Radon Exhalation from Five Wood Species

Five different air-dried wood species that were commonly used in Korean housing materials were used (2 kg each), 4 domestic and 1 imported. The domestic conifers used were: red pine (Pinus densiflora Siebold. & Zucc.), Japanese larch (Larix kaempferi (Lamb.) Carriere), hinoki cypress (Chamaecyparis obtusa (Siebold. & Zucc.) Endl), and sawtooth oak (Quercus accutissima Carruth). The imported species used were: northern red oak (Quercus rubra L. (imported wood)). The density, moisture content, and surface area of each wood specimen are listed in Table 1. A gypsum board made in the 2000s were also tested.

Radon exhalations of each of the wood species were measured using the Closed Chamber Method (CCM) (Petropoulos et al., 2001) along with the Accumulator Method (AM). The self-designed acrylic hermetic chamber (Volume = 0.054 m², I.D. 300mm x 300 mm x 600 mm: width x length x height) was specially constructed for this measurement. The Continuous Radon Monitor (CRM), Model 1028 (Sun Nuclear Corporation, USA, Dimensions = 236mm x 122mm x 74mm), was placed at the bottom of the chamber while an air-dried sample (2 kg) was placed 30 cm above the CRM as shown in Fig. 1. The specimens were made by 297 mm (w) x 297 mm (l), and the heights were variable from 31 to 58 mm (h) because each sample had different density corresponding to its height. The detector was set to have 24 hours of delay time, and the samples were measured total of 12 times every two hours a day for 12 days since the preliminary experimental results had ensured to reach a constant level within 12 days. Lastly, the chamber was hermetically sealed. The temperature and humidity of the chamber were 25 ± 2 °C and RH 50 ± 10 %. Experiments were reflected in conditions representative of the humidity and temperature inside the average home in Korea. Additionally, one more chamber, which did not contain any samples, was located right next to the one with a sample so that radon exhalations not derived from the wood sample can be measured as the blank. Five wood samples and a gypsum board were tested by this method.

Fig. 1. Experiment set-up photograph for the closed chamber method.

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Radon exhalation conversion from the measured concentration values were calculated as detailed below. First equation is of Petropoulos (2001), and it led directly to the second equation by Chen (2010), the third equation is Awhida (2016) combined the concept of measurement time, and the fourth equations have correction terms accounting for each background noise, mass and areal exhalation rate.

where

These results are presented from the measured values of wood as a building material by the equipment (CRM), Model 1028, to identify the characteristics of radon concentration. The analyses were processed by recorded concentration data, curves of measurement results, and U.S. Environmental Protection Agency (EPA) average. The statistical analyses [ANOVA, POST-HOC= SCHEFFE ALPHA (0.05)] were performed using IBM SPSS version 22 to determine the properties of wood species over the half-life with measured concentration of 138 radon counts, because the tendency was unknown only with the curves of the measured data.

As shown in Table 2, the concentrations of exhalation of all the tested wood species were measured to be distributed across an extremely low range of 0.1 - 0.2 pCi/L (3.7 – 7.4 Bq·m^-3) in chamber. The mass exhalation rates of all the species also ranged from only 0.00089 to 0.00181 Bq·kg^-1·h^-1. Moreover, the surface exhalation rates resulted in 0.00677 - 0.01517 Bq·m²·h^-1 range. The average values were corrected by the blanks. There was no striking difference when comparing the averages between coniferous trees and deciduous trees in Table 2. Statistical significance test for group difference between not only softwoods and hardwoods but between wood species could not be determined. Although the detection concentration was seemingly too low, the statistical analysis was still worthwhile because the analysis was useful to confirm any difference of the half-lives, not only difference of tree species.

Based on the findings obtained, one can evaluate the contribution of the radon emanating from the wood materials used in a wooden house to its indoor radon levels. As a wooden house made of 100 percent of wood, for example, we assumed a a 162 m² (width x length = 12 x 13.5), 2.4 m high wooden ceiling, wood-frame structure with wooden floors and wooden wall panels, and an air-exchange rate in the range of 0.5 - 1 air changes per hour (ach). Depended on an average exhalation rate of 0.015 Bq·m^-2·h^-1 (see Table 2), which was the highest outcome among the five species, the contribution of the wood material to indoor radon concentration would be on the order of about 0.017 - 0.034 Bq·m^-3. It is significantly by less than concentrations of 37.0 Bq·m^-3 which observed in structures built from wood materials (KINS 2002). Even for air exchange rates of 0.1 ach, the resulting concentration was calculated as 0.172 Bq·m^-3, which is far below the Korean radon recommendation level, 200 Bq·m^-3 (Ministry of Environment, 2016). On the other hand, to compare our results of the radon exhalation rate from wood materials with those of other construction materials from variable references in Table 3, wood belonged to the lowest group of the radon exhalation among the various building materials. To have a better comparison, the extracts in Table 3 culled from the 5 different cited references. For example, although Lee et al. (2015) had identified radon exhalation rates for 10 framing materials and 16 finishing materials out of 26 total building materials used in the Korean construction industry, only 8 framing materials and 4 finishing materials were extracted for this trend analysis. The extracted radon exhalation values of the same object showed different figures because the values were derived from various countries and origins. Based on the results of Chen (2010), slate and granite generally had higher radon exhalation rates, which was varied widely from non-detectable to about 300 Bq·m^-2·d^-1, than other decorative materials. The average radon exhalation rates were 30 ± 16 Bq·m^-2·d^-1 for slate and 42 ± 63 Bq·m^-2d^-1 for granite, respectively.

