Many socially vulnerable people in South Korea live in semi-underground housing types. However, there is lack of research on hazardous environmental conditions in such housing. This study conducted Rn and bioaerosols exposure assessments of these vulnerable residential environments. The median level of Rn in the houses was 0.7, ranging from 0.43 to 0.80 in this study.
In comparison, the average level of Rn was 1.13 pCi/L in basement parking lots of six governorates of Kuwait in summer (Bu-Olayan and Thomas, 2016), 0.30 pCi/L in the Tokyo subway in Japan (Doi and Kobayashi, 1996), and 0.81 pCi/L in Caracas city subway in Venezuela (Liendo et al., 1997). Moreover, most residential areas in Arab countries have lower Rn on the ground level than basements and areas involved in industrial activities (Alkan and Karadeniz, 2014). Similarly, higher Rn levels were found in basements compared with the first or second floors of buildings (Alkan and Karadeniz, 2014; Abel Ghany, 2008). These results are in line with the current study (Table 3), and they can be attributed to the ventilation and air-conditioning systems, poor building maintenance on the ground floor, and so on (Bu-Olayan and Thomas, 2016).
However, we did not find an association between the number of ventilations, time of ventilation, and Rn levels. The possible reason is that the numbers of ventilations and ventilation times surveyed on the day of sampling were not based on the sampling day but the memory of the residents. A previous study found that increasing ventilation rates by 3–10 times than the natural ventilation rate at home by controlling outdoor air ventilation helped reduce indoor emissions (Hospodsky et al., 2015). Therefore, high ventilation rates are effective in reducing Rn levels.
Since the different sampling methods and conditions of meteorological variables of bioaerosols make level comparisons difficult, we compared only approximate levels of bioaerosols for the indoor environments. In previous research, the levels of bioaerosols sampled in indoor environments such as lecture theaters at universities in China (Dexing et al., 2015) ranged from 7.8 to 1,460 CFU/m3, the mean at an elderly care facility was 310.5 ± 221.3 CFU/m3, and that at a postnatal care center was 302.2 ± 69.4 CFU/m3 (Hwang et al., 2018). These results are fairly close to the range of bioaerosol levels observed in this study. However, in another study, airborne bacteria measured in child day-care centers ranged from 249.6 to 19,000 CFU/m3, with a median of 2,566 CFU/m3 (Hwang et al., 2017), which is higher than any of the values measured in this study. When we compared the same residential indoor environments from other countries, levels of bioaerosols in different Portuguese homes ranged from 350 to 1,618 CFU/m3 with a median of 684 CFU/m3 for CAB, and 119 to 566 CFU/m3 with a median of 250 CFU/m3 for CAF (Madureira et al., 2015).
Despite such results, guidance on bioaerosols has not yet been established by international organizations or agencies in other countries such as the WHO, the United States Environmental Protection Agency, and American Conference of Governmental Industrial Hygienists due to limited data (WHO, 2009; Kim et al., 2018). On the other hand, South Korea established a quantitative guideline of 800 CFU/m3. It should be noted that the average level of bioaerosols (CAB and CAF) did not exceed these guidelines for indoor air quality in this study (Ministry of Environment of Korea. 2014). However, the mean level of CAB was the highest at 609 CFU/m3 in semi-basement houses compared to 1st-floor houses (398 CFU/m3 and 314 CFU/m3, respectively), even though the temperature and RH conditions were similar. These characteristics were similar to a prior research finding that the deeper the underground station platform, the higher the levels of CAB, even in the same underground subway environment (Hwang et al., 2010).
Regarding the ventilation effect on the houses, Kwan et al. (2020) suggested that ventilation was not a significant factor controlling bacterial and fungal levels at homes; they reported that in occupied homes, the average amount of bacteria and fungi in indoor air that is controlled by outdoor air ventilation was less than 10%. With these results, even if the number of ventilations was surveyed on the day of sampling in this study, there might not have been a significant relation between the bioaerosol level and number of ventilations; however, this cannot be guaranteed because the results may vary by region, climate, and residential environments of the countries.
Meteorological parameters and building structures are also important factors that may affect the seasonal variation of Rn levels in indoor environments (Xie et al., 2015). For northern climates, Rn concentration was previously reported to be the highest in winter and lowest in summer (Mose et al., 1992; Lee et al., 2017). This is because the dwellings’ doors and windows remain closed through most of the winter, compared with summer; hence, the ventilation is poor in winter (Duggal et al., 2014). In winter, higher levels of Rn gas develop in indoor environments because of their lower pressure compared to the exterior (Sahoo et al., 2007). Regardless of the season, high Rn levels in semi-basements can be attributed to the entry of Rn through cracks or crevices from the surrounding soil, low temperature, and high RH from outdoor environments (UNSCEAR, 2006).
