The Interaction of Volatile Organic Compounds with Indoor Materials - Consequences for Indoor Air Quality

   
Project leader: Hans Stymne.
PhD student: Peter Hansson.
Supported by: University of Gävle and KK-Foundation

KK-Fpundation logga

Introduction

 
Many gaseous contaminants in indoor air interact with the materials they come in contact with. The prime effect when the concentration of contaminants increases due to release in the air is that the chemical compounds are adsorbed on the material surfaces. The amount adsorbed depends on the chemical and physical characteristics of the surface and the compounds adsorbed. The rate of adsorption depends on how far the concentration in the air is from chemical equilibrium. In many materials, especially those which are porous, the compounds adsorbed continue to penetrate into the interior of the material, and become more or less strongly bonded. This absorption within the material also depends on the chemical and physical properties of the compound and the material. In this way, contaminant depots can build up within materials due to indoor contaminant release. When the contaminant level in the air decreases again due to ventilation, an imbalance in the opposite direction is created. The substances in the depots begin to migrate back towards the surface and are slowly re-emitted to the air. The depot-effect therefore creates an extension of the exposure time for contaminants. Considerable depots may build up within the materials, which then act as more or less continuous secondary sources, especially when the contaminant level increases repeatedly.  

Repeatedly brief releases of contaminants may for example be due to different types of indoor activities. The binding of contaminants by building materials may at first sight be seen as an advantage. The depot-effect initially decreases the concentration in air. The secondary emission, however, continues over a long period of time and may lead to an increased accumulated exposure dose for anyone in the indoor environment.  

Although the existence of these effects has long been known there has been a lack of sufficient physical-chemical data on the interaction between different compounds and materials. Researchers have therefore been unable to draw any certain conclusions as to the importance of these effects for indoor air quality and the corresponding health consequences. The goal of this project is both to develop such a mathematical model, which allows description of the interaction in a satisfactory way, and to determine material parameters, needed for the model. From this, conclusions can be drawn on the importance of the depot-effect on indoor air quality and subsequently also on the risk involved in using various consumer chemicals in relation to the effects of primary emissions from building and other indoor materials - a matter which is considerably more thoroughly investigated.  

Objective


The objective of this study is to investigate the importance of the depot (sink) on the contaminant exposure in real rooms. This shall be achieved through performance of experiments and through development of mathematical models. The results will help in answering questions like: "How much of the contaminant exposure indoors is related to compounds released from depots?" and "Is it possible to decrease the exposure indoors by choosing building materials with small sink capacity?". The methodology and mathematical models developed in an earlier project will be utilised in conjunction with fluid and mass transfer models for real rooms, (CFD and compartment modelling). The models will be verified by performing experiments in a medium sized climate chamber and in a full scale room.

Test specimens


The materials and contaminants chosen for experimental studies shall be regarded as model cases. We will concentrate on the following two common building materials in this study: gypsum board respectively wood particle board with and without paint applied to the surface. The volatile organic compound toluene is chosen as contaminant, because it is a common contaminant in indoor air having consequences for health.

Method


The project is based on an earlier (ongoing) project, aimed at developing an experimental methodology for determining the material parameters of interest. This methodology and additional techniques subsequently developed will be utilised in the project to determine the necessary material parameters for common indoor contaminants and materials.  

The depot effect has been studied experimentally using a dynamical method in a small exposure chamber where the test material is exposed for contaminated air at a given temperature, humidity and velocity. However, additional experiments are planned in the present project, in order to improve the understanding and modeling:

  • Development of a static method for studying the mass transport within a material (i. e absorption and diffusion parameters).
  • Experiments for studying how the pre-history of materials influences the depot-effect.
  • Experiments for studying how sorbed water vapour influences the sorption rate of VOCs.

Much emphasis has been placed on the development of a mathematical model, based on fundamental physical processes, to interpret the experimental results. This model is the basis for quantifying the adsorption/desorption process in the real environment. The depot models will therefore be coupled with compartment and fluid dynamics models (CFD) for ventilated environments in the present project.  

The development of models will be made with support from experiments in a new medium sized climate chamber which is planned to be constructed within the project. Lastly, validation of experiments will be performed in full-scale experimental rooms in the laboratory.

Contributors
The project is carried out in conjunction with the Department for Built Environment, University of Gävle, partly financing the project with funds from the Swedish Council for Building Research (BFR). The project leader Associate Professor Hans Stymne, equipment and laboratory staff services at the department participating in the project, are financed in this way.   The work is carried out in Gävle at the Department for Built Environment. A specially designed laboratory with mass-spectrometry, gas chromatography, exposure chambers etc. has been constructed for the purpose.

Results


The results from the preceding project show that it is possible to interpret the basic physical processes involved i.e. boundary layer diffusion, surface adsorption, surface desorption, internal diffusion and internal absorption, through fitting of the model parameters to experimental data. Gypsum board is a building material which shows high diffusion for toluene vapour, the high porosity of gypsum board being the probable explanation. The propensity for accumulation of toluene is small compared to other VOCs. The large permeability for toluene may have a more significant effect on the air quality in real rooms.
The propensity for accumulation and diffusion is reduced, when a layer of paint is applied to the surface of the gypsum board.

Expected achievements


A substantial contribution to the understanding of the importance of the depot-effect for indoor air quality.
A good foundation for conclusions regarding the risk factor in using consumer chemicals in the indoor environment.
A possibility to incorporate mathematical models for the depot-effect in simulations of air quality in ventilated buildings.
The work is planned to produce several scientific publications as well as a doctoral thesis and PhD examination for Peter Hansson.

Published papers

  • Peter Hansson, Hans Stymne (1998). "Adsorption and desorption of air contaminants on indoor surfaces". Proceedings of the CIB World Building Congress 1998, June 7 - 12, Gävle - Sweden, Symposium B pp 1255 - 1263.
  • Peter Hansson, Hans Stymne (1998) "Determination of material parameters for sorption of gaseous contaminants on indoor surfaces". Proceedings of the 6th International Conference on Air Distribution in Rooms, Roomvent ´98, June 14 - 17, Stockholm - Sweden, vol 1 pp 513 - 520.
  • Hansson P., Stymne H. (1999) On the sink effect of VOCs on indoor materials —consequences of repeated adsorption. Proceedings of the 8th International Conference on Indoor Air Quality and Climate, Indoor Air ´99, August 8 - 13, Edinburgh -Scotland, vol 5, pp 223 - 228.
  • Hansson P. (1999) The interaction of gaseous contaminants with indoor materials -modelling and development of an experimental technique. Licentiate thesis, Centre for built environment, Roy. Inst. of Technology, ISBN 91-7170-432-9.
  • Hansson, P. and Stymne, H. (2000) "VOC Diffusion and Absorption Properties of Indoor Materials —Consequences for Indoor Air Quality". Proceedings of the 6th International Conference on Healthy Buildings, HB 2000, August 6 - 10, Espoo - Finland, vol 4 pp 151 - 156.
Publicerad av: Liselotte Laurila Sidansvarig: Gunilla Mårtensson Sidan uppdaterades: 2013-11-26
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