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Volatile organic compounds (VOCs) are all organic compounds with a boiling point between 50°C and 250°C (WHO, 1989), although there is no clear and widely accepted definition. Although more than 900 VOCs have been identified at detectable levels in indoor air, about 250 chemicals have been recorded at concentrations higher than 1 ppb (Nathanson, 1993). The presence and magnitude of a wide variety of VOCs can be affected by different factors, which increase the complexity of indoor air quality. The VOCs most commonly detected in indoor air are benzene, ethylbenzene, tetrachloroethylene, trichloroethylene, toluene, o-xylene, and m,p-xylenes (Etkin, 1996).

Many researchers are currently using the concept of total volatile organic compounds (TVOC), since the identification and measurement of individual VOCs are expensive and time-consuming and some compounds are difficult to identify or measure because of their very low concentrations. Indoor TVOC and VO Cs concentrations are often significantly higher than those outdoors (Salonen et al., 2009; Brown et al., 1994; Wallace et al., 1991). Many indoor VOC sources exist, including outdoor sources, human activities, building materials, furniture, and other indoor products (Nazaroff and Weschler, 2004; Ekberg, 2003; Edwards et al., 2001; Hodgson et al., 2000; Wolkoff, 1995). Due to various source strengths and ventilation conditions, estimation of indoor VOC concentrations is difficult.

TVOC levels are generally associated with general indoor air quality (Molhave et al., 1997). VOCs are frequently investigated when bad indoor air quality is suspected. Many VOCs are known to have acute and chronic adverse effects on human health and comfort (Molhave, 1991). Some VOCs are associated with the perception of odors. Adverse health impacts include the irritation of mucous membranes, mostly of the eyes, nose, and throat, and long-term toxic reactions of various kinds (ECA-IAQ, 1991). However, it is difficult to conclude that TVOC is a predictor of health risks as they represent only the sum of the mass concentrations of VOCs at the low exposure levels typically encountered in nonindustrial indoor air (Wolkoff and Nielson, 2001; Andersson et al., 1997; Molhave et al., 1997). The results of the few reported controlled human exposure studies and epidemiological studies have confirmed that health effects and outcomes were often inconsistent (Molhave et al., 1997).

Although the TVOC concept is widely used, information on the individual VOCs concentrations typically present in public spaces is needed for the interpretation of indoor air measurements. The purposes of our study were to establish profiles for 91 individual VOCs in indoor public places in Korea and to determine seasonal variations in their concentrations. In addition, statistical analyses were conducted to determine correlations between individual VOCs.

The mean TVOC concentrations in public spaces were 782±1084 μg/m³ in summer and 540±380 μg /m³ in winter. The mean TVOC concentrations were slightly higher in summer than in winter, although they were not significantly different (Paired t-test, p=0.14). Cumulative distributions of TVOC in summer and winter are shown in Fig. 1. In summer, the highest concentrations were observed in the preschool (1718 μg/m³), health care facility (1709 μg/m³), art gallery (1667 μg/m³), and elderly welfare facility (1046 μg/m³). In winter, the highest concentrations were observed in the airport (2096 μg/m³), underground market (854 μg/m³), and health care facility (839 μg/m³).

Fig. 1. 
Cumulative distribution of TVOC in public places in summer and winter.

In summer, acetone, methylethylketone, benzene, toluene, ethylbenzene, m,p-xylenes, styrene, and naphthalene were detected in all samples from the 50 locations. Another 33 chemicals were detected in more than 80% of samples, 11 chemicals were detected in less than 20% of the samples, and 21 chemicals were not detected in any samples. The individual VOC concentrations of 10 μg/m³ or more were toluene (316.7 μg/m³), acetone (44.1 μg/m³), hexane (41.5 μg/m³), isopropyl alcohol (36.6 μg/m³), m,p-xylenes (19.2 μg/m³), ethyl acetate (14.5 μg/m³), ethylbenzene (13.6 μg/m³), methyl ethyl ketone (13.6 μg/m³), and nonanal (10.1 μg/m³). These nine compounds comprised 64.8% of TVOCs. The individual VOC levels in summer are shown in Table 2.

In winter, toluene, acetone, m,p-xylenes, ethylbenzene, benzene, and styrene were detected in all samples from the 49 locations. Another 18 chemicals were observed in more than 80% of the samples, 14 chemicals were found less than 20% of the samples, and 27 chemicals were not detected in any samples. Compounds with individual VOC concentrations of 10 μg/m³ or more were toluene (109.9 μg/m³), isopropyl alcohol (36.6 μg/m³), acetone (31.0 μg/m³), m,p-xylenes (18.8 μg/m³), tetrachloroethylene (17.6 μg/m³), limonene (16 μg/m³), ethylbenzene (15.9 μg/m³), hexane (11.0 μg/m³), and methyl ethyl ketone (11.2 μg/m³). These nine compounds comprised 49.6% of TVOCs. The individual VOC levels in winter are shown in Table 3.

Some compounds had occasional high values. This was apparent for isopropyl alcohol in which the mean was about 8 and 10 times larger than the median. Three compounds with medians greater than 10 μg/m³ were toluene, acetone, and m,p-xylenes. Another six compounds had medians greater than 5 μg/m³ in summer: methyl ethyl ketone (8.1 μg/m³), nonanal (7.5 μg /m³), ethyl acetate (7.0 μg/m³), ethyl benzene (6.8 μg/ m³), 1-butanol (6.7 μg/m³), and hexane (5.9 μg/m³). Four compounds had medians greater than 5 μg/m³ in winter: benzene (6.7 μg/m³), ethylbenzene (6.4 μg/m³), limonene (5.9 μg/m³), and ethyl acetate (5.6 μg/m³).

