Building materials are a major source of indoor air contaminants. Volatile organic compounds (VOCs) are an important class of contaminants prevalent in indoor air. Attempts have been made to model the emission of VOCs from building materials. Diffusion has been shown to control the rate of mass transfer within certain types of building materials. The primary objective of this research is to develop a fundamental diffusion-based model for single and double layer building materials. The single-layer model considers a slab of material located on the floor of a chamber or room with the material acting either as a source or a sink for VOCs. The behavior of the model is governed by the material phase diffusion coefficient (D), the material/air partition coefficient (K), the concentration of VOC in the influent air stream, and the initial concentration within the material phase. The single-layer model extends a previously developed version, incorporating the non-uniform initial concentration inside the building material and a transient influent concentration. Experimental work is performed to check the validity of the model. A steel chamber housing a piece of vinyl flooring is used to simulate building material within a room. D and K values for two representative VOCs, n-dodecane and phenol, are available from earlier experiments. These parameters are used in the model to predict the VOC concentration inside the chamber. The predicted values compare very well to the observed experimental data. A double layer version of the model is developed and studied from a theoretical perspective. The model also permits a time dependent influent concentration and a non-uniform initial concentration profile within each of the two layers. A parametric analysis is performed varying the ratio of the diffusion coefficients, the partition coefficients and the thickness of the two layers. Three cases of practical interest are studied using the double-layer model. The use of a thin low-permeability barrier layer placed on top of a building material is shown to hold considerable promise for reducing the emission rate of VOCs into indoor air. / Master of Science

The adverse effects of contaminated outdoor air have been recognized and subject to control for many years. More recently environmental engineers and health professionals have become cognizant of the hazards associated with contaminated indoor air. It is now understood that contaminated indoor air negatively impacts human health, worker productivity, and physical property. Volatile organic compounds (VOCs) are a common class of indoor air pollutants. Building materials such as treated wood, pressed-wood products, wallboard, sealants, adhesives, floor coverings, and paints can be sources of VOC emissions. The knowledge-base necessary to develop effective solutions to indoor air quality problems requires an understanding of the emissions behavior of indoor materials. Environmental chambers are often utilized to characterize indoor material as sources of VOC emissions to indoor air. Chamber studies, although expensive and time consuming, can be utilized to provide estimates of the rates at which a particular material emits VOCs under a specific set of environmental conditions. By fitting curves to emissions data obtained through chamber studies, VOC emissions models have been constructed. These models are frequently empirical and as a consequence, 1) apply only to the specific material and environmental conditions investigated, 2) provide little understanding of the source/sink characteristics of the material, and 3) provide little knowledge of the mass transfer processes governing emissions behavior. As a result, our understanding of the mechanisms that control VOC emissions from indoor materials remains rudimentary. Physically-based models that describe the emissions characteristics of building materials would greatly facilitate the process of improving indoor air quality. Evidence exists suggesting well-established fundamental mass transfer mechanisms govern emissions from indoor materials. Of the various mechanisms governing emissions behaviors, diffusion appears to be one of the most significant. The primary objective of this research was to demonstrate that the VOC emissions source behavior of a diffusion-controlled homogenous building material could be predicted using a mechanistic mathematical model. A commercial grade sheet vinyl flooring (VF) was selected for study because VF is present in many residential and commercial buildings, is relatively homogenous, and has been shown to emit hazardous organic chemicals. If successful, this research would demonstrate that the proposed strategy could be generalized to other VOC sources using appropriately constructed mathematical models. Satisfying the research objective required development of a physically-based model to predict gas-phase VOC concentrations resulting from exposure to a diffusion-controlled material. Key parameters for this model are the solid-phase diffusion coefficient, D; the solid/air partition coefficient, K; and the initial solid-phase VOC concentration, C0. D and K have been previously quantified for only a few indoor materials and methods for determining C0 are rudimentary. Therefore, this research project required development and execution of methods for quantifying D, K, and C0. D and K were quantified using a recording microbalance. C0 was evaluated using a new technique of cryogenic milling followed by fluidized bed desorption. The model was validated by exposing a VF sample in an environmental chamber and directly measuring gas-phase VOC concentrations resulting from mass transfer from the solid material. Further model validation was achieved by directly measuring the VOC concentration profiles after exposure in environmental chambers. Because the key model parameters were quantified independently of chamber studies, the model validation process provided a rigorous test of the validity of the mass transfer model in particular and of the source characterization strategy in general. The results of this research contribute to our understanding of the fundamental mechanisms that govern emissions of VOCs from vinyl flooring and provide a sound theoretical foundation for characterization of a wide range of other sources of indoor VOCs. This understanding could facilitate product reformulation strategies aimed at preventing or reducing indoor air contamination. Mass transfer models could also be utilized to develop standards for the environmental performance of indoor materials. The proposed approach will prove useful in conjunction with broader studies on sick building syndrome to identify sources that may have a critical impact on the health and comfort of building occupants. / Ph. D.

This thesis describes the development of a generic model for predicting the emissions of organic compounds from materials used in the manufacture of various goods and products. Many products contain organic substances that are not bound to the matrix formed by their constituent materials and are thus able to dissociate from the material and become transferred into the surrounding environment. A wide range of materials and products are used in modern societies, and many compounds deriving from these materials are regarded as emerging pollutants in both indoor and outdoor environments. The model uses three components to describe the transfer of compounds from materials to the surrounding environment: partitioning of the compound between the material and its surroundings based on linear free energy relationships, diffusion within the material based on the Piringer equation, and convective mass transfer in air or water based on an empirical flat surface model. The model’s predictive capacity was tested against three experimental case studies: emissions of plasticizers from vinyl flooring and triphenyl phosphate from LCD screens into the air, and leaching of organophosphates from concrete into water. The rates of emission from vinyl flooring were clearly affected by the number of layers comprising the material. Triphenyl phosphate was found in the front surface of all tested flat screens and its rates of emission were related to the nature of the screen and its operating temperature. The model accurately predicted emissions into the air and leaching from concrete into water once modified to include modules that describe dissolution from surfaces and diffusion in water-filled pores. The model was then used to investigate emissions on the national scale. It was found that the rates of emission from vinyl flooring are not changing over time, and that the total mass of emitted material is dependent on annual sales volumes and the expected life span of the vinyl flooring. Moreover, the additive used has a large effect on the emitted mass. Emissions from flat screen displays depend strongly on their operating temperatures: displays with high working temperatures that are active for extended periods of time produce more emissions. The model was also used to study the release of organophosphates from the concrete used to make a bridge, which depended on the flow of water under the bridge, the temperature, the porosity of the concrete, and the additive’s water solubility. Data on annual sales volumes and the total surface area of sold goods are essential when studying emissions on a national scale. National retailers’ organizations are valuable sources of such information. When adequate data are not available, it is necessary to perform uncertainty analyses to determine the impact of uncertainty in the modeling of different stages of the emissions process in different scenarios.