The assessment of TD-GC-FID method and the results obtained from indoor investigations in residential buildings are presented and discussed below.
3.1. TD-GC-FID Method Optimization
The temperature program used to separate VOC analytes was optimized to obtain the best peak resolution. Under the optimized parameters, chromatographic separation was satisfactory, with acceptable tailing observed for all the compounds of interest (Table 3
). Also m
- and p
-xylene, which typically have overlapping peaks and are more difficult to separate [18
], were separated at baseline, making their individual quantification possible (Table 3
Linearity and sensitivity.
For each compound of interest, the method linearity was evaluated within the mass ranges reported in Table 3
. All VOCs exhibited satisfactory results, with correlation coefficients (R2
) of the multi-point calibrations always >0.99 (Table 3
). LOD and LOQ values calculated as mass (ng) and concentration in air (µg/m3
) are shown in Table 3
. Results for our GC-FID method were slightly higher than those reported in other studies using GC-MS instrumentation, as expected [27
]. Despite this, these figures would enable the quantitative determination for all the analytes of interest at the concentration levels typically expected in real samples for the investigated indoor environments.
Tube desorption evaluation. To evaluate the performance of VOCs desorption process from Tenax tubes, a subsequent re-analysis of the already desorbed tubes was carried out, both with spiked tubes and sampled tubes, in order to verify the effective complete desorption and, if necessary, remove any remaining analytes. A desorption efficiency >98% was accounted for all the investigated compounds. The GC chromatograms obtained from a subsequent GC analysis of Tenax tubes after desorption were indeed comparable to the blanks performed on the same tubes immediately prior to their use.
The whole measurement method—from ambient sampling to quantitative analysis—was assessed by calculating the percent relative standard deviation (%RSD) of concentration levels obtained from parallel measurements on four Tenax tubes exposed at an outdoor urban background site. Results revealed very low inter-tubes variability for all the investigated pollutants, with RSDs quite always lower than 5% (Table 3
). Slightly higher values, although in a satisfactory range, were found for benzene (10.4%) and 2-ethylhexanol (12.2%). This could be attributable to the presence of other peaks—probably due to solvent impurities or substances released from Tenax at high temperatures—which eluted in the immediate vicinity of the target molecules, and made inhibited the integration of peak areas, albeit in an insignificant way.
3.2. Vocs Concentrations in Residential Buildings
All the target compounds were quantified above their respective LOQs, with the exception of styrene and benzyl alcohol, that were not detected in the indoor air of two ambient samples. The statistic parameters associated with the mean, standard deviation, median, minimum and maximum values for the investigated pollutants are presented in Table 4
The most abundant VOC in all dwellings was d-limonene, which registered concentration levels often >100 µg/m3
, with a mean of 231 µg/m3
and a maximum equal to 611 µg/m3
. This was expected, since the air monitoring was deliberately performed during the dishwasher washing cycle, and d-limonene is frequently used as fragrance in dishwasher tablets or liquid detergents (Table 2
) and in cleaning products in general because of its pleasant odor [32
All the other compounds were found at concentration levels one or two orders of magnitude lower than d-limonene, and ranged from 3.2–63.1 µg/m3
(toluene), 1.7–38.6 µg/m3
-xylene) to <LOD-5.0 µg/m3
(benzyl alcohol) and <LOD-1.5 µg/m3
(styrene). Benzene, ethylbenzene, o
-xylene, EGBE, 2-ethylhexanol, and α-pinene showed comparable values, with average concentrations between 4.0 and 6.0 µg/m3
Boxplots shown in Figure 1
provide a more detailed view about the variability in the indoor VOCs concentrations for the nine investigated homes. The highest 75th percentile was observed for d-limonene, followed by toluene and m
-xylene (363.0, 48.1 and 20.7 µg/m3
, respectively). All the target compounds showed a certain concentration variability among homes (coefficients of variability always >60%) and, in some cases (e.g., α-pinene, 2-ethylhexanol, p
-xylene), the observed variability was emphasized by the presence of one or two concentrations in the data that were clearly higher than all the other corresponding values (Table 4
and Figure 1
). Nevertheless, the available information (concerning, for example, the occupants’ habits or the specific household activities carried out during sampling) were not enough to interpret and explain these higher values, which, anyway, have been reported in residential or other environments [3
]. For this reason, all data were included and presented in Figure 1
As previously stated, the high concentrations and variability shown for d-limonene could likely reflect the heterogeneous daily habits of occupants, in particular in their use of cleaning products during the dishwasher washing cycle and also for the general cleaning of homes and through the use of personal care products.
