In the case of acetic acid, for example, the peroccupant emissions rate in the classroom was about 0.3 mg/h. That value, if applied to the two occupants of the house studied by Liu et al., would account for less than 10% of the inferred total emissions rate of 12 mg/h and 20 mg/h . Similarly, the per-person emission rate from the classroom study for formic acid, 0.05 mg/h, suggests that occupant associated emissions are only a small portion of the total indoor generation rates of 2.3 mg/h and 4 mg/h determined in the study house. An interesting and important lesson can be extracted from the data in Table 16, when considered in the context of how physiological response varies across compounds in a homologous series. Cometto-Muñiz et al. measured the odor thresholds for five carboxylic acids. The results spanned 5 orders of magnitude from formic acid to octanoic acid . The reported average concentrations of formic acid and octanoic acid in Table 16 differ by about two orders of magnitude. Here is a key point: focusing on the most abundant organic compounds, which naturally emerges from chemical analyses, can readily mask the prevalence of compounds that are more important with regard to human physiological response. In this particular instance, the measured average concentration of formic acid is a few orders of magnitude below its odor threshold. However, the much smaller measured concentration of octanoic acid exceeds its odor threshold by an order of magnitude. Several studies have assessed carboxylic acid emissions from woods, emphasizing acetic acid as a prominent species. For example, Risholm-Sundman et al. reported that “some hardwoods give a high emission of acetic acid.” The highest reported emissions of acetic acid in their study were from cherry and oak. Manninen et al. found that the temperature history of the wood mattered, writing “in the emissions of heat-treated wood,trim bin tray the most abundant individual compounds, 2- furancarboxaldehyde, acetic acid and 2-propanone, made up about 60% of the total VOC emission. … None of these compounds was found in the VOC emission from air-dried wood.”

Gibson and Watt272 stated that, “acetic acid is known to emit from all natural woods with hardwoods, e.g. oak, being thought to emit the highest concentrations of acetic acid …”. They found that emissions were sensitive to temperature, being much higher at 45 °C than at 20 °C, and also to humidity, with lower emissions for drier conditions. Carboxylic acids can be generated through the oxidative decomposition of higher molecular weight fatty acids. Linoleic acid is a prominent ingredient of linoleum, a common flooring material. Jensen et al. modeled the concentrations of propanoic acid utilizing emission measurements from a linoleum flooring sample. They predicted an indoor concentration of 56 µg/m3 one month after installation, only 2´ below the odor threshold. Other processes in atmospheric oxidative chemistry also can generate formic and acetic acids. For example, summed over the global atmosphere, the dominant sources of formic and acetic acids are believed to be “photochemical oxidation of biogenic organic compounds, in particular isoprene.” Zhang et al. conducted experiments in a Teflon test chamber designed to explore the production of formic and acetic acid from oxidative chemistry. In that work, ozonation of limonene using indoor-relevant concentrations was found to generate acetic acid. Formic acid was produced in each of the three systems tested: ozonation of styrene, of limonene, and of 4- vinylcyclohexene, respectively. Destaillats et al. quantified formic and acetic acid levels in chamber studies of the ozonation of three household consumer products: a pine-oil based cleaner, an orange-oil based degreaser, and a plug-in air freshener. Median reported concentrations in 11 experiments were 14 ppb for formic acid and 22 ppb for acetic acid.Interior paints can be a source of carboxylic acid emissions. Reiss et al. studied ozone reactions with latex paints. They did not find formic and acetic acid to be generated by ozone reactions. However, they did report that both compounds off-gassed from the latex paints themselves. They also found that the rates of emissions of these compounds were higher at higher relative humidity.

Investigating finishing materials that might be used for preserving cultural artifacts, Schieweck and Bock reported that “low-VOC” and “zero-VOC” paints “released heightened acetic acid levels and are therefore not favored for the use in sensitive environments.” Incomplete combustion and/or high-temperature volatilization from fuels can be another source of carboxylic acids. For example, Kuo et al. determined an emissions factor for acetic acid from incense use to be 840 ± 520 µg per g of incense burned, based on experiments with four popular brands. Christian et al. measured emission factors of formic acid and acetic acid from biomass combustion. Considering open wood cooking fires, they reported 0.25 ± 0.12 g of formic acid to be emitted per kg of wood burned. The corresponding emission factor for acetic acid was 1.8 ± 1.3 g/kg. Let’s next consider the phase state of formic and acetic acid. In the presence of condensed water, there are three potentially important states: gaseous, aqueous and undissociated, and aqueous in the form of the conjugate base . As we have already described, the partitioning among these three states depends on two key properties of the volatile acid: Henry’s law constant and the acid-dissociation constant . Influential features of the indoor environment include the relative abundance of condensedphase water and the pH of that water. Factors influencing the pH of indoor condensed water include the abundances of all of the indoor air acids and bases plus the properties of any material substrate in contact with the water. The presence of any gas-phase carboxylic acid would tend to acidify condensed water. For the present analysis, let’s assume that the condensed-water pH is externally regulated, independent of the influence of carboxylic acids. That could apply, for example, in the limit of a small abundance of the carboxylic acids. As described in §2.1, liquid water abundance can be quantified as a volume fraction, with dimensions liters of water per m3 of air. We use the symbol L* to signify an equivalent volume fraction that is chemically equilibrated with indoor air. We restrict analysis here to fixed common indoor conditions of pressure and temperature . In §3, we presented equations describing equilibrium quantitative partitioning of a monoprotic acid considering the three phase states.

