Effects of Ventilation and Forced Air Movement on the Activity Concentration of ²²²Rn and ²²⁰Rn

1. Quick Background

Radon (²²²Rn) and thoron (²²⁰Rn) are naturally occurring radioactive gases, respectively belonging to the primordial decay chains of uranium and thorium embedded in our Earth’s crust. Invisible, odorless, and tasteless, they are prevalent in indoor environments, particularly basements and ground floors. Radon and thoron have half-lives of 3.8 days and 55.6 seconds, respectively, a significant difference that dramatically influences the behavior of the isotopes.

2. Distribution of Radon and Thoron in an Indoor Setting

Radon, with a half-life of days, is more likely to emanate into the open air and accumulate as a gas, making the assumption of uniform radon concentration in a room usually valid. By contrast, the distribution of thoron within a room is significantly affected by its short half-life. Assuming no supplemental air mixing and that emanation occurs through purely diffusive means, the simplest model suggests an exponential reduction of thoron concentration with distance from the emanating wall [1]. This behavior often necessitates a tailored approach to measuring its presence in indoor environments; however, mitigation strategies for thoron have largely not been considered necessary.

3. Identified Health Risks and Mitigation Strategies

Radon has been a long-standing public health concern. Dating back to the early 20^th century, when the existence of uranium, its radioactivity, and decay products was a fresh discovery, a correlation between lung cancer risk and prolonged exposure to radon gas was identified in miners [2]. While the risks associated with radon have been extensively studied and mitigation strategies have been recommended at a high level of authority and regulation for some time [2–5], thoron has only recently been receiving a similar level of attention [6, 7]. This lack of attention in the past may be attributed to the distribution of thoron gas concentration as discussed prior, not reaching far from an emanating surface, and making it unlikely for an individual to consistently inhale thoron or its progeny. However, this assumption might not always hold, posing potential risks. Especially as, for the same activity concentration, thoron and its progeny are generally contributing more dose if inhaled than radon and its progeny [1, 8]. This paper aims to explore scenarios where thoron gas distribution does not follow this assumption. We have employed tests of forced air movement and varying air ventilation rates to achieve these scenarios.

Ventilation in particular is a common mitigation strategy for radon [3–5]; however, its effects on thoron concentration are unclear. As both isotopes will often be present in tandem, an analysis of how existing ventilation strategies interplay with thoron concentration can help better refine radiation safety models for hazards previously unrecognized [9]. This study prompts a broader discussion on the importance of accurate thoron measurement for optimizing conditions in places with higher likelihoods of radon and thoron presence, such as underground indoor dwellings, mines, and recreational caves. However, within the scope of the present work, we seek to determine if the existing strategies are effective, neutral, or counterproductive in managing thoron.

1. Study Period and Design

This study consisted of a series of controlled experiments to understand the behavior of radon isotopes (²²²Rn and ²²⁰Rn) in various simulated environments. Across all experiments, the parameters included: (1) forced air movement, a direct air current induced by an electrically powered fan; (2) the exhaust state, varying between either a closed loop circulation of air, or open to air outside the experiment space; and (3) ventilation speed, an external air pump attached to the airflow system and routed into the experiment space, experimentally measured in liters per minute but reported in units of air changes per hour (ACPH) for better association with other literature. In this text, ACPH refers to the number of times the amount of air in a volume of a space being ventilated and replaced with fresh air in 1 hour.

The calculation used to convert from air flow rate in L/min to ACPH is a straightforward calculation involving the volume of the space being ventilated and the ventilation flow rate. Let V be the volume of the space in cubic meters, and let Q be the air flow rate in L/min, as Equation (1):

2. Experimental Settings and Equipment

The experiments were conducted in two primary settings: a closed room of concrete walls, floor, and ceiling in the basement of a building at the Australian National University in Canberra, Australia, and in a laboratory environment in the radiological lab of Safe Radiation Pty. Ltd. in Brisbane, Australia. All measurements of ²²²Rn and ²²⁰Rn concentrations were carried out using Durridge RAD7 monitors.