Compounds including benzene, toluene, ethylbenzene and xylenes (BTEX) are generally ubiquitous in the indoor environment. The Spearman correlation analyses performed on the target pollutants considering the nine investigated homes always revealed high and statistically significant associations among BTEX (Spearman correlation coefficients > 0.75; p
< 0.02), suggesting a potential common source for these compounds, which is most likely outdoor air pollution. Indeed, different studies clearly showed the great influence of outdoor emissions (e.g., fossil fuel combustion by traffic-related or industrial sources) on the indoor occurrence of these chemicals [37
]. Nevertheless, at the same time, specific indoor sources of BTEX may also exist. Benzene has been widely used as an industrial solvent in paints and varnishes, and the most relevant indoor sources are tobacco smoke and incense burning [40
]. Toluene was found in a variety of household products such as paints, cleaning agents, adhesives, nail polishes and other cosmetics. Xylenes are widely used in the chemical industry for products such as paints, inks, dyes, adhesives and detergents [41
], and they are also emitted as result of cigarette smoking [40
]. Also, styrene is a widely-used VOC in a number of products, such as plastics and almost all carpets that have a synthetic backing [42
]. Ethylexhanol is a typical secondary VOC derived from the alkaline hydrolysis of some compounds (e.g., carboxylic acid esters and polymers) present in different materials as carpets and vinyl floorings [43
In terms of health concerns, among all the target VOCs, only benzene is classified as human carcinogen of Group I (genotoxicity and acute myeloid leukemia) from the International Agency for Research on Cancer [14
]. For this genotoxic compound, no safe levels of exposure can be recommended, and according to the WHO, the unit risk of leukemia per 1 µg/m3
of benzene in air is equal to 6 × 10−6
]. Therefore, from a practical standpoint, since there is no known exposure threshold, it is expedient to reduce indoor exposure concentrations to as low as possible, by eliminating or at least reducing the human activities that release benzene (e.g., smoking tobacco). Also adequate ventilation systems could help to control the indoor penetration of outdoor benzene, for example, by positioning inlets for fresh air on the least polluted side of the building [14
The other investigated compounds, such as terpenes and aromatic organic molecules, could cause irritant effects after dermal contact, and their inhalation may be related to respiratory endpoints like nose and throat irritation, palpitation, dizziness, chest pain, bronchitis or nephritis [46
]. For such chemicals, specific WHO air quality guidelines are not available, but different health-based limit values above which humans should not be exposed to avoid the occurrence of such effects exist. Table 4
shows, for example, the available Derived No-Effect Levels (DNELs) related to short- and long-term exposure via the inhalation route for the general population (consumers and humans exposed via the environment) [47
]. The field monitoring campaign presented in this paper was just a simple case-study, limited in terms of sampling period, number of samples and representativeness of indoor exposure levels. Nevertheless, obtained results (some µg/m3
) were always from three to four orders of magnitude lower than their respective DNELs (some mg/m3
) (Table 4
In the framework of the EU EPHECT Project, a detailed health risk assessment was performed and health-based Critical Exposure Limits (CELs) were derived for five selected pollutants of respiratory health relevance, including d-limonene and α-pinene [49
]. Regarding d-limonene, a short-term CEL of 90 mg/m3
was established for sensory irritation, whereas the long-term limit of 9 mg/m3
was derived by extrapolation from short-term data, applying an assessment factor of 10, following a conservative based-approach. For α-pinene, the short-term and long-term CELs were established to be 45 mg/m3
and 4.5 mg/m3
, respectively [49
]. Also in this case, the concentration values obtained in our study for both compounds were well below the proposed CELs.
