Indoor Radon Research in Asia-Pacific Region

Indoor Radon Research in Asia-Pacific Region: History

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Indoor radon is a major hazard to human health; it is one of the leading causes of lung cancer. Therefore, radon research in Asia has intensified due to the growing awareness of the harm that radon poses. An analysis of the collected literature data showed that in Asia–Oceania, some regions have—or are believed to have—little indoor radon problems due to climate and low Rn ground. It can be concluded that countries have their own approaches, techniques, and protocols.

radon
regulation
Asia
HDI

1. Introduction

Radon (Rn), a radioactive gas naturally emanating from Earth’s crust, accounts for about one-half of the effective dose of ionizing radiation received by humans. The most relevant isotopes are

^{222}

Rn, hereafter referred to as ’radon’, which is a decay product of the

^{238}

U decay chain, and

^{220}

Rn, hereafter referred to as ’thoron’, which is a decay product of the

^{232}

Th decay chain. Radon measurements in various uranium and non-uranium mines began in the early 20th century. Based on this, it was soon assumed that radon could be responsible for lung cancer among underground miners ^[1]. Two studies with quantitative analyses among the US and Czechoslovakian miners concluded that the lung cancer risk increased monotonically with cumulative exposure to radon progeny ^[2]^[3]. Motivated by these studies, further efforts were made to develop more reliable methods for monitoring radon progeny in mines. It was found that radon progeny ranges from several to thousands of kBq m

^{- 3}

^[4].

The results of the first set of indoor radon measurements in Sweden indicated high levels of radon in several houses built with radium-rich alum shale concrete ^[5]. Since then, large indoor radon surveys have been carried out in several countries, for example, the USA, many European countries (e.g., the UK and Czech Republic), and Japan. In some countries, such studies are currently being conducted, e.g., in India and China.

General findings from numerous epidemiological studies, based on data from miners and the general population, support the conclusion that, following cigarette smoking, prolonged residential radon exposure is one of the leading causes of lung cancer in the general population ^[6]^[7]^[8]^[9]^[10].

However, contrary to these findings, there are observations that do not confirm that statement. A series of cancer mortality studies near radon hot springs were conducted through the collaborative efforts scattered throughout different Japanese cities. At present, there is no definite evidence indicating an increase in cancer mortality in the Misasa radon hot spring area. Moreover, lower risks were found for stomach cancers in both radon and non-radon hot springs ^[11]^[12]^[13]. In Ramsar, Iran, inhabitants are exposed to levels of natural radiation that are about 150 times higher than the average global dose rate; indoor radon concentrations are up to 3700 Bq m

^{- 3}

. It was concluded that the lung cancer rate may show a negative correlation with the natural radon concentration ^[14].

In addition, the results of a meta-analysis involving thirty-two case-control studies and two ecological radon studies on lung cancer, focusing on radon concentrations below 1000 Bq m

^{- 3}

, do not support the finding that radon may be a cause of a statistically significant increase in the incidence of lung cancer ^[15]. The discussion about the effects of low radon exposure as well as low-level ionizing radiation is ongoing ^[16]^[17].

In any case, internationally, indoor radon is recognized as a health issue and a preventable risk factor that can be handled through effective national policies and regulations ^[18]. Consequently, due to the potential effect of radon on human health, it has been the subject of many studies worldwide. Currently, work is being conducted on both radon monitoring and epidemiology. Radon monitoring and control strategies focus on indoor and underground spaces where radon can accumulate due to limited ventilation.

On the other hand, due to their physical properties and dynamic behavior, radon and its progeny can be employed as tracers in geohazard and atmospheric studies, for example, in the prediction of earthquakes and volcanic events, as well as in the analysis of mass transport and mixing processes.

It can be concluded that radon should be treated by its negative and positive aspects, i.e., not only as a health threat but also as a useful research tool, as summarized in Table 1.

Table 1. Positive and negative aspects of radon.