In this study, each experiment was divided into collections of runs. Furthermore, each run was split into a set number of accumulation cycles. The accumulation period, the time taken to accumulate, was the same for each accumulation cycle. Activity concentration for radon and thoron was reported at the end of each accumulation period.

All reported values of activity concentration for radon and thoron were corrected for humidity using the RAD7 Capture software (Durridge). Additionally, the RAD7 calculates and reports a two-sigma uncertainty value of the measured radon and thoron concentration over the accumulation cycle [10]. For the mean activity concentrations reported in this paper, the uncertainty of the mean value was obtained through propagating the uncertainty of the individual measurements. More specifically, the uncertainty of the mean was evaluated as the sum of squares of each individual uncertainty, then square-rooted and divided by the total number of measurements. In Equation (2), as:

where n is the total number of individual measurements, △x_i is the uncertainty associated with the i^th measurement, and △m is the uncertainty of the mean activity concentration.

3. Basement Room with Forced Air Movement

Two probes were strategically placed in a closed room, one in the geometric center and the other 0.1 m away from the center of a wall (Fig. 1). The setup also included a floor fan positioned adjacent to the door, which could be operated from outside the room. The primary focus of this experiment was to analyze the behavior of radon in a closed room and how it changes under induced air movement.

Fig. 1.

The two probes placed in the geometric center of the room (orange), and 10 cm away from a wall (blue).

A total of three runs were carried out for both detectors running simultaneously with an accumulation period of 8 hours. This longer accumulation period was chosen in order to allow the detector to accumulate enough to produce statistically significant results. The first and third runs involved a static airflow state, with the fan switched off. That is, the fan was switched on in the second run and then switched off again in the third run to validate that the measurements revert to a static airflow state. For both devices, the first and second runs had 20 accumulation cycles, and the third run was programmed to have four accumulation cycles. The detector measuring in the center completed all four during the third cycle; however, the device measuring 0.1 m from the wall could only complete the first two accumulation cycles and crashed due to an error.

4. Laboratory-Based Apparatus

Due to the limited access time with the basement room, the second set of experiments was conducted in our own radiological lab at Safe Radiation to attempt to reproduce the results from the basement experiment and to explore more experimental conditions, such as simulating forced ventilation. A custom-machined chamber designed to simulate radon emanation from a surface was utilized. This chamber contained mineral salts with uranium and thorium traces and featured an externally operable fan. Specifically, the mineral salts used were a zircon sample from a mineral sand extraction operation in North Stradbroke Island, QLD, Australia. Safe Radiation carried out a compositional analysis of naturally occurring radioactive materials within this sample, which is given in Tables 1 and 2. A single RAD7 monitor was used for these measurements.

Table 1.

Mass Concentration of Naturally Occurring Radioactive Material

Table 2.

Activity Concentration of Various Isotopes in the Zircon Sample Used in the Laboratory Experiment Sets

The experiments carried out in the laboratory were split into two parts. The first part focused primarily on the state of the exhaust system (open/closed) and wind conditions created by a stationary fan. The second part introduced forced ventilation through an air pump as a varying parameter. Further through the text and within the tables, this separation is written as ‘Lab experiment part 1’ and ‘Lab experiment part 2.’

In the first part, six runs were carried out, all with an accumulation period of 3 hours and eight accumulation cycles for each run, with the exception of the first run, which had seven due to a detector error. Each run involved a change between the two fan flow states (‘On’ and ‘Off’), governed by the small, externally operable fan at the top of the chamber and the two exhaust states of closed and open.

A shorter accumulation period of 3 hours (compared to the previous experiment) was used to get finer data points on the change of radon and thoron concentration. Additionally, the choice of shorter accumulation was influenced by this experiment having a stronger source relative to its chamber size compared to that of our previous experiment in the basement dwellings. As such, for the basement dwelling, the longer accumulation period was necessary to obtain statistically significant results. However, this was not necessarily the case in the laboratory experiments.