Nevertheless, in this regard, it is noteworthy that the concentrations of terpenes, although well below the derived DNELs or CELs, should not be totally underestimated—especially in the case of d-limonene—because of the possible gas-phase reactions which these compounds can readily undergo in the presence of ozone, with the formation of different chemical species which are potentially harmful for the human respiratory health [6
]. Indeed, as explained in the introduction section, terpenes—and, for instance, limonene—are able to rapidly react with ozone in the atmosphere to produce a complex mixture of gaseous oxygenated compounds like formaldehyde, acetone and hydrogen peroxide, as well as different gases having a low vapor pressure (e.g., multifunctional carbonyls and acids). The low vapor gases can subsequently self-nucleate to generate ultrafine particles (<0.1 µm) or condense on existing larger particles, leading to the formation of secondary organic aerosols (SOAs) [6
3.3. Comparison of Vocs Concentrations with Other Studies
VOC concentrations from some previous studies performed in residential buildings worldwide are presented in Table 5
. Unlike our survey, in which a specific case-study related to short-term exposure to dish washer emissions was examined, the reported investigations generally referred to long sampling intervals (from some hours to some days), because this approach is generally assumed to be more representative of the actual personal exposure than short-term measurements. Moreover, looking at the scientific literature, similar investigations on IAQ during and following dishwasher cycles were not found. Therefore, from this point of view, the comparison could be somewhat impaired, at least for those chemical species whose concentration levels are expected to be influenced by this type of domestic activity.
Regarding terpene compounds, d-limonene and α-pinene can be found in the literature in a wide range of concentrations, with mean values that can vary from few to hundreds of µg/m3
. In a large survey performed in England, Raw and collaborators registered concentrations of d-limonene from 0.1 to 308 µg/m3
, with an average geometric mean of 6.2 µg/m3
]. A more recent study carried out in English residential buildings showed concentration values between 18 and 1439 µg/m3
, with one of the highest domestic values reported in the literature for d-limonene [4
In the AIRMEX study, the authors identified α-pinene and d-limonene as being predominantly derived from indoor sources, with mean concentrations of 14.5 and 29.2 µg/m3
observed in homes, respectively [18
]. Similarly, 22 studied homes in Puertollano, Spain, registered average concentrations of d-limonene and α-pinene equal to 17.1 and 18.5 µg/m3
], while in 53 indoor environments in Michigan, d-limonene was monitored with mean and maximum values of 25.7 and 258.5 µg/m3
]. Also Schlink et al. observed in German residential rooms a mean value of d-limonene equal to 32.9 µg/m3
]. In our survey, the average level monitored for d-limonene was generally one order of magnitude higher (231.5 µg/m3
) than those in the cited literature because of the experimental approach used, which led this result to be more representative of short-term peak concentrations reached during a specific household activity.
For all other compounds, the indoor measurements in Como homes were generally in the same range of other cities around the world [37
]. Only indoor concentrations of toluene were slightly higher than those registered in Spain [52
], Argentina [56
] and in different cities across Europe [18
] (Table 5
3.4. Strengths and Limitations of the Study
As extensively highlighted in the scientific literature, research is needed to evaluate potential risks associated with the inhalation exposure to consumer products, to protect and promote health and well-being in indoor environments. Moreover, it is also underlined that product emission rates, but also typical household uses, should be investigated, following not only the combined use of consumer products during the day but also any single product use, in order to obtain an accurate risk assessment in case of consumer products [49
The present work focused on the specific use of cleaning agents which, to our knowledge, has never been examined before as an individual case-study in terms of potential exposure to indoor volatile compounds.
Nevertheless, the experimental design is affected by some drawbacks and a lack of background information, which did not allow us to provide quantitative relationships or estimations (about the indoor VOCs variability, for example). The nine investigated homes were randomly selected, without considering differences in house location, house design and ventilation systems. Information concerning the occupants’ habits, the specific household activities carried out during sampling; as well as outdoor measurements, air exchange rates and deposition factors were not gathered. The number of collected samples was limited. Finally, from an analytical point of view, more in-lab experiments and inter-laboratory comparisons should be added to better assess the method’s reproducibility, precision and accuracy.
Therefore, for all these reasons, the obtained results should be taken as part of an exploratory survey, which could represent the starting point for more in-depth IAQ investigations. Moreover, relevant information in terms of short-term exposure to irritants could also be obtained from punctual peaks of concentrations, for which direct-reading instruments and lower time-resolution studies would be optimal.