Indoor/outdoor air exchange on indoor radon concentration
was investigated. We evaluated the effect of air extraction versus air
introduction at different flow rates on equilibrium 222Rn activity
concentrations in a scale model room of 62 cm × 50 cm × 35 cm (inner length
x width × height), made of a porous, radium and thorium-rich lithoid
ignimbrite (Tufo di Gallese) from Vico volcano (Lazio, central Italy).
Experiments were carried either with the inner walls of the chamber covered
with a plasterboard shield or without any inner coating. Air introduction
was always more effective than air extraction to reduce indoor 222Rn
and, in both cases, higher flow rates produced higher 222Rn decreases.
The presence of the plasterboard enhanced 222Rn reduction when outdoor
air was introduced in the chamber. Main results were that, with
plasterboard, maximum reductions of 89.5 % and 25.0 % were obtained
introducing and extracting air, respectively; without plasterboard, we found
maximum radon decreases of 33.2 % and 26.6 %, namely with air
introduction or extraction. The diffusion of 222Rn through the walls of
the scale model room was modelled with a modified version of Fick's second
law, where a term considering air flow velocity was added. These findings
suggested that the combined use of proper coatings on the inner walls of a
house and outdoor air introduction at suitable rates are a good strategy to
approach radon mitigation actions.
Introduction
Indoor radon accumulation is considered the main source of human exposition
to ionizing radiation (NCRP, 2009; Radulescu et al., 2022). High levels of
radon are generally due to the isotope 222Rn, characterized by the
longest half-life (about 3.8 d) and only occasionally the contribution of
the isotope 220Rn becomes significant: (i) in the basement of buildings
rising over thorium-rich bedrocks; (ii) when thorium-rich building materials
are used for construction. In these cases, 220Rn with a very short
half-life (56 s) may reach large activity concentrations.
Many studies dealt with experimental monitoring and mathematical modelling
of indoor radon from soil and building materials in test houses (Capra et
al., 1994; Font and Baixeras, 2003; Shaikh et al., 2003; Mancini et al.,
2018) or modelled indoor radon concentration from experimental radon
exhalation rates of building materials (Tuccimei et al., 2006; Kumar et al.,
2014; Syuryavin et al., 2020).
The impact of environmental parameters on radon entry into buildings was
addressed by many papers (Arvela et al., 2013; Vasilyev et al., 2015; Shen
and Suuberg, 2016). Collignan and Powaga (2019) demonstrated that indoor
environmental conditions (depressurization and air exchange rate) strongly
affect radon entry and indoor radon activity concentrations. McGrath and
Byrne (2018) experimentally validated model predictions for radon –
ventilation relationships in retrofit buildings.
Tuccimei et al. (2009) proposed a classification of building materials based
on the 222Rn and 220Rn exhalation rates required to attain
predetermined indoor radon levels in a standard confined environment (the
model room of 56 m3, 4 × 5 × 2.8 m, reported in EC,
1999) completely covered with the investigated material. The classification
was to some extent modified years later (Cinelli et al., 2019). To our
knowledge, no studies worked on a purpose-built scale model room to survey
the contribution of building materials, radon proof membranes and
air-exchange rate on indoor radon activity concentrations.
A scale model-room of 62 cm × 50 cm × 35 cm (inner length × width × height)
was created with a very porous, 226Ra-, 232Th- and 40K rich
lithoid ignimbrite, called “Tufo di Gallese” to evaluate the contribution
of building materials to indoor radon accumulation (Lucchetti et al., 2020).
The model-room allows to study the effect of: (i) different types of covers
(inner and outer) and (ii) air ventilation on indoor radon.
The outcomes of preliminary experiments (Lucchetti et al., 2020) indicate
that equilibrium 222Rn activities were reached rapidly (in just two
days) when the chamber was left to exchange air through its walls consisting
of a very porous rock (43 %). If the walls were externally coated with a
transparent film used to conserve food, the air exchange was strongly
reduced enhancing radon accumulation. Further tests demonstrated that inner
covers (such as plasterboard and different kind of paints) partially reduced
indoor 222Rn, but entirely cut the short-lived 220Rn. Finally,
decreases of ambient temperature reduced radon exhalation from the building
materials and, in turn, indoor activity concentration. In this paper, we
present new experiments where indoor radon is monitored as a function of
indoor/outdoor air exchange (air introduction versus air extraction at
different flow rates), with or without inner plasterboard coating, to: (i) evaluate how the ventilation regime associated with the air permeability of
a building affects indoor radon levels, and (ii) model the radon diffusion
trough the chamber walls.
