Radiation exposure of workers in storage areas for building materials

Peer review under responsibility of Taibah University

Abstract

Radon levels and radioactivity were measured in 50 shops and storage areas for building materials in Sudan. Charcoal canister and gamma spectrometry systems were used to measure radon in 55 types of natural material, and concentrations of 71–292 Bq/m³ (mean, 154 ± 38 Bq/m³) were found. The concentration of radium (²²⁶Ra) ranged from 2.8 to 182.5 Bq/kg, of thorium (²³²Th) from 1.2 to 302 Bq/kg and of potassium (⁴⁰K) from 82.3 to 1413.3 Bq/kg. Porcelain, ceramic and marble showed high values, while gravel types had low radioactivity. Radium in building materials was well correlated with radon (r² = 0.77). The average annual dose of workers at these sites due to inhalation of radon was estimated to be 2.8 mSv. The activity index of building materials ranged between 0.33 and 1.97 (mean, 0.77).

Keywords:

• Building materials
• Naturally occurring radioactive material
• Radon
• Uranium
• Thorium
• Exposure
• Dose

1 Introduction

Use of modern types of building materials has increased following the “urban revolution”. Some of these materials may contain naturally occurring radioactive materials, and in countries such as Sudan these materials are stored in closed rooms, generally with poor ventilation; the concentration of radon gas inside these buildings can therefore be expected to be high. Many workers spend much of their time inside these buildings, which may result in occupational exposure to radon. The health effects of indoor radon and risk assessments of exposure of radon in homes have been determined in many studies [1–3].

Increasing attention is being paid to the effects of natural radioactivity, yet exposure in stores of building materials has not been considered. The exposure of the general public to natural sources of radiation has been estimated by the United Nations Scientific Committee on the Effect of Atomic Radiation [4]. Monitoring of any release of radioactivity to the environment is important for environmental protection, and studies of natural radioactivity are necessary not only because of the radiological impact but also because it is an excellent biochemical and geochemical tracer. Although natural radioactivity is found in rock and soil throughout the earth, accession in specific areas varies within narrow limits. The new types of building materials are brought from various places around the world, and some may be extracted from areas with high background radiation. The contribution of building materials to indoor radon has been investigated in some previous studies in different regions [5–7].

Inhalation of radon and its short-lived daughter products is a major contributor to the total radiation dose of exposed people. In uranium mining, the lung dose due to radon progeny may be sufficiently high to increase the occurrence of lung cancer [8]. Building materials also represent a source of radon in houses. The general recommended permissible limit of exposure to indoor radon, 100 Bq/m³ [9], can be used as a guideline. Naturally occurring raw building materials and processed products have radionuclides of three series, uranium, thorium and ⁴⁰K isotopes [10]. As high concentrations of natural radionuclides in building materials can result in high indoor doses, setting a reference radon level for building materials will account for contributions from other sources without exceeding the hazard level. Special attention has been paid to the radioactivity of brick components of building structures [11–13]. The level of radon also depends on the ventilation of the room: Abdallah et al. [14] showed that poor ventilation increased exhalation rates of radon and increased the level.

With recent developments in building in Sudan, little is known about the natural radioactivity of the building materials used. In general, the building materials used in Sudan are derived from rocks or soil without consideration of the radon content. The aim of the present study was to evaluate the radon levels in storage sites of building materials, the radioactivity of building materials used in Sudan and radiological risk.

2 Materials and methods

2.1 Gamma-ray spectroscopy

Two types of gamma spectrometry system were used: 3″ × 3″ sodium iodide and high-purity germanium detectors in a standard arrangement that included multichannel associated amplifiers and data readout devices [15]. The systems were calibrated against a known source, mixed gamma standard from Amersham. The high-purity germanium detector (p type), with a relative efficiency of 20%, was validated with the International Atomic Energy Agency (IAEA) reference materials RGU-1 and RGTh-1 and calibrated in terms of energy, efficiency and resolution against a mixed radionuclide standard (Amersham Buchler B1575) in the same geometry as the samples. The gamma background contribution was determined with an empty Marinelli container.

