airflow could not be determined, but it established a fundamental difference between the radon concentrations in the two galleries. No airflow was detected in Gallery A. The monitoring of Gallery A was conducted at two different points: at the initial part behind the access door and a more distant point. For Gallery B, a single point was established behind the access door. The sensors and data recorded at each point are described in Sections 2.1 and 2.2. Figure 8represents the levels of radon (a) and pressure (b) in the natural state without any external action. The pressure reference for the differential pressure measurement was located on the office floor. Atmosphere 2022,13, 1692 9 of 18 Atmosphere 2022, 13, x FOR PEER REVIEW 9 of 18 (a) (b) Figure 8. (a) Levels of radon in main wing spaces. Natural state (reference state). (b) Pressure in main wing spaces. Natural state (reference state). Pressure reference on office floor. (a) Figure 8. ( a ) Levels of radon in main wing spaces. Natural state (reference state). ( b ) Pressure in main wing spaces. Natural state (reference state). Pressure reference on office floor. Figure 9a shows a large difference between the radon levels in Galleries A and B. This may have been due to the higher aeration detected in Gallery B. As can be seen in Figure 8a,b, the pressure and radon values developed a cyclical daily period. Both galleries had very similar pressure trends. The galleries were not isolated, and there is no information about the connections to other underground spaces or city galleries. Cyclical periods can be influenced by ventilation or connection to other underground spaces, heating in the building, or human activity. On the other hand, radon levels decrease when the outside pressure is lower than the inside pressure (outside—P8 pressure sensor). The pressure recorded outside shows an influence of the wind during those days. The office window was placed leeward, and the wind created suction. In opposition, the windward corridor was affected by positive pressurization. High wind rates affected the air in the galleries and indoor spaces, changing the radon levels. The pressure range of the interior spaces was around 4 Pa. This range increased to 25 Pa between the indoor and outdoor spaces. The pressure in the galleries tended to be lower than in the adjacent spaces. The fact that the pressure differential is very small indicates that a large entry of radon into the living spaces was prevented. Better slab insulation, higher air turnover, and the presence of this pressure gradient explain the lower radon levels in living spaces, as shown in Figure 9a. Atmosphere 2022,13, 1692 16 of 18 galleries, and another office and the spaces surrounding the reverberation chamber (a corridor and a shed). The radon concentrations and differential pressures were measured while applying different mitigation techniques: sealing, ventilation, pressurization, and depressurization in each space. Both scenarios—the galleries and the reverberation chamber—show quite similar results. The pressurization test was the best way to reduce the radon concentration. The introduction of fresh air and the increase in pressure diluted the radon concentration and prevented gas ingress. It was also observed that the pressurization of the spaces was not affected by the adjacent spaces. In the main wing, this may have been due to the isolation between the gallery and the upper office. In the acoustic wing, the reverberation chamber and the adjacent office were not attached, so it was not possible to determine the absence of influence. On the other hand, the application of the depressurization technique was the least effective mitigation technique. This may have been due to the displacement of radon from the soil to the suction point. Therefore, it can be concluded that these are spaces with unique characteristics in which the possible mitigation actions must be conducted in a conservative way within each, as it is not possible to apply correction measures for the insulation of the floors and walls. Although the galleries studied are non-habitable spaces, the results of the study can be applied in actions to mitigate radon gas in unique buildings. Such would be the case, for example, of buildings with historical heritage, in which crypts, caves, and other interior spaces are found in contact with the ground, with a strong limitation for the application of protection measures against radon gas. Although the application of overpressure inside is usually effective for the protection of closed spaces where it is not possible to act otherwise and prevents the entry of gas from the ground, the application of one or another measure should be studied on a case-by-case basis. Studies must consider both the space to be remedied and the possible effects of the adjacent spaces, which may lead to the choice of a less effective technique, but one that provides a better overall result to all the spaces. Author Contributions: I.S.: Conceptualization and methodology, validation, formal analysis, investigation, writing—original draft, writing—review and editing, visualization. S.A.: Conceptualization and methodology, software, validation, investigation, writing—original draft, writing—review and editing, visualization. M.G.: Conceptualization and methodology, resources, writing—review and editing, supervision, project administration, funding acquisition. J.J.A.: Conceptualization and methodology, resources, writing—review and editing, supervision, project administration, funding acquisition. B.F.: Conceptualization and methodology, resources, writing—review and editing, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Project RadonFlow PID2019-109898RB-100 funded by MCIN/AEI/10.13039/501100011033, Spanish National Research Council (CSIC). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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