Materials and methodsThe scale model-room
As reported in Lucchetti et al. (2020), the model room was built with sixty
blocks (15 cm × 10 cm × 5 cm per block) of “Tufo di Gallese” ignimbrite.
Table 1 summarizes the main properties of this material. A cement mortar
with very slight 222Rn and 220Rn exhalation rates was used to
paste blocks. Only the walls of the room consisted of ignimbrite stone
(surface area of 0.78 m2), whereas the floor and the roof were made
from Plexiglas boards. The apparatus has been conceived to focus on the role
of the walls made of a homogeneous radon-emitting building material on
indoor radon concentration and considering its prevalent interaction with
the outdoor environment. The inner volume of the chamber was about 0.110 m3. Two taps were applied on the upper Plexiglas board (Fig. 1) to
connect the room with the input and output openings of the RAD7 monitor via
the vinyl tubing provided by Durridge Company Inc.
Porosity, 232Th,
226Ra and 40K specific
activities and 222Rn and 220Rn
exhalation rates of “Tufo di Gallese” ignimbrite.
The scale model-room of 62 cm × 50 cm × 35 cm (inner length × width × height), made of “Tufo di Gallese” ignimbrite (Caprarola area,
central Italy). Two taps are installed on the Plexiglas roof of the room to
introduce and/or extract air from the chamber.
Indoor radon determination
Two kinds of experiments were accomplished: (1) introduction of outdoor air
in the model room; (2) extraction of indoor air from the model room. The
tests were repeated either with internal coatings on the chamber walls or
without them. Just 222Rn, hereafter called radon, was considered.
In the first type of experiments, indoor radon activity concentration was
measured using the AER PLUS (Algade Instrumentation) radon monitor. It is a
small sized commercial solid-state radon detector, with local storage for
temperature and relative humidity data. It is battery supplied with an
autonomy of one year. The instrument was calibrated at Istituto Nazionale di
Geofisica e Vulcanologia (INGV, Roma, Italy) and compared with other
instruments such as RAD7. The effect of water molecules on AER PLUS
efficiency was evaluated (Galli et al., 2019).
In the second type of experiments, indoor radon activities concentration was
measured using either AER PLUS (Algade Instumentation) or RAD 7 (Durridge
co.). RAD 7 is an electrostatic chamber equipped with a solid-state silicon
detector, operated at a nominal voltage of 2000–2500 V for the
collection of radon daughters onto its surface. The sensor detects and
separates alpha particles on their energy basis. This allows to select only
the short-lived 218Po (with a half-life of about 3 min) to measure
222Rn, reaching the radioactive equilibrium between them in just 15 min. This option (the Sniff mode, according to RAD7 protocols) allows to
change experimental conditions fast and perform rapidly the test.
Temperature and relative humidity are recorded inside the instrument and a
pump guarantees the air circulation in the set-up. A cylinder of desiccant
(drierite) was placed before the inlet of RAD7 to reduce air humidity.
Indoor air was sampled from the model room and then transferred to the radon
monitor. 222Rn activity concentration was corrected for effect of
water molecules on the electrostatic collection of 218Po ions onto the
surface of the silicon detector (neutralization) according to De Simone et
al. (2016).
Experiments with the inner coating
As anticipated in Sect. 2.2, two types of experiments were carried out to
evaluate the relative change of indoor radon in the model room under
different experimental conditions. Firstly, the model room was internally
covered with a shield of plasterboard with a double coat of paint to
simulate the walls of a building where the bricks are covered with plaster
and paint.