2.2 Measurement characteristics

2.2.1 Radon concentration in air

The air concentration of radon was determined by counting the gamma ray emission of radon decay products adsorbed on activated charcoal enclosed in an aluminium cylindrical canister. The canisters were heated in an oven at 60 °C for about 10 min to anneal the charcoal. At the sampling location, the cap of the canister was removed and radon allowed to adsorb onto the charcoal for 4 days. The canister was then closed, sealed and returned to the laboratory for gamma spectrometry.

Samples were collected from 50 randomly selected ceramic shops and storage sites in Khartoum State. At each site, radon was sampled about 1 m above the ground and measured directly by placing it on top of the detector with counting for at least 3 h, depending on the concentration. For radon analysis, a chamber was constructed for calibration, which contained a radium source of known activity and a set of canisters exposed to radon under secular equilibrium. A calibration factor was then obtained and used in this study [16].

2.2.2 Building materials

The shops and storage sites contained ceramic from Sudan, Saudi Arabia, Egypt, China and Spain. The other materials collected were basic building materials such as cement, ceramic, porcelain and fireclay bricks (local production); traditional materials such as clay bricks, gravel, sand and fire bricks; and material collected from major local producers in Sudan. Samples of raw materials were taken from locations representing the major sources of these materials, such as areas of clay brick production and local markets. The materials were either in powder form (e.g. cement and clay) or solid. The latter, such as bricks and ceramic were crushed into small pieces or powdered. The samples were then weighed, sealed in Marinelli beakers and stored for 30 days before counting to allow ²²⁶Ra and its short-lived decay products to reach secular equilibrium.

The activity concentration (Bq/kg) of ²³²Th was determined from the photo-peaks of ²⁰⁸Tl (583 keV) and ²²⁸Ac (911 keV), and that of ²³⁸U was obtained from the gamma-lines of ²¹⁴Pb (352 keV) and ²¹⁴Bi (609 keV), whereas ⁴⁰K was measured directly from the photo-peaks at 1460 keV.

2.2.3 Building materials activity index

The activity index I for materials intended for use in building construction was calculated from the following equation:(1) $I=\frac{C_{\text{Th}}}{200}+\frac{C_{\text{Ra}}}{300}+\frac{C_{\text{K}}}{3000}$(1) where C_Th, C_Ra and C_K are the activity concentration values of ²³²Th, ²²⁶Ra and ⁴⁰K in the product expressed in Bq/kg. If the activity index I is 1 or less, the material can be used in building as far as radioactivity is concerned, without restriction. The factors 200, 300 and 3000 Bq/kg were calculated for a dose criterion limit of 1 mSv/y.

3 Results and discussion

3.1 Building materials

The objectives of this analysis were to evaluate the radioactivity content and then derive a radioactivity index for the purpose of radiation protection and safe use of the materials. Radium (²²⁶Ra) was evaluated mainly from gamma lines of ²¹⁴Bi (assuming secular equilibrium). It is recognized that, under this assumption, the radioactivity of radium represents the activity of uranium and its decay series. Thorium (²³²Th) was evaluated as its daughter ²⁰⁸Tl and potassium ⁴⁰K from the gamma line 1460 keV.

The average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K measured in the building material samples are presented in Table 1 and Fig. 1 in eight categories. The distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples was not homogeneous, and the activity concentration values vary (Fig. 2), from 2.8–108.2 Bq/kg for ²²⁶Ra, 3–394 Bq/kg for ²³²Th and 82–1413 Bq/kg for ⁴⁰K. Fig. 1 also shows the range of radionuclide concentrations for all categories. The highest activity concentrations of ²²⁶Ra and ²³²Th were measured in the high-surface materials, while the highest activity concentration of ⁴⁰K was measured in fire-clay brick samples. The activity concentrations account for 30% of the total activity of surface material and 7% that of cement. For comparison with global data, the average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K measured in the earth’s crust are 32, 45 and 412 Bq/kg, respectively [17] (Fig. 1). Thus, the ²²⁶Ra concentration in the surface material samples was significantly higher than the average value for the earth’s crust, while that of ²³²Th in surface material and porcelain samples was slightly higher. Blocks and sand-gravels had significantly lower values of the three radionuclides than the average for the earth’s crust.