Several compounds were closely correlated with other compounds. The criteria for determining correlations between individual VOCs were an R>0.9 and a P-value (or significance probability value) <0.001. Based on these criteria, the correlations of different individual VOCs in summer and winter are summarized in Tables 4 and 5, respectively. Among the 91 targeted VOCs in this study, 18 VOCs showed strong cor-relations with other VOCs in summer, including ethylbenzene, m,p-xylenes, o-xylene, isopropylbenzene, tert-butylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, heptane, toluene, 2-ethyltoluene, 3-ethyltoluene, 4-ethyltoluene, ethyl acetate, dodecane, tridecane, naphthalene, and 2-chlorotoluene. In winter, 13 VOCs showed strong correlation with other VOCs, including m,p-xylenes, o-xylene, isopropylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, 2- ethyltoluene, 3-ethyltoluene, 4-ethyltoluene, n-propylbenzene + 4-chlorotoluene, decane, and limonene.

In Korea, the indoor air quality of public places is regulated by the Indoor Air Quality Control in Public Use Facilities Act. The recommended TVOC level is 500 μg/m³. VOC levels should be measured once every 2 years and maintained below the guideline. When the VOCs were measured at 50 locations during the summer, 25 locations exceeded the guideline. When the VOCs were measured at 49 locations during the winter, 17 locations exceeded the guideline. Thus, a significant proportion of indoor public places were noncompliant with regulations.

In this study, 72 individual VOCs were identified from indoor air samples. The number of individual VOCs in one indoor air sample can be as many as 250 (Nathanson, 1993). However, 20-30 compounds account for 50-75% of the TVOC in indoor air samples (Molhave et al., 1997). In this study, nine VOCs with more than 10 μg/m³ accounted for 64.8% of the TVOC in summer and 49.6% in winter. Seven VOCs (toluene, acetone, hexane, isopropyl alcohol, m,p-xylenes, ethylbenzene, methyl ethyl ketone) were detected at levels of more than 10 μg/m³ in both seasons. Ethyl acetate and nonanal were included in summer and tetrachloroethylene and limonene were included in winter. In particular, the source of the limonene may have been the high consumption of tangerines in Korea.

Six VOCs (toluene, acetone, m,p-xylenes, ethylbenzene, benzene, styrene) were detected in 100% of the samples in summer and winter. In summer, methylethylketone and 1-butanol were also detected in all samples. Although many VOCs are present in indoor air, the dominant VOCs in indoor air are toluene, m,p-xylenes, ethylbenzene, and benzene, and the dominant VOC profile recorded in this study agreed with those reported for nonresidential spaces (Salonen et al., 2009; Tang et al., 2005; Chao and Chan, 2001; Baek et al., 1997). Based on the location and type of building, some other VOCs may be present. In mechanically ventilated buildings in Hong Kong, chloroform and trichloroethylene were also found in 100% of the samples (Chao and Chan, 2001). When VOCs were measured in problematic buildings, the most abundant VOCs were 2-(2-ethoxyethoxy)ethanol, acetic acid, 1,2-propanediol, and toluene (Salonen et al., 2009).

Several studies have reported on individual VOCs in public spaces. We summarized the indoor concentrations of selected VOC species and compared them with those in other regions, as shown in Table 6 (Eklund et al., 2008; Tang et al., 2005; Chao and Chan, 2001; Kim et al., 2001; Baek et al., 1997). The VOC concentrations showed similar trends. In our study, the toluene concentration during summer was the highest. A shopping mall in Ghangzou reported high concentrations for almost all species (Tang et al., 2005). In Korea, new apartment buildings have guideline levels for benzene (30 μg/m³), toluene (1000 μg/ m³), ethylbenzene (360 μg/m³), xylenes (700 μg/m³), and styrene (300 μg/m³) before occupation. Currently, no specific guidelines have been established for individual VOCs in public spaces in Korea. As BTEX was dominant in both detection frequency and concentration levels, considering the implementation of air quality guidelines may be necessary.

When we determined correlations between individual compounds, 19 and 14 VOCs showed strong correlations with other VOCs in summer and winter, respectively. Many compounds were included in both seasons: m,p-xylenes, o-xylene, isopropylbenzene, 1,2,3- trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, 2-ethyltoluene, 3-ethyltoluene, and 4-ethyltoluene. Alkylbenzenes are well known as anthropogenic chemicals coming from the vehicular emissions of gasoline burning in spark-ignition engines (Chao and Chan, 2001). The main components of these gasoline emissions are benzene, toluene, ethylbenzene, m,p-xylenes, o-xylene, p-ethyltoluene, and 1,2,4-trimethylbenzene (Oelert et al., 1974). Therefore, we suggest that indoor spaces in Korea are being affected by the infiltration of polluted outdoor air.

The sampling of VOCs can be affected by various conditions. One of the critical factors is the adsorbent tube used. When two types of adsorbent tube [a single adsorbent of 300 mg Tenax-TA and a double adsorbent of Tenax-TA in front (100 mg) and Carbotrap (200 mg)] were compared, the recorded TVOC levels were comparable, but the single tube showed slightly higher levels. Relative percent differences between the two methods indicated that the single tube may collect larger amounts of VOCs. A tube with Tenax-TA and Carbotrap was validated in experiments and field study (Kuntasal et al., 2005). The tube showed high recoveries, in the range of 80-100% and MDL from 0.01 to 0.14 ppb. The sampling method also showed good linearity (R²>0.99) and precision (<8%) values (Kuntasal et al., 2005). Two adsorbent tubes showed precisions of 20-30% for most aromatic VOCs (Baek and Moon, 2004).