Positive Aspects	Negative Aspects
Earthquake prediction ^[19]^[20]^[21] Atmospheric studies, climate research ^[22]^[23]^[24]^[25]^[26] Volcanic surveillance ^[27] Lunar science ^[28] Mineral exploration ^[29]^[30] Geothermal energy prediction ^[31]^[32] Balneotherapy in radon spas (USA, Japan, and Europe) ^[33]^[34]^[35] Search for organic pollutants in the ground ^[36]	Health effects, i.e., cancer and leukemia ^[37]^[38] Contributor to radiation doses in NORM, e.g., mineral factory ^[39] Background in laboratories, e.g., Super-Kamiokande neutrino observation laboratory ^[40]

2. Radon Regulations

In Europe, the EURATOM Basic Safety Standards, which were published in 2013, include binding requirements (to be implemented into national legislation) for protecting against indoor radon exposure at work, home, and in the manufacturing and use of building materials ^[41]. Therefore, many radon projects are underway in Europe. One of the achievements of European radon research is the development of the European Indoor Radon Map, which is part of the European Atlas of Natural Radiation, created by the Joint Research Centre of the European Commission ^[42]. Other achievements include the EU-funded Metro Radon project (http://metroradon.eu, accessed on 1 February 2023) which focuses on QA, from radon measurement to the delineation of radon priority areas; moreover, the traceRadon project (http://traceradon-empir.eu, accessed on 1 February 2023) provides a necessary scientific base for measuring atmospheric radon activity concentrations and radon fluxes ^[25]. According to the recommendations of the WHO ^[43] and the European Commission ^[41], a reference level (RL) of 300 Bq m

^{- 3}

has been established, and the annual average indoor radon concentration should not exceed this value. National RLs vary across different countries due to the variations in regional levels of indoor radon, which usually range from 100 to 300 Bq m

^{- 3}

In the USA and Canada, the action level, i.e., a threshold for recommending or requiring mitigation of exposure to harmful elements, is set at 148 Bq m

^{- 3}

(4 pCi/L) and 200 Bq m

^{- 3}

, respectively ^[44]^[45], with no distinction between existing and new dwellings. In Asia and Oceania, the countries with radon regulations are limited, according to the WHO Radon Database ^[46] and other collected references; see Table 2.

Table 2. Reference levels (RL) in dwellings, mitigation, and prevention actions. AM—national mean indoor Rn concentration (Bq m

^{- 3}

Country	RL	AM $^{a}$	Mitigation	Prevention	Reference
Australia	200	10.5	No	No	^[46]
Bahrain	300	- $^{b}$	-	-	^[46]
China	300	40	-	-	^[46]^[47]
Georgia	300	-	-	-	^[46]
Mongolia	100	-	-	-	^[46]
Turkey	400	81	Yes	No	^[46]
Turkmenistan	200	-	-	NA	^[48]
Korea	200	62	-	-	^[49]^[50]
Israel	200	31	-	-	^[51]
Kazakhstan	100 (new), 200 (old)	-	-	-	^[52]

^{a}

value from the latest survey,

^{b}

not available.

There is no country in Asia or Oceania with a national radon risk communication strategy; however, in some of these countries, strategies against pollutants other than radon exist, as listed in Table 3.

Table 3. Radon communication and linkage to other national strategies (source: WHO).

Country	Cancer Control Strategy	Lung Cancer Reporting/Screening Strategy	Indoor Air Quality Strategy	Energy Conservation Strategy
Australia	X	X
China	X	X	X	X
Iran	X
Mongolia	X
Turkey			X	X

3. Radon Surveys

Surveys can be classified according to their design characteristics. Among the criteria are their objectives (assessments of geographical or demographic means, i.e., means per area unit or per person living in an area, which are generally different), their coverage (which part of a country does a survey cover, distinguishing between local, regional, and national surveys), and the degree of representativeness. The latter indicates whether derived statistics, such as the empirical mean, can be assumed to coincide with the respective true value of the sampled quantity.

Here, the researchers do not make a distinction between surveys according to their design characteristics, in particular, whether they are regional or national surveys. Details, where available, are given in the relevant Section 3.1. See Section 4.3 for further information on this important subject.

Based on 2019 WHO data ( https://www.who.int/data/gho/data/themes/topics/topic-details/GHO/gho-phe-radon-database, accessed on 1 February 2023) only Australia, China, Turkey, and Syria conducted national radon surveys. However, as will be discussed later, there are (or have been) large-scale radon studies in several countries, namely India (ongoing), Iran, Israel, Japan, Korea, New Zealand, and the Philippines. Russia is not discussed here since it is usually categorized with Europe. On the other hand, Turkey, Armenia, Georgia and Azerbaijan are discussed here, although they are often counted with Europe.

A summary of radon levels is presented in Table 4. The countries are presented by region, i.e., central Asia (I), eastern Asia (II), southeastern Asia (III), southern Asia (IV), and western Asia (V), according to the methodology introduced by the United Nations ^[53]. The list might not be complete because sometimes survey results are published in literature that is difficult to access. Therefore, it may also be that in countries not mentioned here, radon data exist. The first meta-survey on radon in Asia and Oceania was presented in 2019 at the 16th AOGS conference in Singapore ^[54]. New data have been added since then.