Fig. 1 Ranges and mean activity concentrations of radionuclides (Bq/kg) in the most commonly used raw building materials in all samples in eight categories.

Table 1 Geometric mean activity concentrations of radionuclides (Bq/kg) in the most commonly used raw building materials in eight categories.

Category	^232Th	^226Ra	^40K	Total activity (%)	Radioactivity index
Bricks (clay, lime, fired clay)	58.1 ± 3.1	20.4 ± 2.1	459.6 ± 116.8	13	0.54
Sands and gravels	13.30 ± 1.6	10.4 ± 0.4	352.2 ± 69.6	9	0.21
Pure cements	32.8 ± 3.9	15.2 ± 8.8	229.54 ± 174.0	7	0.35
Cement blocks	14.98 ± 1.4	11.5 ± 1.4	378.37 ± 42.0	10	0.23
Porcelains (marble, mazzaiko)	59.9 ± 25.1	31.3 ± 41.5	486.4 ± 542.5	14	0.58
Gypsum ceiling	20.59 ± 1.32	18.2 ± 1.1	293.23 ± 120.9	8	0.26
Light mud (on roofs)	20.91 ± 1.32	16.8 ± 1.5	271.16 ± 44.6	8	0.25
Surface materials	213.4 ± 30.4	76.8 ± 27.9	900.3 ± 407.5	30	1.62
Range	3–394	2.8–108.2	82–1413
Geometric mean	52.4	20.6	206.9
Standard deviation	55.2	17.6	113.6

Most concentrations were within the range of normal levels. The main causes of variation include the presence of small amounts of naturally occurring radioactive materials in most building materials, reflecting natural distribution, as noticeable differences in concentration were observed between tiles (mazzaiko) and porcelain derived from similar sources.

Fig. 1 also shows the radioactivity indexes of the eight categories of building material. The lowest value (0.21) was in sand and gravel, while the maximum value (1.62) was in surface materials. The radioactivity index for all samples is below the acceptable level (<1.0) except in surface materials [8].

3.2 Radon

The average indoor radon concentration in the main ceramics storage sites in Khartoum State was normally distributed (Fig. 3), with a geometric mean of 154 ± 38 Bq/m³, which is over the acceptable limit of 100 Bq/m³. The highest concentration was 292 Bq/m³ and the lowest 71 Bq/m³. The storage sites were classified into two main categories, unventilated (closed) and poorly ventilated (semi-closed). Table 2 lists descriptive statistics for the two categories. No significant difference was found, except for one data point considered to be an outlier (292 Bq/m³), which was excluded for comparison purposes. The measured values (average, 126 Bq/m for poorly ventilated and 123 Bq/m for unventilated rooms) can be used to estimate the exposure of workers to radon; the difference between them is ±2%.

Fig. 2 Histograms of radioactivity (²²⁶Ra, ²³²Th and ⁴⁰K) in building materials.

Fig. 3 Histogram of indoor radon concentration, showing a normal distribution.

Table 2 Descriptive statistics of radon concentrations in poorly ventilated and unventilated storage areas and shops.