Equation describes the fraction of the total abundance that is in the gas phase. Figure 11 displays the results for formic acid and acetic acid ,pollen trim tray showing gas fraction in relation to the liquid water volume fraction for four different values of pH .The thermodynamic properties used in these calculations are reported in Table 14. These plots show that the overall behavior of formic and acetic acids is qualitatively similar with regard to phase partitioning between air and water. Even a small amount of condensed water can be a major sink for these carboxylic acids if the water is maintained by external factors to have a high pH . Conversely, at a low pH , most of these carboxylic acids will remain gaseous provided that the equilibrated liquid water abundance remains small . In the event that the condensed water is relatively abundant , there is the opportunity for substantial partitioning to water, even for acidic pH conditions.Because of the material damage risks posed, many studies of carboxylic acids have been conducted in museums and archives. The nature of the specific risks from formic and acetic acid in damaging cultural artifacts is well described by Brimblecombe and Grossi, including“Byne’s disease,” which refers to efflorescence of calcareous materials owing to their dissolution upon exposure to organic acids. Prosek et al. provide a useful introductory overview of corrosion risks associated with volatile carboxylic acids and also describe the development of a direct monitor “to assess small changes in air corrosivity in real time.” Graedel described the corrosive nature of organic acid vapors for lead, indicating that “acetic acid [is] five to ten times as aggressive as formic acid.” In an interesting application of corrosion concerns, Niklasson et al. reported that “high concentrations of acetic and formic acid vapours are present in the wind system of the corroded [church pipe] organs. … The main source of acetic acid is the wood from which the wind system is built. In contrast, formic acid is generated in the church environment outside the wind system.” Reinforcing the idea of wood as an important emission source, Kontozova-Deutsch et al. measured levels up to 450 µg/m3 of formic acid and up to 1050 µg/m3 of acetic acid in enclosed showcases at the Metropolitan Museum of Art in New York. Much lower levels were found in the galleries. Concern about material degradation risks posed by organic acids has spurred efforts in the development of novel control technologies. For example, Dedecker et al. have developed a metal-organic framework for removing “low concentrations of acetic acid from indoor air at museums.” Among the challenges in sorbent performance that MOF technology has the potential to overcome is poor selectivity for polar compounds compared to the much more abundant water vapor.Hodgson et al. assessed the performance of an air cleaner utilizing ultraviolet photochemical oxidation .

They reported a strong caution: “formaldehyde, acetaldehyde, acetone, formic acid and acetic acid were produced … due to incomplete mineralization of common VOCs.” Truffier-Boutry et al. assessed photocatalytic paints and found “that the degradation of the organic matrix [of the paint itself] leads to the release of organic compounds into the air….” This evidence supports a finding that partial oxidation of organic molecules can generate formic acid and acetic acid at levels of potential concern for indoor environmental quality. In contrast to formic and acetic acid, which have been extensively studied indoors, there is little published work reporting on the higher molecular weight carboxylic acids in indoor environments. However, absence of evidence isn’t the same as evidence of absence. The limited available information does point toward the potential for these compounds to be of interest indoors, as highlighted by the following observations. Liu et al.69 characterized the organic matter found in films extracted from interior surfaces of the windows of various building types, including a residence, a restaurant, and an office. They found that monocarboxylic acids dominated among polar compounds, with C11-C31 monoacid densities in the range 6.5-100 µg m-2. Fang et al.309 reported on chemical characterization of dust extracts collected from homes, a gymnastics studio, and office environments. In the portion of dust extracts most associated with agonism of human peroxisome proliferator-activated nuclear receptor gamma, “fatty acids … including oleic acid, stearic acid, palmitic acid and myristic acid, were the primary chemicals identified.” Higher molecular weight n-alkanoic carboxylic acids, especially palmitic and stearic acids, have been identified as important markers of the impact of cooking emissions on urban air quality. For example, in an atmospheric monitoring study in the Los Angeles area, carboxylic acids were quantifiable contributors to fine particulate matter. In that study, monthly average values of palmitic acid in atmospheric fine PM were in the range 0.10-0.25 µg/m3. In a study of atmospheric fine particulate matter in Beijing, averaged airborne concentrations were reported for lauric , myristic , palmitic , and stearic acid . Several studies have reported quantitative emission factors for particle-phase carboxylic acids from commercial or institutional-scale cooking activities, including western-style meat cooking, stir-frying and deep-frying vegetables with seed oils, and various Chinese styles of cooking. Palmitic and stearic acids are prominently featured among the emitted chemicals in all of these studies. Candle burning has also been characterized as a source of particle-phase carboxylic acid emissions. 316 Emissions from paraffin candle wax were predominantly palmitic and stearic acid, as a “result of unburned wax volatilization.”For beeswax candles, the most prominent emissions of particle phase n-alkanoic carboxylic acids were palmitic and lignoceric acid . A few studies have reported on the indoor carboxylic acid abundance in “quasi-ultrafine” and fine particulate matter. Arhami et al. studied the abundance and sources of organic compounds in quasi-UF PM in four retirement homes in the Los Angeles basin. They found that the “n-alkanoic acids were likely to be influenced by indoor sources.” They also reported that, for outdoor air, “hexadecanoic, octadecanoic, and phthalic acids were the most dominant measured acids in quasi-UF PM.”