Table 4. Average and maximum indoor radon concentrations (Bq m

^{- 3}

) with the number of measurement points.

Country	ISO Code	Rn Survey Average (WAM)	Rn Max	Subregion	Number of Measurement Points
-	-	Bq m $^{- 3}$	Bq m $^{- 3}$	-	-
Afghanistan	AF	65	2064	IV	16
Armenia	AM	*	400	V	800
Australia	AU	12	423	Oceania	3413
Azerbaijan	AZ	84	1100	V	2404
Bangladesh	BD	113	2616	IV	308
Brunei	BN	**	**	III	1
China	CN	37	1244	II	144,937
Georgia	GE	114	376	V	28
Hong Kong	HK	155	700	II	1580
India	IN	32	373	IV	895
Indonesia	ID	96	1015	III	394
Iran	IR	198	3700	IV	3194
Iraq	IQ	38	239	V	175
Israel	IL	90	200	V	45,415
Japan	JP	18	1256	II	11,360
Jordan	JO	70	1532	V	3904
Kazakhstan	KZ	114	37,650	I	246
Korea	KR	91	2810	II	11,106
Kuwait	KW	33	595	V	1108
Kyrgyzstan	KG	200	1996	I	68
Lebanon	LB	39	343	V	65
Malaysia	MY	22	196	III	183
Myanmar	MM	17	84	III	65
Nepal	NP	123	2206	IV	108
New Zealand	NZ	21	302	Oceania	977
Oman	OM	21	39	V	9
Pakistan	PK	40	191	IV	3041
Palestine	PS	98	984	V	88
Papua New Guinea	PG	13	18	Oceania	60
Philippines	PH	21	58	III	2626
Qatar	QA	16	42	V	84
Saudi Arabia	SA	26	195	V	2955
Singapore	SG	15	80	III	10
Syria	SY	44	524	V	1435
Taiwan	TW	11	51	II	274
Tajikistan	TJ	76	2000	I	70
Thailand	TH	36	405	III	1541
Turkey	TR	81	1400	V	7293
United Arab Emirates	AE	40	71	V	61
Uzbekistan	UZ	219	1050	I	25
Vietnam	VN	79	634	III	142
Yemen	YE	43	890	V	293

* Data not available, ** Problem with data, explained in the text.

The mean values shown in Table 4 are weighted average values (

W A M

) in case that in a country several surveys have been performed. The weighted average was calculated by multiplying the average radon concentration by the number of measurements for a given survey, then dividing by the total number of measurements using following equation:

W A M = s u m (A M_{i} * n_{i}) / s u m (n_{i})

, where

A M_{i}

is the average radon concentration from the

i - t h

survey and n is the number of measurement points during

i - t h

survey.

4. Identification of Problems

As previously shown in Table 4, the results of indoor radon measurements in some Asian countries are relatively old, dating back to the late 1980s and early 1990s. In this case, three problems can be recognized.

4.1. Bias Due to the Thoron Interference

The first problem involves the quality assurance of results due to old types of detectors, i.e., bare detectors, and/or thoron influence. The problem was widely discussed by Tokonami ^[55]. It was concluded that some old detectors have a high sensitivity to thoron, while others have a low sensitivity. It should be noted that the influence of thoron may be large if the detector is placed near the wall, even when low-sensitive detectors are used. As a consequence, the calculated annual dose can be overestimated.

4.2. Tendency toward “Green” Construction

The second problem is connected to the house construction. Modern technology and trends toward low, “green”, energy houses can lead to tighter dwellings and reduced natural ventilation. Recent studies from Russia have shown an increasing trend of radon levels in buildings ranked with high energy efficiency indices ^[56]. In contrast to this, the results presented by McCarron ^[57] support the hypothesis that certified passive house buildings present lower radon levels.

4.3. Survey Design and Evaluation

In many papers reporting on the means of regional surveys, the ’sample representatives’ issue is poorly (or not at all) discussed. Deviations from representative sampling can introduce biases in statistics, such as the mean, which renders the results questionable. See IAEA (2013), Section 3 of that report ^[58], and European Commission (2019), Section 2.4.5 ^[42] of that report, for further discussions of this very important subject. Moreover, the reporting of results that meet statistical standards is sometimes suboptimal, and uncertainty budgets are rarely addressed. In order to deliver reasonable results that can be internationally recognized, it is important to employ certified and QA-ed procedures, including calibration, sampling designs, individual measurements, and statistical evaluation. In many papers, QA is poorly reported.