Ventilation	Number	Radon concentration (Bq/m)			Standard deviation
		High	Low	Average
None	28	247 (292)	72	126	45.4
Poor	22	247	71	123	44.5

3.3 Correlation between radon and radium

An attempt was made to correlate radon concentration in storage rooms and radium in building materials. The materials were grouped according to type as shown in Table 1. Fig. 4 shows a plot of the correlation between radon and radium, with average, minima and maxima for each category. A linear relation was found between indoor radon and radium in all types of building material (r² = 0.74); exclusion of surface materials yielded a correlation coefficient of r² = 0.8, indicating that the main source of indoor radon is building materials. The emanation coefficient of radon in storage rooms can be obtained from C_Rn = 2.1C_Ra + 97.1 (r² = 0.74) or, when the outlier in Fig. 4 is omitted, C_Rn = 5C_Ra + 43 (r² = 0.81), with an overall uncertainty up to ± 19%, where C_Rn and C_Ra are indoor radon and radium concentration in the materials expressed in (Bq/m³) and (Bq/kg), respectively. The fate of indoor radon depends on the exhalation rate of the materials, which in turn depends on factors such as material bulk density and porosity. The radon exhalation rate from building materials has been the subject of many investigations [18,7,19], which show that the radon concentration (predicted by models or measured) is well correlated with the radioactivity in materials, in good agreement with our findings. The issue of radon due to building materials has been studied widely [20–22] and reviewed by Pacheco-Torgal [9], radon being considered an indoor air contaminant.

Fig. 4 Correlation between radium (²²⁶Ra) in building materials and indoor radon (²²²Rn).

3.4 Risk estimates

The concentration of indoor radon can be converted into an effective dose for workers at storage sites due to inhalation of radon and its decay products, in theoretical models [23]. If we assume that the indoor breathing rate is 0.75 m³/h, the mean effective dose can be estimated from a dose conversion factor [7] and the estimated time spent inside (8 working hours for 280 days/year) from Eq. (2)(2) $D_{\text{eff}}=C\cdot Q\cdot t$(2) :(2) $D_{\text{eff}}=C\cdot Q\cdot t$(2) where D_eff is the effective dose (Sv/year), C is the radon concentration (Bq/m³), Q is the dose conversion factor (8 × 10⁻⁸ per Bq/m³ h), and t is exposure time (h).

The calculation gives an average value of 2.75 ± 0.7 mSv/year, which is higher than that of people not classified as radiation workers. According to the recommendations of the International Commission on Radiological Protection (ICRP) and the IAEA [24], the limit for the general public is 1 mSv/year and that for radiation workers is 20 mSv/year; intervention is required at 6 mSv. These limits have been adopted in many countries. The activity index I for building materials, calculated from Eq. (1)(1) $I=\frac{C_{\text{Th}}}{200}+\frac{C_{\text{Ra}}}{300}+\frac{C_{\text{K}}}{3000}$(1) , ranged from 0.33 and 1.97, with a mean of 0.77; the index was <1.0 for basic materials and <6 for surface materials.

4 Conclusion

We found notable variation in the radioactivity of building materials and in indoor radon levels. The main causes of variation are that most building materials contain small amounts of naturally occurring radioactive materials, which reflect the geology of their site of origin. A good correlation was found between indoor radon concentration and radium in building materials in shops and storage areas (r² ∼ 0.8). For radiation protection, there are national and international regulations and guidelines for indoor radon, such as the recommendations of the IAEA and ICRP [23,25] and the recommendation of the European Commission (action level, 400 Bq/m). Countries including Saudi Arabia and Sudan have adopted the WHO level of 100 Bq/m [26]. Our findings are in good agreement with those of other studies in the region, such as in Saudi Arabia [27] and Libya [5], which drew attention to building materials as a source of indoor radon. Indoor radon and radioactivity in building materials are commonly treated as exposure, with controls based on dose criteria and exemption levels. The dose criterion is established on the basis of overall national circumstances, but it is widely accepted that external doses exceeding 1 mSv/year should be taken into account. Employees with such doses are classified as non-radiation workers, although they receive 2.8 ± 0.7 mSv/year. The present study adds valuable information that can be used in future studies, such as of normal exposure to ionizing radiation, or for developing national exposure limits.

Notes

Peer review under responsibility of Taibah University