How should we define an indoor surface?

An "indoor surface" is a commonly employed term in our community but one for which there is wide latitude given to its use. Consider the following expressions that all refer to an aspect of indoor surfaces: surface chemistry, surface-area-to-volume ratio, surface reservoir, surface film, and partitioning to surfaces. In each case, we understand that surfaces represent the elements of the solid and liquid entities within the indoor environment that interact with indoor air and aerosol particles but the exact nature of the surfaces and the interactions under consideration are often not defined clearly. Nevertheless, it is implicitly understood that surfaces can influence contaminant concentrations in indoor air. In the chemistry community, the term surface is sometimes used to describe the (roughly) molecule-thick interface between the gas phase and a solid or liquid material to which chemicals can adsorb. For crystalline porous materials, such as zeolites and metal-organic frameworks, internal interfaces represent the majority of the surface area that is responsible for adsorption and catalysis and is often quantified by a BET surface area. Similarly, the building science community has long recognized the dynamic interactions that occur between the bulk material below the interface and the air above it; that is, these materials act as both sources and sinks for chemicals in indoor air.1 Should we also refer to this bulk material, which can be centimeters thick, and the overlying boundary layer, as a component of the surface? And, what about the surface area term? It is frequently reported that the surface area-to-volume ratio of an indoor space has values of a few per meter.2 But what surface area is being referred to, given that the surface area at the microscopic scale is much larger than this value and may contain internal void space which is difficult to quantify? Although these concepts and questions are not new, the goals of this editorial are to (i) highlight the importance of a common understanding of indoor surfaces, and (ii) place into discussion recommended terms to describe this important component of the built environment. To start, why does it matter? At the broadest level, loose definitions can lead to ambiguities that complicate our understanding of the fundamental processes at play. To illustrate, the degree of gas-to-surface partitioning sets the abundance of many chemicals in the air and in surface materials, thus directly affecting estimates of pollutant inhalation and dermal exposure. Which partitioning compartments should we use to assess these exposures, and what is the temporal responsiveness of the chemicals in each surface compartment to environmental perturbations? It is also important to describe correctly the rates of indoor multiphase chemistry that can both remove and produce harmful pollutants. As an example, we now know that reactive chemistry occurs not only on surfaces such as carpets, walls, and windows, but also on humans.3 The reactive surfaces on humans—their skin, hair, and clothing—are chemically and morphologically complex. A clear description of these complex surfaces is needed to accurately represent such processes. Consider the interactions that occur between indoor air and indoor surfaces (see Figure 1). First, a gas-phase molecule that passes through the gas-side boundary layer encounters an interface, a few tenths to a few nm thick, where there is an abrupt transition from a (low) gas-phase density to a (high) condensed-phase density. Depending on the interaction, molecules can physi- and chemi-sorb to interfaces4 and, given that the interfacial structure and composition may differ from the underlying bulk materials, adsorbed species may react in an interface-specific manner. Interfaces of accessible internal voids present in porous solids, such as wood, may also exist. As indicated in Figure 1, the movement of molecules can be both from the air to the surface and vice versa. Impermeable surface materials are those that do not experience mass transfer into their interiors over relevant timescales. Examples are silica present in glass windows, quartz in kitchen countertops, and stainless steel on our appliances. By contrast, permeable surface materials undergo substantial diffusion of molecules beyond their interfaces into the bulk. Mass transfer can occur either within the condensed phase, as for liquids such as skin oil or semi-solids such as resin or polymer coatings, or within the gaseous voids of porous solids, such as concrete, wood, textiles, carpets, or wall board, into which indoor air gases can diffuse. The depths to which molecules penetrate determine the extent to which surface materials interact with air.5 The effective penetration depth can extend from nanometers to hundreds of microns or more, with its value controlled by time, and a combination of diffusive and reactive factors. Semi-volatile molecules, such as most organics and water vapor, partition in significant quantities to indoor condensed-phase materials.6, 7 Surface films arise when these molecules accumulate to thicknesses greater than one monolayer. At the microscopic level, these accumulations may be continuous or exist as islands, be morphologically rough, and contain deposited particles.8 They are likely thicker on impermeable materials (eg, semi-volatile organic films with thicknesses of nanometers to tens of nanometers develop over weeks or months)6, 9 than on permeable materials, because of diffusion into the bulk in the latter case. If exposed to the same levels of semi-volatile species in the gas phase, the film composition of semi-volatile species may be uniform from room to room when deposited onto an impermeable surface material over a period of a few weeks.9 The term surface reservoirs collectively refers to all the condensed-phase compartments that interact with the gas phase and aerosol particles over relevant time scales.1, 10 These include interfaces, surface films, deposited particles, and all the accessible components of bulk materials that are present in our indoor environment (building materials, furnishings, humans, liquid water, etc.). The timescales for gas-surface interactions are important to consider. Molecules adsorbed to interfaces typically undergo very rapid exchange with the gas phase (seconds or less)4 whereas the timescale for diffusion of molecules through paint layers to the underlying wallboard is on the order of hours or more11 and PCBs can take years to partition into concrete and other building materials.12 We note that in this editorial we distinguish between deposited particles and suspended (ie, aerosol) particles for two reasons that differentiate their impact on indoor air. One, the residence time of suspended particles is very much shorter than deposited particles, with removal times determined by air exchange and settling. Two, the partitioning capacity of aerosol particles is orders of magnitude smaller than that of immobile surfaces. Nevertheless, important reactive and partitioning processes occur with aerosol particles, occurring both at the air-particle interface and via diffusion below the interface. Important quantities such as the indoor surface area-to-volume ratio and partitioning capacity have clear meanings using these definitions. Specifically, when evaluated over a macroscopic length scale, the geometric surface area-to-volume ratio is useful to quantify the rates of gas-surface interactions using a deposition velocity approach. However, to determine the degree to which molecules partition to interfaces via adsorption, a much larger surface area determined at the molecular length scale is required. The total degree to which partitioning occurs reflects both the molecules that adsorb and those that absorb, by diffusing beyond the interface into the bulk of all the surface reservoirs present. The indoor partitioning capacity is so large that even highly volatile species, including many VOCs and HONO, are predominantly partitioned to indoor surface reservoirs, with a much smaller abundance in the gas phase.10 We note that the effective partitioning capacity is a time-dependent value, increasing as longer times are considered.11, 12 As an everyday example, third-hand smoke presumably penetrates deeply into surface reservoirs, like upholstery or paint, in residences with long-term smokers. These definitions are not necessarily new but, with the inherent complexity of indoor surfaces, the use of these terms remains inconsistent.13-16 In particular, it is important to recognize the wide range of distance and time scales that characterize interactions of indoor air with the different surface reservoirs. It is hoped that this editorial encourages discussion on this topic as we continue to evaluate the important roles that surfaces play in the indoor environment. No conflicts of interest declared. This article was conceptualized by JA and GM. The original draft was written by JA. The final draft has been reviewed and edited by all authors. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.