Starting from a reference equilibrium radon level (Rnno ventilation)
reached with the two taps on the chamber roof closed (see Fig. 1), air
exchange between the room and the outer environment was appropriately
induced and controlled. The average 222Rn level in the laboratory was
30 Bq m-3, with no detectable 220Rn. Then, outer air was
introduced, or indoor air extracted from the model room at different flow
rates and indoor radon concentration was monitored up to new equilibrium
concentration (Rnair ventilation). Relative change of Indoor Radon
(RIR) was expressed following Eq. (1):
RIR=Rnair ventilation-Rnno ventilationRnno ventilation×100
The value is negative in case of radon reduction, positive for radon
increase.
In the first set of tests, we evaluated the effect of outer air introduction
in the model room trough a tap and with the other closed, to simulate the
effect of the room overpressure on indoor radon levels. The pump of RAD7 was
employed to drive air in with variable air flow rates (AFR): 0.82, 0.64,
0.50, 0.30 and 0.15 L min-1. These conditions were obtained and
controlled using two flowmeters appropriately coupled to a T-joint, with one
flowmeter connected to RAD7 and used to check the air flow and the other
just employed to reduce the air flow. Indoor radon was measured with the AER
PLUS placed in the model room, with RAD7 acting just as a pump.
In the second group of experiments, indoor air was extracted from the model
room trough a tap, with a flow rate of 0.82, 0.50 and 0.15 L min-1. The
experimental set-up was like that of the first set of trials, with the
second tap closed and RAD7 used to monitor indoor radon levels. The goal of
these experiments was to evaluate the influence of enhanced indoor
depression on radon activity concentrations. In order to make the results of
the experiments more exportable, the air flow rates (L min-1) were expressed
as air exchange rates (ACH, h-1) by dividing air flow rates (AFR) by
the volume of the model room (V) and multiplying by 60.
ACH=AFRV×60
Experiments without the inner coating
Finally, the plasterboard shield was removed from the inner walls of the
room; outdoor air was introduced, and indoor air was extracted from the
chamber at 0.50 and 0.82 L min-1. In both cases, indoor radon was compared
with the equilibrium value reached without ventilation. Outdoor radon
concentration in contact with the room walls, at 5 and 10 cm distance was
monitored. The results of these tests were modelled with the Fick's second
law.
Modelling radon diffusion through the room walls
In order to model radon diffusion through the room walls (Sasaki et al.,
2007; Savović and Djordjevich, 2008; Savović et al., 2011; Urošević
and Nikezić, 2008), we applied a modified version of Fick's second law, where we
added a term (second term of Eq. 3) to consider the air flow velocity, thus
modelling a diffusive-advective transport (Chakravertya et al., 2018):
dC(x,t)dt=Dd2C(x,t)dx2-vdCx,tdx+g-λC(x,t)
where:
C(x,t) is the radon concentration in the pore space of the building
material (Bq m-3) that depends on location x and time t,
D is the radon diffusion coefficient (m2 s-1),
x is the distance from the inner side of the wall to the outdoor direction
(m),
v is the air velocity through the wall (m s-1),
g is the radon creation rate per unit size of the pores (Bq m-3 s-1), and
λ is the radon decay constant (s-1).
Results
Results of all experiments are reported in Table 2.
Relative change of Indoor Radon (RIR) in the model room
with and without the plasterboard inner covers in a series of experiments
where outdoor air was introduced in the model room or indoor air was
extracted from the model room at different air flow and air exchange (ACH)
rates.
An example of indoor radon monitoring during the experiments and data
treatment is provided by experiment 1 (Fig. 2), where radon activity
concentration was measured in the model room using the AER PLUS instrument.
In the first four days, 222Rn was recorded in the room with the two
taps closed. After two days, equilibrium level (Rnno ventilation)
was almost reached (about 641 Bq m-3) and at the end of the fourth day,
outdoor air was introduced in the room with a flow rate of 0.82 L min-1. After ten hours new equilibrium 222Rn concentration
was attained (about 67 Bq m-3) and RIR was calculated (-89.5 %, see
Table 2). The same approach was applied to all tests.
222Rn activity concentration versus time during experiment 1
(first group of experiments). Radon data were recorded hourly. The relative
standard deviation is about 40 % at 100 Bq m-3, about 18 % at 500 Bq m-3
and about 13 % at 1000 Bq m-3.
The grey band shows data uncertainties (one standard deviation).