Korea Institute of Nuclear Safety (KINS) in 2002 reported that wood materials yielded second highest radon concentration levels among other commonly used materials in Korean architecture. In 2010, KINS also reported a list of radon concentrations emitted by various common construction materials: wood (37.0 ± 21.9 Bq/m³), red brick (27.9 ± 7.2 Bq/m³), concrete (26.8 ± 19.4 Bq/m³), cement block (25.9 ± 10.8 Bq/m³), stone (24.7 ± 11.1 Bq/m³); tested at the Nationwide Survey of Indoor Radon in Korea (2) - Public facilities. Wood was reported to have significantly high amount of radon concentration emission; however, the radon exhalation rate of wood itself proved the lowest group among those of the building materials from the present experiment. Also, Ingersoll (1983) reported the same results that wood was the weakest emanatory in building materials. He showed that the mean radon emanation rates of wood were 0.02 pCi·kg^-1·h^-1 for two wood samples of unknown species from the western states of the U.S.A. by the gamma-ray spectroscopy techniques. These rates were very similar to 0.1 - 0.2 nBq·g^-1·s^-1, proven by Zikovsky (1992), for two wood samples of unknown species purchased from stores in the Montreal area by the method of internal proportional counter. The measured rates from wood used in the U.S.A. and Canada were very close to the present results as shown in Table 3. The radon exhalation rate of Quercus accutissima Carruth (white oak) which has the highest density showed the highest value among the five wood species, on the other hand, the rest of four species stayed at the very low levels. National Research Council (US) Committee (1999) noticed that Radium, chemically similar to calcium, is imbibed from the soil by plants. The radium content of plants varies on its soil because it originates in soil and is variable. As a result, it is reasonable to expect that different radon emanation rates of wood should be detected with variety of resources according to their regions. The present results confirmed that radon exhalation rate of wood itself considered the lowest group in building materials. The data from scientific reports mentioned above and the results from our experiment shows the potentially hazardous influx of radon exhalation from the surrounding of wood building, not the wood material itself. Therefore, preventions of influx of radon and selection of construction-finishing materials should be taken into consideration in architectural construction methods. Furthermore, it is highly desirable that wood will be required for furniture and even as interior finishing material in indoor environment.

Stranden (1988) noted that Closed Chamber Method (CCM) is the most popular way to estimate the emanation rate of a building material. Radon concentration growth in the closed chamber reaches its peak and equilibrium conditions dependent on radon emanation through half-lives of several times. To compare with wood and gypsum board, which is commonly known as possessing high radon emanation, Fig. 3 shows that high radon concentration of gypsum board made in the 2000s, and it certainly shows what the tendency is about from the experimental result. As a result, equilibrium was preserved in 4^th half-life. In contrast, wood stayed at the low radon concentration, and there was no difference between half-lives as shown in Fig. 2 about Larix kaempferi (Lamb.) Carriere. In the Fig. 2 and 3, the results of radon concentrations for both materials in the chamber without reflecting its background effect are shown.

Fig. 2. Radon concentration of Larix kaempferi (Lamb.) Carriere. in chamber.

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Fig. 3. Radon concentration of gypsum board in chamber.

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Using CCM and accumulator method (AM) the radon concentrations of wood species were compared with up to the 3^rd half-life. More measurements were not taken since the average peaks of the concentrations were reached at the 2^nd half-life, as shown in Table 4. The measurements of the concentration were taken only 8 days, unlike other reports previously mentioned (Lee et al., 2015; Najam et al., 2013) that took 2 to 3 months. When the EPA averages in Table 2 were calibrated by the average of correction value within the same species, the statistical significance test for group difference between half-lives indicated the same results because there were no big differences between the corrected average values and original non-corrected measured values. In addition, the reason why the correcting background effect was not performed is that each of the samples and background recorded differently at different times. The correcting values were not able to be proceeded every single count of sample and blank. Due to this difficulty, the average values were corrected by calibration.

As a result, statistical analysis was necessary because it is different, whether the maximum values are reached quickly or not, by the half-lives and the characteristics of the samples. The original measured values without the correction were used for group difference among half-lives. As a result of the statistical processing data, four species had the same results on the 1^st, 2^nd, and 3^rd half-lives. Therefore, the measurement was enough until the 1^st half-life period, but only one species was measured up to the 2^nd half-life period although the experiment was proceeded until the 3^rd half-life period to compare between 2^nd and 3^rd half-lives.