As noted, the pH of sorbing surfaces can influence the capacity for nicotine and other chemically basic reduced-nitrogen constituents of third hand smoke . Spectroscopic measurements indicate that sorbed nicotine and amines are present on surfaces primarily as the monoprotonated species rather than the neutral species, consistent with their basicity. The amount of nicotine on a surface is relevant to health concerns, because, among other reasons, sorbed nicotine can subsequently react with indoor nitrous acid to generate carcinogenic nitrosoamines that would otherwise not be present. It can also react with ozone, with a half-life of ~ 6 days at 40 ppb O3, to generate various oxidation products including formaldehyde, N-methylformamide, nicotinaldehyde, cotinine, myosmine, and nicotyrine. Given that pH plays a significant role in nicotine’s sorption to surfaces, a related issue for future study is how the reactivity of non-ionized nicotine compares to that of monoprotonated nicotine.Despite their importance, indoor surfaces and interfaces are poorly defined, especially porous materials such as wood, gypsum board, paint films, vinyl, and carpets. While there is a large body of literature addressing chemistry and catalysis on ideal surfaces, there is a dearth of such information for real-world surfaces. Hence, the information summarized in this section is not as strongly grounded in physical science as that in the previous sections of this review. Despite this limitation, we aim to provide the reader with a clear sense of what is currently known and call out areas that warrant further investigation.Indoor surfaces influence the concentrations, dynamic behaviors, and fates of indoor acids and bases. Gas-phase acids and bases partition to surfaces; acidic and basic constituents of airborne particles deposit on surfaces; acid-base chemistry occurs on surfaces. When we speak of surfaces, we are referring to the exposed surfaces of building materials,hydroponic stands coatings and furnishings, as well as surfaces within these materials that are accessible to indoor air via relatively rapid mass transport.

We include the air-surface interface of bulk water and organic films. Indoor surfaces should be understood to have a third dimension, i.e. a thickness that is generally much smaller than the areal dimensions but may be much larger than molecular scale. Scientifically, the extent of surface thicknesses that can interact meaningfully with indoor air composition remains not well resolved. It certainly varies with material properties, such as porosity, permeability, and viscosity. The thickness also varies with the time scales of concern, as the time needed for diffusive transport through a thin layer of a homogeneous substance scales with the inverse square of the layer’s thickness. Near one end of the range of possibilities, an organic film on an impermeable indoor surface with a thicknessof ~50 nm would likely be fully accessible to interact with gaseous species without meaningful transport restrictions. A permeable and porous paint layer of 50-100 µm thickness might also be substantially accessible. On the other hand, the materials that comprise gypsum wallboard, with typical overall thickness of 13 mm, might not be fully accessible to interact with gaseous species indoors because of lengthy transport times, even though the material is porous and somewhat permeable. For low-porosity and highly impermeable materials, such as tile and glass, the scale of interaction of gaseous species with indoor surfaces may only extend through a few molecules thickness into the surface because of the slow molecular diffusivity into such solids. In a typical room, indoor surfaces comprise floor, walls, ceiling and furnishings. Through air exchange with hidden spaces, other materials such as wood framing and insulation may also influence indoor air pollutant dynamic behavior. The surface in contact with room air may differ from the underlying material that constitutes the bulk of the floor, walls or ceiling. Wood floors are frequently varnished; concrete floors are often covered by tiles or synthetic flooring , and synthetic floors may be polished, waxed or coated with a polymer. Some floors are partially or completely covered with carpeting. Walls are commonly painted or papered. In China, walls are often “limed,” i.e. coated with layers of aqueous Ca2. External walls have windows, which may have adjustable coverings, such as blinds, shades or drapery. Ceilings may be painted or may be finished with ceiling tiles.

Occupants also contribute to indoor surfaces with their clothing, skin, and hair.An important feature of indoor environments is the high ratio of surface area to volume of air in contact with those surfaces. Surface-to-volume ratios are orders of magnitude larger indoors than outdoors. Hence, processes that are impacted by surfaces are of much greater consequence indoors than outdoors. Two moderate-scale studies plus several smaller ones have reported surveyed surface-to-volume ratios. Hodgson et al. measured S/V in 33 rooms in nine residences in San Francisco, CA. All objects with a surface area > 300 cm2 were included in these surveys. Surface areas were based on shape and dimensions, but did not attempt to adjust for fleeciness, roughness or porosity. The average ± standard deviation S/V values for all rooms was 3.6 ± 1.0 m2 m-3 . Bathrooms averaged 4.9 ± 0.3 m2 m-3 ; bedrooms/offices averaged 3.7 ± 0.9 m2 m-3; and common rooms averaged 2.8 ± 0.3 m2 m-3 . More recently, Manuja et al. measured S/V ratios in ten bedrooms, nine kitchens and three offices in Blacksburg, VA. Including contents, the average ± standard deviation S/V values for all rooms was 3.2 ± 1.2 m2 m-3; offices averaged 3.6 ± 0.4 m2 m-3 ; bedrooms 3.0 ± 0.4 m2 m-3; and kitchens 3.2 ± 1.8 m2 m-3. These two studies considered only macroscopic surface area. Neither attempted to account for the additional area associated with rough or porous surfaces . Microscopic surface area is anticipated to be much larger than these reported S/V values. As one illustration of the scale of effect that might be expected, Morrison and Nazaroff reported that the microscopic surface area of carpet samples exceeded the floor area covered by factors of 30 and 33 for two commercial loop carpets and by factors of 46 and 66 for two residential, cut-pile carpets.483The surface area of indoor airborne particles contributes negligibly to total indoor surface area. Consider a 30 m3 room with a total superficial surface area of 95 m2. An extensive dataset for outdoor air pollution collected in the Ruhr Valley, Germany,grow table found a median PM10 concentration of 20 µg/m3 and a median lung-deposited particle surface area concentration of 36 µm2 /cm3 . 484 Using this surface area concentration as a magnitude estimate for indoor environments, the corresponding total particle-associated surface area in the 30 m3 room would be ~10-3 m2, or about 5 orders of magnitude smaller than the superficial area associated with the fixed interior surfaces.