The first group of experiments clearly shows that the introduction of
outdoor air in the model room, internally covered with a layer of
plasterboard, strongly reduces indoor radon concentration from -63.8 %
with air flow of 0.15 L min-1 to -89.5 % with flow of 0.82 L min-1 (Table 2),
demonstrating that RIR is directly proportional to the air flow. On the
other hand (second group of experiments), the extraction of indoor air from
the room with the plasterboard coating moderately cuts radon level from -4.7 % with air flow of 0.15 L min-1 to -25 % with flow of 0.82 L min-1 (Table 2).
Finally, we removed the plasterboard from the inner side of the model room
and performed four further experiments (experiments from 9 to 12, see Table 2). In this experimental condition (third group of experiments), RIR values
were extremely reduced in the “introduction of air” experiments compared
with corresponding tests with the plasterboard coating (group 1, Fig. 3).
For example, experiment 1 provided RIR of -89.5 % with air flow of 0.82 L min-1 and experiment 6, RIR of -33.2 with the same flow. A direct
correlation of RIR and air flows is also confirmed in tests of group 3.
Experiments of group 4 (“extraction of air” type, without the
plasterboard) provided results that correspond within the error range to
those of group 2 experiments (Table 2 and Fig. 3), suggesting that the
presence of the plasterboard does not improve the effectiveness of the air
exchange like in the “introduction of air” experiments.
Relative change of Indoor 222Rn (RIR) at different air flows with
or without a plasterboard coating the inner walls of the model room. RIR is
equal to (Rnair ventilation-Rnno ventilation)/(Rnno ventilation)×100. Green and blue curves on the right stand for RIR obtained in the
“introduction of air” experiments of groups 1 (from test 1 to 5) and 3
(experiments 9 and 10); red and orange curves on the left side of the plot
correspond to RIR recorded in the “extraction of air” experiments of
groups 2 (from test 6 to 8) and 4 (experiments 11 and 12).
DiscussionThe influence of indoor/outdoor air-exchange on radon
levels with the inner coating on the room walls
These experiments allowed us to simulate the effect of indoor/outdoor
air-exchange on indoor radon levels. The scale model-room simulates the
effect of building materials with high radon exhalation rates on
indoor radon activity concentration. The high porosity of “Tufo di
Gallese” ignimbrite, when the chamber is not coated with inner covers,
enhances the effect of natural ventilation on indoor radon levels.
A common practice to reduce indoor radon makes use of air fans which extract
indoor air from the room. If air exchange rates (ACH) are low and radon
concentrations are very high, radon-rich air is more effectively pumped in,
producing a modest effect on radon concentration (see results of second
groups of experiments).
According to the experiments, a better result is reached if radon-free
outdoor air is injected into the room, producing an overpressure that
strongly reduced the entry of radon-rich air from the building materials
(see first group of experiments), a porous ignimbrite with high 222Rn
and 220Rn exhalation rates (see Table 1).
The presented tests show that indoor radon is strongly affected by air
ventilation and that the higher the flow rates the stronger the radon
decrease. This finding agrees with measurements carried out by Syuryavin et
al. (2020) that investigated the effect of air exchange on indoor radon and
thoron from a low dense building materials placed in a perfectly sealed
chamber. They obtained 222Rn decreases up to 66 times from experiments
with ACH of 0.50 h-1 compared with correspondent no air exchange
measurements. This reduction is much higher than those obtained exchanging
air at the same rate from the presented model room, because “Tufo di
Gallese” ignimbrite is very porous, even if internally covered with the
plasterboard (Table 2).
The influence of indoor/outdoor air-exchange on radon
levels without the inner coating on the room walls
Experiments of groups 3 and 4 carried out without the plasterboard on the
inner walls of the room lowered the ability of air ventilation to reduce
indoor radon concentration. In the “introduction of air” tests (group 3)
the decreased efficiency of air exchange compared with that of the
experiments of group 1 (with the plasterboard) is due to the enhanced air
exchange through the very porous building material that reduce the
overpressure and the radon discharge outwards. Also in this case, the
limited reduction of indoor radon is directly correlated with air flow
(Fig. 3).
The comparison between the “extraction of air” experiments accomplished
with the plasterboard (group 2) and without it (group 4) show no significant
differences in the results because the air exchange is not strong enough to
produce a large underpressure and remove indoor radon. On the contrary, the
porosity of the “Tufo di Gallese” seems to favour, even if only slightly,
the radon release outward in the experiments without the plasterboard.