A sponge-like foam tubing was added to the chamber, placed above the mineral salts (Fig. 2), aiming to adjust the activity flux ratio between radon and thoron to better correspond to what may occur in a natural situation [11].

Fig. 2.

Diagram of the chamber used in laboratory experiments at Safe Radiation.

In the second part, an external pump system was integrated (Fig. 3), and ventilation speeds were introduced as a variable in the experiments. An accumulation period of 3 hours was continued throughout this part of the experiment, however over six accumulation cycles. This setup was devised to investigate the effects of varying air exchange rates on radon and thoron behavior. The experiment utilized pumps of different capacities (5, 10, 20, and 30 L/min) and a flow control valve for fine-tuning ventilation rates. In order for ventilation to occur, the air must be vented out of the chamber into another space. Thus, in this second part, all runs take place with an open exhaust system. Additionally, for each ventilation rate, measurements were repeated with the externally operable fan switched on to see the additional effect that may occur when a direct air current is applied onto the emanating source. Across a total of 13 continuous runs, the ventilation rate was increased. For each ventilation rate, radon and thoron concentrations were measured with or without airflow from the externally operable fan.

Fig. 3.

An (a) external air pump pushes air through (b) an air flow control valve, which is then fed into (c) the chamber housing the mineral salts containing traces of ²³⁸U and ²³²Th. From there, the egress of air from the chamber is connected to the (d) Durridge RAD7 radon monitor for counting. The air from the RAD7 air outlet is finally sent to the (e) an external exhaust system.

1. Basement Room

For the static air (air mixing fan of radon and thoron), average activity concentrations at the center of the room were 161±2 Bq/m³ and 1.3±0.4 Bq/m³, respectively. Static air radon activity concentrations ranged between 161 Bq/m³ and 197 Bq/m³, which decreased between 78 Bq/m³ and 89 Bq/m³ with air movement. Conversely, thoron concentrations were between 1.3 Bq/m³ and 1.4 Bq/m³ in the static conditions but dramatically escalated to 47.0–50.0 Bq/m³ range when the air was moving. The results reported are averaged across the total period of its experiment type. Namely, the static air type runs are averaged over 24 accumulation cycles, with an 8-hour accumulation period, totaling 192 hours. Whereas, for the moving air type runs are averaged over 20 accumulation cycles, with an 8-hour period, totaling 160 hours. Fig. 4 demonstrates this change in activity concentration between static air and air movement. The mean activity concentration for radon and thoron grouped by air flow state is provided in Table 3.

Fig. 4.

Average activity concentrations of radon and thoron in the basement room corresponding to the probes situated at (A) the geometric center and (B) 10 cm away from the wall (as shown in Fig 1).

Table 3.

For the Basement Room Experiment, the Mean Activity Concentration across Fan Flow States

2. Lab-Made Apparatus for Simulating Radon and Thoron Emission

1) Moving air with the externally operable fan (Lab experiment part 1)

For the first part of the laboratory-based experiments, the data demonstrates that in closed systems, radon accumulates towards an equilibrium with 3.8-day half-life, with observed concentrations up to 2,281 Bq/m³. As expected, opening the exhaust appears to significantly mitigate radon accumulation through passive ventilation. However, most notably, the data also indicate that moving air significantly elevates thoron concentrations in both open and closed systems, with thoron concentration in the range of 2,270–2,520 Bq/m³ for a closed system and 1,990–2,200 Bq/m³ for an open system. This potentially suggests air flow as a factor in thoron dispersion regardless of the system’s state. This behavior is displayed in the plots of Fig. 5, and the mean activity concentration and range of activity concentrations of radon and thoron for this part of the experiments are shown in Table 4. For all runs except the first, the results reported are averaged across a total run period of 24 hours: a 3-hour accumulation period with eight accumulation cycles. The first run’s results are averaged across the total run period of 21 hours, with a 3-hour accumulation period with seven accumulation cycles.