5. Recent Developments

5.1. Thoron

In some regions of the world, thoron and its progeny contribute more to radiation doses than radon ^[59]^[60]^[61]. Kanse et al. ^[62] worked on developing a method that uses the exhalation rate of Tn from indoor surfaces as the basis for estimating the average concentration of Tn in indoor air. Taking this thoron concentration and appropriate conversion factors into account, the inhalation dose can be calculated.

5.2. Calibration Chambers

Karunakara et al. presented an innovative technique of using soil gas as a source of radon in a calibration chamber ^[63]. Constant radon concentrations in the range of 0.5–31 Bq m

^{- 3}

with a deviation of 5–15% were obtained by periodically injecting soil gas into the chamber. The time needed to obtain stable conditions is approximately 30–120 min depending on the required concentration.

As mentioned in Section 2, some Asian countries have (or will introduce) regulations on radon concentration levels in residential buildings and workplaces. For this purpose, measurements should be made and maintained; for the results to be reliable, the measurement systems must be checked and validated periodically. One method of maintaining quality is to conduct intercomparison tests. Janik et al. ^[64] presented the results of an experiment conducted in five radon and thoron measurement systems located in four Asian countries (China, India, Japan, and Thailand). They obtained good results when comparing the radon systems (chambers). Deviations from the average concentrations did not exceed 5%. They also showed that the systems for testing and calibrating thoron devices still require further research.

5.3. New Detectors

One method of dividing radon detectors is to distinguish between passive detectors that integrate radon (e.g., SSNTD, active carbon, electret) and active detectors that measure radon continuously, based on, e.g., semiconductors, PIN photodiodes, etc. ^[65]^[66]

Although these detectors are used successfully, new methods are being developed and tested. One example involves a new detector presented by Hassanpur et al. ^[67]. They explored the possibility of alpha spectroscopy in detecting radon and its progeny using a microstrip gas detector. Experiment data were validated by using a MCNPX code and the spectrum from the microstrip detector was compared to the one obtained by the Atmos device. Results showed that the microstrip detector can measure radon and its progeny and it has the ability to extract the spectrum obtained from it.

Another gas-type detector, a micropattern gas detector (MPGD), was tested in order to measure radon and progeny ^[68].

Another example of the development of measurement techniques and methods is the system for measuring radon in the soil, as presented by Wang ^[69]. One of the challenges in measuring radon in soil is that moisture interferes with the results. The presented system attempts to avoid this problem by using a suitable waterproof membrane and a calculation algorithm.

5.4. Soil Radon as Tracer

A topic widely discussed in recent publications is the relationship between radon in soil and geohazard research. As a recent example, the purpose of the study presented by Ma et al. ^[70] was to show the mechanism that generates soil Rn anomalies by means of studying the geochemical behaviors of radionuclides in karst environments. They confirmed higher soil radon concentrations in karst compared to non-karst areas. They also found a significant positive correlation between Ra and MnO

_{2}

^{2}

= 0.86), which implied that Ra mainly occurred in manganese oxide minerals.

5.5. Advanced Data Preprocessing and Evaluation

One important topic in the collection and interpretation of data, which mainly applies to active detectors, is imputation. Mir et al. presented a new imputation methodology (by feature importance) to generate an imputed dataset when dealing with soil gas radon concentration time series data. This approach provides more accurate mean value predictions ^[21].

A study by Rafique et al ^[71] investigated the complexity of radon, thoron, temperature, and a relative humidity time series via entropy, along with fractal dimension analysis techniques. Their results showed that the dependence and complexity of the time series data of soil gases are greater in the winter than in the summer.

5.6. Radon Awareness and Risk Communication

In addition to measuring radon and assessing the dose, increasing radon awareness and communication are very important tasks. Based on the survey results, it was concluded that the level of radon awareness among the people of Bahrain is low, with only 32.6% being aware of radon and its health hazards ^[72].

5.7. Radon Therapy

Currently, radon is being explored as an additional method of treatment for various diseases related to the respiratory system, pain, or rheumatism.

Review studies on radon therapy were compiled and presented by authors in Japan ^[73]. A comparison of the published research results shows that active oxygen in combination with radon gas has great potential in suppressing disorders and various types of diseases.

This entry is adapted from the peer-reviewed paper 10.3390/atmos14060948

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