A framework to interpret passively sampled indoor-air VOC concentrations in health studies
Introduction
A crucial factor in epidemiological studies of environmental health effects is the correct quantification of personal exposure. In particular, when exposure to indoor-air contaminants, such as volatile organic compounds (VOC), is considered, it is common practice to assess the exposure only indirectly by studying participants’ responses to questionnaire items; advanced analyses utilize measurements made at certain time-points. In view of the variability of VOC concentrations, a frequent exposure misclassification in health studies is likely.
Information regarding the respiratory health risks of indoor-air volatile organic compounds (VOC) in children is scarce and conflicting [1]. Fuentes-Leonarte et al. [2] found just three epidemiological studies and concluded that these studies, firstly, report inconsistent results. Diez et al. [3] and Rumchev et al. [4] found adverse effects, while Venn et al. [5] could not observe significant effects. Franck et al. [6] identified significant associations between floor covering during pregnancy and the risk for physician treated wheeze in children suffering from atopic predisposition. Deng et al. [7] observe an increase of childhood asthma with new furniture and home decoration and pose the question of an adverse exposure time-window. Secondly, these studies often (but not always) only used proxy variables (such as recent renovation or painting activities) as an indicator for exposure.
Important reasons for the ambiguous findings are methodological difficulties of the performed studies [8]: VOC exposure is often estimated qualitatively and measurements are, at best, of short duration only and rely on a sampling period of several hours (with active sampling) to several days (with passive sampling). As queried health complaints usually refer to a longer time-basis (e.g. yearly disease prevalence), this can lead to random misclassification of exposure levels and to an inaccurate estimation of health risks [9]. Franck et al. [6] demonstrated that VOC concentrations registered during pregnancy are more significantly associated with wheezing symptoms than VOC levels measured during 12 months after birth.
Necessarily, a reliable analysis of health effects has to consider the VOC exposure levels not only over a short period, but also the peak and cumulative levels throughout the past year and, therefore, the variability of VOC concentrations have to be taken into account. Such a thorough consideration of exposure would be in line with the novel “exposome” approach to studying and preventing non-communicable diseases [10].
In the indoor environment, VOC concentrations are influenced by (i) building materials, furniture, or other equipment [11]; (ii) humans themselves or human activity, like cleaning, usage of copying machines, smoking, cooking, renovation or construction activities [12]; and (iii) outdoor concentrations, e.g. from car traffic or industry [13]. Among the factors that have an important impact on the VOC pollution in a flat are the socioeconomic status of the inhabitants [14] and renovation or redecoration activities ([15], [16]). In the aftermath of such renovation actions, a decay of concentrations is expected. While this effect was analyzed in some experimental and chamber studies ([17], [18], [19]), qualitative and quantitative information about the decay of VOC concentrations in real and occupied apartments is scarce in the international literature ([20], [21]) but necessary for a reliable exposure assessment. Although many factors influencing the indoor-air concentration of VOCs have been identified ([22], [23], [24], [25], [26], [27], [28], [29], [30]), the consequences of the variability of concentrations for an assessment of cumulative and peak personal exposure and respiratory health risks have not yet been considered. A framework is needed for the adequate utilization of VOC measurements in epidemiological studies.
The present study aims to bridge the different time-scales between exposure and health records and suggests a pre-processing of VOC concentrations that can be used by a public health researcher before assessing the respiratory health risks in an epidemiological study. For that purpose, the temporal course of the exposure must be either measured continuously, or, at least, approximately reconstructed on the basis of influencing factors and single measurements. To discuss this exposure-reconstruction in a quantitative manner, we use data of 47 VOCs and 54 influencing factors (Table 1, Table 3) repeatedly gathered in flats in Leipzig, Germany.
The purposes of our study are (1) to quantify the temporal change of VOC concentrations after redecoration activities, such as renovation, new furnishing, and painting, (2) to estimate decay rates and half-life times for 47 airborne organic compounds, (3) to specify relevant influencing factors and to estimate their impact, and (4) to suggest and demonstrate a framework for assessing peak and cumulative exposure levels from field data measured in occupied apartments for use in health effect studies and risk assessment.
Section snippets
Study design
The present analysis was carried out as part of an on-going birth cohort study in Leipzig, Germany, in which the children and, therewith, the apartments have been randomly selected (details in supplementary material S1). Participation in the study was on a voluntary base; we did not make any exclusion on grounds of the inhabitants’ activities, such as smoking or recent renovation, or due to health problems or complaints of the occupants. Study participants were followed up regularly with VOC
Ranking of factors governing the VOC’s variability
For the identified 13 relevant factors we analyzed their influence on the VOC concentrations (supplementary material S3). To compare the effects of these factors, we quantified their variances in an ANOVA table (Table 2). Results show that the different flats cause the predominant part of variance (e.g., ∼89% for α-pinene and ∼61% for dodecane). Such strong inter-apartment variations are comprehensible. We conclude from this finding that, for each flat, at least one measurement is needed to
Conclusions and limitations
Though many influencing factors for indoor-air concentrations of VOCs have been researched during the last decade, information about respiratory health risks of VOCs is still insufficient and conflicting. Having in mind that VOCs accumulate in the indoor environment due to emissions from building materials, furniture and human activities, it is desirable to come to a conclusion about potential health risks caused by exposure to concentration levels existing in occupied apartments [46]. As a
Acknowledgements
This research was partly financially supported by the European Union (European Commission, FP6 project NoMiracle, grant number 003956). The study was approved by the Ethics Committee of the University of Leipzig (file ref. 046–2006). The technical help of Melanie Nowak and Brigitte Winkler is gratefully acknowledged.
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Current address: Institute of Forensic Medicine, University Hospital Jena, Friedrich-Schiller-Universität Jena, Bachstraße 18, 07743 Jena, Germany.