When we count the hours spent sleeping, working or at school, we find that humans spend most of their time in confined spaces. This time amounts to around 90% of the time for people in developed countries, and is even greater for vulnerable sectors of the population (young children, people with weakened health, or seniors). Therefore the indoor air quality (IAQ) is a significant problem that needs to be addressed [
1,
2]. Personal exposure to VOCs has been investigated in a limited number of studies. Gokhale et al. studied the exposure to VOCs in homes, offices and outdoor in Leipzig (Germany). The highest proportion of personal exposure was from households (42–73%), followed by the outdoor environment (18–34%), and offices (2–38%). Benzene, dodecane, decane, methylcyclopentane, triethyltoluene as well as trichloroethylene prevailed in the outdoor environment, while methyl cyclohexane, triethyltoluene, nonane, octane, tetraethyl toluene, and undecane had the highest concentrations in the offices and whereas a group of terpenoides such as 3-carane, limonene, α-pinene, β-pinene, and the aromatic compounds toluene and styrene had the greatest impact in households [
3]. Several other studies [
4,
5,
6] have also pointed to the fact that at home we spend about 60–70% of our time at home. Due to this fact, households are among the most investigated microenvironment in terms of IAQ (resp. the occurrence of VOCs and other pollutants). However, the prevalence of individual VOCs as well as their levels are different and vary not only within countries or cities, but also within households themselves (i.e., in different rooms). The most abundant VOCs the indoor air of Puertollano in Spain were formaldehyde and hexanal, followed by butanal, acetone and acetaldehyde in the indoor air of Puertollano in Spain. On the basis of the indoor/outdoor ratio (I/O ratio) it has been found that the presence of sources in indoor environment is common for limonene, α-pinene, hexanal, formaldehyde, pentanal, acetaldehyde, o-xylene, nodecane, and acetone [
7]. The risk level for 93 chemicals polluting the indoor air was calculated in Japan. Formaldehyde, acrolein, 1,4–dichlorobenzene, benzene, tetrachloroethylene and benzo (a) pyrene were ranked in the highest risk category [
8]. TVOC concentrations were low, however, about four times higher than in outdoor air, indicating the dominant influence of indoor sources in the established apartments in Melbourne (Australia). Much higher concentrations were observed in new or renovated buildings [
9]. High concentrations of VOCs are more often reported in newly built than in already occupied residential buildings [
9,
10,
11,
12]. However, VOCs levels in many new residential buildings are declining over time due to the fact that the emission strength of structures and furniture decreases with time [
13]. This is supported by several articles [
10,
12], which have dealt with the long-term course of VOCs concentrations in new residential buildings after the users moved in. In these studies, VOC measurements were repeated for three years with an annual interval. The results showed that most VOC concentrations in new households did not show similar levels to older households after two years. Järnström et al. [
11] repeated their VOC measurements in newly completed residential buildings during more than a year with an interval of less than six months and found that the most significant decrease in concentrations occurred during the first six months. In addition, the VOCs coming from the construction phase were replaced by new ones the longer the building was use. The same issue was also addressed by Shin and Jo [
14], who monitored the development of VOC concentrations for 24 months with a monthly measurement interval in 25 households in different new residential homes. Both TVOC and VOC concentrations showed a decreasing trend during this period. The average TVOC concentration in the first month of measurement was 881 µg/m
3 while in the last month was 432 µg/m
3. Floor coverings/coatings were the most influential indoor source of VOCs followed by cleaning agents, wood paneling/furniture, paints and moth repellents.
TVOC concentrations in newly decorated rooms ranged from 650 to 690 µg/m
3 in Hangzhou (China). The characteristics of the emission source were a key factor influencing the concentration. In addition, the levels were influenced by temperature, humidity, time from the end of the decoration to sampling as well as the time at which the windows and doors were closed before sampling while temperature and humidity were less important factors [
15]. Noris et al. investigated the impact of building reconstruction in order to reduce the energy demands on the indoor environmental quality (IEQ) in 16 apartments (eight apartments with continuous mechanical ventilation and eight apartments without mechanical ventilation). Their results indicate an improvement in the levels of chemical pollutants including VOCs in the indoor air. In general, apartments with continuous mechanical ventilation showed a more pronounced improvement in IEQ than apartments without this system [
16]. The indoor air in 20 new passive houses and 21 new regular houses in Sweden were evaluated by Langer et al. [
17] in Sweden. Significant differences in TVOC and formaldehyde concentrations between passive houses and regular houses indicate the presence of substantial TVOC sources in passive houses, while source of formaldehyde may be more pronounced in regular houses.
In another study, Langer and Bekö [
18] investigated the Swedish Housing Stock as well as the relationship between building characteristics and IAQ. Higher concentrations of TVOCs as well as formaldehyde were found in family houses than in dwellings, as well as in dwellings built in between 1955 and 1980 than in new or older apartments. TVOC concentrations were higher in rural areas comparted to cities and in the apartments with natural ventilation compared to those with mechanical ventilation. A significant negative correlation between air exchange and TVOC as well as formaldehyde concentrations reflects the ability of ventilation to reduce the indoor exposure to these compounds. The sum of VOCs in New Delhi (India) ranged from 33.6 to 107.2 µg/m
3. Higher concentrations were found in the living rooms, followed by kitchens and bedrooms [
19]. Dodson et al. [
20] investigated the impact of cellars, garages, and common corridors on VOCs in households. Concentrations in garages were 5–10 times higher than the median concentrations of benzene, toluene, ethylbenzene and xylenes in the indoor environment. The ratio of concentrations in cellars/households was significant for methylene chloride, ethylbenzene, m/p-xylene, o-xylene, and summer ratios tended to be higher than winter ratios. Approximately 20–40% indoor concentrations were associated with petrol sources, such as methyl t-butyl ether, benzene, toluene, ethylbenzene and xylenes for households with attached garages. The cellars contributed to approximately 10–20% of the indoor concentrations. For apartments, approximately 5–10% of indoor concentrations were associated with air from corridor. The use of LPG stoves had more significant impact on VOC concentrations than the use of natural gas stoves in Hong Kong [
21]. Guo et al. [
22] found that formaldehyde concentrations correlated with the age of the building, while this trend was not observed for VOC concentrations. In the study [
23], observed that the type of ventilation system and flat placement correlated with formaldehyde concentrations.
More than 70% of total VOC concentrations in indoor air in Edmonton were assigned to indoor sources where households products were the main contributor, followed by combustion process with tobacco smoke, deodorants, and construction materials. The main outdoor sources of VOCs were the oil and gas industry, transport emissions and biogenic emissions [
24]. Building materials were the largest contributor to VOC concentrations in households in Hong Kong, followed by air freshener products, household products, mothballs and painted wood products [
25]. Increased concentrations of VOCs in Michigan (USA) were associated with eight sources or activities: the presence of attached garage, recent renovations, older residences, smoking inside, fewer windows and doors, higher CO
2 concentrations, and a lower level of ventilation [
26]. Ohura et al. [
27] compared the quality of indoor (living rooms, bedrooms and kitchens) as well as outdoor air in Japan and China during the summer and winter season between 2006 and 2007. Concentrations of many target VOCs (benzene, toluene, ethylbenzene, xylenes and trimethylbenzene) were significantly higher in China than in Japan. Indoor VOC levels in Japan were similar to those outdoor, while in China they tended to be higher. Outdoor sources, including transport and industrial emissions, as well as human activity, were significant sources of VOC pollution. As can be seen, the results from different countries, cities differ in their conclusions, identified VOC or TVOC levels. Overview of TVOC levels in residential buildings in different countries/cities is illustrated in
Figure 1.