Fig. 5.

Activity concentrations of radon and thoron in the lab chamber, for each accumulation cycle across the 6 day (approximately 145 hours) experiment period. Experiment runs were divided first in exhaust state (open or closed), and then in fan state (static or moving).

Table 4.

For Lab Experiment Part 1, the Mean Activity Concentration and the Range of Activity under Varying Exhaust State and Fan Flow

2) Forced ventilation through air pump (Lab experiment part 2)

In the second part of the laboratory-based experiments, where ventilation speeds are introduced as a varying parameter, the data elucidates a discrepancy in ventilation requirements for mitigating both radon and thoron effectively. While ACPH values within a range of 1–10 substantially reduce radon levels, reducing from 1,558 Bq/m³ (with no ventilation) down to 13 Bq/m³ (at 1 L/min). However, these rates are insufficient for thoron mitigation, which only reduces from 410 Bq/m³ with no ventilation to 74 Bq/m³ at 7.5 L/min; demanding significantly higher ventilation speeds exceeding 100 ACPH to achieve comparable reduction. This distinction may underscore the necessity for more aggressive ventilation strategies to effectively manage thoron concentrations in closed environments. The influence of the exhaust state and fan state also concurs with the previous experiments, which is apparent when comparing plots, Fig. 6. This is particularly true for the values less than 100 ACPH. A table displaying the mean activity concentrations for varying ventilation speeds is given in Table 5. In contrast, for induced air movement, Fig. 6A indicates an overall increase in thoron concentration for the same ACPH values of the other system configurations. For all runs, the results reported are averaged across a total period of 18 hours, a 3-hour accumulation period with six accumulation cycles.

Fig. 6.

Averaged activity concentration of radon and thoron across each run against the corresponding ventilation rate it was tested at. Plots are grouped by the value of airflow state and exhaust state. Listed in order of airflow state respectively: (A) fan flow on; (B) fan flow off. ACPH, air changes per hour.

Table 5.

For Lab Experiment Part 2, Mean Activity Concentration against Varying Ventilation Speeds

3. Experimental Pitfalls, and Where to Go from Here

The results show a significant relationship between thoron concentration and mechanical ventilation, suggesting a range in which certain ventilation rates can inadvertently increase airborne radiation hazard by increasing thoron activity concentration while successfully reducing radon concentration. We hypothesize that the enhancement in thoron activity concentration in the room air could be due to pressure variations between the room’s void and the porous gaps in structures like those in the concrete walls, which in turn may affect the diffusion length of thoron particles. Nonetheless, our data displays that there likely exists a threshold in ventilation efficiency that is effective for both radon and thoron.

A subsequent step to take in this research would be to identify how equilibrium factors would change under different airflow conditions, which in turn requires observing how radon and thoron progeny change under those conditions. In doing this, we will be able to provide a more refined model of equilibrium factors to provide more accurate dosimetric analysis of complex scenarios.

4. Comparison to Other Reported Ventilation Rates

A few studies indicate a range of 0.5–2 ACPH as an effective ventilation rate for radon mitigation [12, 13]. In comparison to our results, radon had indeed been consistently reduced in that range of ventilation rates, while thoron remained prevalent in concentration until much higher rates of 50 ACPH and greater (2 L/min and greater in our chamber). However, it should be noted that ACPH as a quantity is dependent on the volume of the space. In our experimentation, the small volume of the chamber allowed for achieving a high ACPH with a relatively slow speed of air flow. This, combined with the available air pumps used, meant that the first measured ventilation speed value was 21 ACPH (1 L/min in our chamber), which is a large leap that is difficult to compare with available literature. Though the comparison of ACPH to other literature is done under the assumption that our experimental ventilation speeds correlate, at least proportionally, to what may happen in a larger volume, which may not necessarily be true. To rectify this ambiguity, further experiments in a larger volume and greater specificity of ventilation rates should be carried out. One radon study was found to have reported values of ACPH in the ranges of 50–100 ACPH [14].