Two surveys of indoor surface materials important for moisture uptake and humidity buffering were mentioned in §2.4.In addition to surface-to-volume ratios, Hodgson et al. and Manuja et al. catalogued the different materials that constituted the surfaces in their surveyed environments. These material descriptions were used by Hodgson et al.: metal, glass, ceramic/porcelain/tile, finished wood, unfinished wood, painted wood, PVC, other plastic, painted/papered plaster and wallboard, thin fabrics, upholstery/carpet, and paper. Finished/painted surfaces accounted for a substantial fraction of total surface area in all room types: finished wood with median contributions of 0.54 m2 m-3 in common areas, 0.73 m2 m-3 in bedroom/offices, and 0.26 m2 m-3 in bathrooms; painted wood with median contributions of 0.38 m2 m-3 in common areas, 0.34 m2 m-3 in bedroom/offices, and 0.44 m2 m-3 in bathrooms; painted/papered plaster and wallboard accounted for a substantial fraction of total surface area in all room types, with median contributions of 1 m2 m-3 in common areas, 1.2 m2 m-3 in bedroom/offices, and 1.6 m2 m-3 in bathrooms. Impermeable surfaces were extensive in bathrooms, with median S/V for metal, glass, ceramic, porcelain and tile summing to 1.3 m2 m-3 . Such impermeable surfaces were less abundant in common areas and bedrooms with median S/V values of 0.16 m2 m-3 . In all room types, vinyl and other plastic surfaces had median S/V values of 0.38-0.98 m2 m-3 , whereas textiles and fibrous materials had median S/V values of 0.45-0.55 m2 m-3 . Manuja et al. used somewhat different material categories: cardboard, concrete, fabric/fiber, glass, metal, paint, paper, plastic, wood , or other. The dominant material was paint covered surfaces , chiefly walls and ceilings, followed by stained wood . Fabric/fiber surfaces were abundant in bedrooms; plastic and metal surfaces were abundant in offices and kitchens. Glass surfaces comprised a small proportion of total surface area. Exposed concrete contributed slightly to indoor surfaces in kitchens and was barely present in bedrooms and offices. It is noteworthy that together painted surfaces and stained/finished-wood surfaces accounted for almost two-thirds of the surfaces in the rooms evaluated in these two studies. The substrates beneath the paint are often gypsum wallboard or pressed wood composites . In these cases, both the paint and the substrate are somewhat permeable, suggesting that painted surfaces and stained wood could serve as substantial sinks for gas-phase species that interact strongly with their chemical constituents. As summarized in §2, at 50% RH the moisture content of painted gypsum board is 0.5-1.1%, while that of wood is notably higher, at 8- 10%. The abundance of such permeable surfaces with significant moisture content may help explain the large indoor reservoirs that are seen for nitrous, formic and acetic acid, as discussed below. Human occupants can contribute meaningfully to the total surface area of the rooms they occupy, and acid-base chemistry can occur on exposed skin, hair and clothing.

A typical adult has a total body surface area of approximately 2 m2 . If two adults occupy a 30-m3 room with an S/V of 3.5 m2 m-3 , the human surfaces contribute about 4 m2 to the total surface area. Skin and hair are covered by surface lipids, about 25% of which are organic acids. Human skin has a pH in the range of 4.5 to 6. Bodies are most commonly clothed; certain fabrics in clothing can have substantial moisture content at typical indoor humidity. For example, at 50% RH, the equilibrium moisture contents of nylon, cotton and wool are approximately 3%, 5%, and 10%, respectively . As discussed in §2, water sorbs to indoor surfaces. When impermeable surfaces have water coverage larger than about five equivalent monolayers, the nature of the surface interacting with room air is closer to that of water than that of the underlying substrate. Given the importance of surface interactions influencing indoor air constituents, including gaseous acids and bases, the limited available evidence from surveys of indoor surface materials is striking. Surveys similar to those undertaken by Hodgson et al.23 and Manuja et al. are needed in homes, schools and offices in other US cities and in other countries. It would be especially valuable to have results from such surveys conducted in different cultures with large populations and high population densities, such as China and India.All surfaces become soiled. Three processes are primarily responsible for soiling: contact transfer by occupants , partitioning of semivolatile species from the gas phase, and particle deposition. Whereas partitioning and particle deposition impact all exposed surfaces, contact transfer only influences surfaces that are commonly touched by occupants . As a consequence of contact, chemicals are transferred from a surface to occupants, and also from occupants to surfaces. Fingerprints are a prime example of skin oils left behind on a surface that has been touched. Indeed, Zhou et al. relied on human touch to transfer skin oils to glass capillaries prior to investigating their oxidation by ozone. Experimental measurements have provided information about film growth and particle deposition on impermeable surfaces. Based on results from such studies, coupled with modeling, we can estimate approximately how long a surface must be exposed to indoor air before soiling has substantially altered the nature of its surface. Evidence is emerging that soiling imposes a degree of commonality among indoor surfaces that can be quite different from one another when clean.Absorption of semi-volatile organics to an impermeable surface requires a few layers of organic species on the surface to kick-start the partitioning process. How this might occur has been considered by Eichler et al., but a full description of the processes that initiate absorptive partitioning on indoor surfaces remains to be elucidated.