In this study, the Rn level was significantly correlated with levels of CAB (r = 0.33, p < 0.01) and CAF (r = 0.22, p < 0.01); the correlation between these three variables is visualized using color in Fig. 1. Lee et al. (2020) reported that Rn radiation could instigate microbial metabolic activity depending on the Rn levels when they were exposed, which indicates that the threshold concentration present in the ecosystem is relevant to both microbial diversity and population density. Mumataz et al. (2013) also found that microbes could grow in mediums containing a high concentration of uranium in the Ranger Uranjum mine. Moreover, microbial density and dehydrogenase activity, such as of Bacillus sp., Brevibacillus sp., Lysinbacillus sp., and Paenibacillus sp., increased as they came closer to the natural Rn source (Lee et al, 2020).
Moreover, this study found that CAF levels correlated with the levels of CAB and RH. A recent study analyzed the correlation between bacteria and fungi in sugarcane tops silage before and after aerobic exposure, finding that Pichia was positively correlated with genera Lactobacillus and Pediococcus, but negatively with genera Acinetobacter, Citrobacter, and Serratia (Zhang et al., 2019). This indicates that the correlation between CAF and CAB levels depends on the species of microorganisms. Unfortunately, this study could not identify the bioaerosols, making it difficult to explain the correlation between CAF and CAB. Moreover, RH is known to affect the correlation among microorganisms in various studies (Takahashi, 1997; Tang, 2009; Hwang et al., 2018;). The current study showed that among other environmental factors, such as illumination, height, and temperature, RH had the greatest impact on fungal richness (Li et al., 2020).
There was no correlation between the levels of Rn and the residences’ temperature in this study (r = 0.08, p > 0.05; Fig. 1). This result was the same as that reported by Xie et al. (2015), who found no clear correlation between indoor Rn levels and indoor temperature. A previous study reported that this may be because variations in indoor temperature in the premises were substantially lower than variations in outdoor temperature, and indoor Rn levels strongly depend on outside temperature (Baltrenas et al., 2020). We noticed that the correlations between indoor Rn levels and RH in this study were not significantly different, in line with a previous study (Baltrenas et al., 2020), but RH was still an important factor that could be correspondingly high with increasing Rn levels (El-May et al., 2004; Schnell et al., 2012).
Another factor that can influence indoor Rn levels is the underlying geology. A significant positive correlation was found between indoor Rn levels and the geological composition of the ground (Je et al., 1999; Sundal et al., 2004; Moreno et al., 2008). A previous study reported that high Rn levels are present in underground station platforms based on granite areas compared with those based on non-granite areas (Hwang et al., 2018).
Further, we found that people living in semi-basements had higher health condition scores than those living in ground residences (Table 3). This result reveals that people living in semi-basements have weaker health conditions with relatively higher levels of Rn exposure compared with ground living residents. Modern construction practices have produced many buildings that capture, contain, and concentrate Rn to unsafe levels (Cholowsky et al., 2021). This continues to worsen as newer residential buildings are constructed, increasing the innate risks of producing high Rn levels, which are disproportionately impacting younger individuals with children (Gogna et al., 2019; Lorenzo-Gonzalez et al., 2019). Recently, the WHO emphasized that Rn levels are higher indoors and in areas with minimal ventilation, with the highest levels found in places such as caves and mines; this is especially true for some homes where people spend much of their time, thus risking significant exposure to Rn (WHO, 2021). According to WHO (2021), some common ways to reduce Rn levels are increasing under-floor ventilation and sealing floors and walls.
The present study has several limitations. First, the assessment period covered only the summer and the fall seasons because of resource limitations. Thus, evaluation of the influence of seasonal differences, such as those during the winter season, is required to identify seasonal variations in Rn levels in underground environments. Second, air samples of Rn and bioaerosols were not collected outside the residences for comparison between indoor and outdoor levels. Third, we could not identify the specific bioaerosols for CAF and CAB. Thus, further studies should concentrate on the types of bioaerosols that could affect the levels of Rn in residential environments. Despite these limitations, this study found a correlation between the levels of bioaerosols and Rn, although further large-scale studies are needed.