This physical behaviour was modelled applying the Fick's second Law (see
Eq. 3) to experiments of groups 3 and 4. The graphical solutions of the
modelling are reported in Fig. 4a (air introduction) and 4b (air
extraction). Table 3 reports the values of parameters used for modelling.
Radon diffusion through the wall of the scale model room according
to Eq. (3). (a) Introduction of outdoor air in the model room at 0.5 and
0.82 L min-1, compared with the no ventilation experiment. The curve at 0 L min-1 of year 2020 does not take into consideration the real measurements of
laboratory background at increasing distances from the chamber walls, but
just an average value of 30 Bq m-3. (b) Extraction of indoor air from the
model room at 0.5 and 0.82 L min-1, compared with the no ventilation
experiment.
Parameters used in the modelling using the Fick's second
law (Eq. 3).
a Narula et al. (2010), datum referred to a porous material with a density like that of “Tufo di Gallese”ignimbrite.
b Righi et al. (2006). c Lucchetti et al. (2020).
Figure 4a shows a first curve of radon diffusion through the room walls,
labelled “0 L min-1, year 2020”, where the real measurements of laboratory
background at increasing distances from the chamber walls were not taken
into consideration, but an average value of 30 Bq m-3 was hypothesised.
The shape of this curve does not reproduce the real shift of radon from the
room walls outward. On the other end, the other curves at air flow rates of
0, 0.5 and 0.82 L min-1 rely on the experimental measurements of radon levels
at increasing distance from the walls. These outdoor data are much higher
than 30 Bq m-3 and display a decreasing trend moving away from the wall.
The diffusion curves trough the building material are characterised by a
first part with increasing values and then by a decreasing trend. These
results demonstrate very well the outward forced flow of radon, induced by
the air introduction.
Figure 4b reports the modelling of experiments of group 4. The shape of the
curves is much different from that of group 3 tests (Fig. 4a) since
external air is drawn through a very porous material into the chamber. As a
result, the outdoor air is enriched in radon along its way to the room.
Conclusions
These experiments demonstrate that the application of air exchange and the
contemporaneous use of coatings applied on the inner walls of a buildings
made of radon-rich materials enhance indoor radon reduction. Outdoor air
introduction increases the indoor pressure of the room, reducing radon entry
and pushing radon outwards. If the outer walls are not sealed by cement or
mortar, radon diffusion out of the building is even more facilitated.
Conversely, the extraction of indoor air is less effective than air
introduction, regardless of the plasterboard cover, because the room
pressure is slightly decreased, and radon exhaled from the building material
more easily enters the room.
This suggests the use of a combined mitigation strategy to cut indoor radon
levels, consisting of effective air exchange and the application of suitable
radon-proof membrane on the inner walls of a room to prevent radon entry
from the room walls. It is also recommended to avoid unnecessary coatings on
the external wall surfaces, to make the reduction of indoor radon more
effective when a room overpressure is applied. Findings of this work
directly apply to radium-rich building materials and to the use of air
exchange. Mixed mitigation strategies should consider the specific radium
content of building materials and guarantee: (i) a low degree of noise since
ventilation should not be so annoying, (ii) comfortable indoor temperature
and humidity conditions either in winter or summer, and (iii) a sustainable
energy consumption.
Future studies will focus on the determination of radon diffusion
coefficients of commercial and new radon-proof materials that will be then
applied on the presented model room where proper air exchange will be
reconstructed.
Code and data availability
All data and equations are reported in the manuscript. Equations are in Sect. 2, data in Sects. 3 and 4. The reader is referred to Table 2, Figs. 2, 3 and 4 for data presentation.
Author contributions
CL, GG and PT designed the experiments and carried them out. GG and PT
developed the model code and performed the simulations. GG and PT prepared
the manuscript with contributions from CL.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Geoscience applications of environmental radioactivity (EGU21 GI6.2 session)”. It is a result of the EGU General Assembly 2021, 19–30 April 2021.
Review statement
This paper was edited by Gerti Xhixha and reviewed by two anonymous referees.
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