The release of chloroform from indoor use of chlorinated drinking water along with associated exposures has been extensively studied. For example, Weisel et al. measured chloroform in exhaled breath following showering among 49 female subjects throughout New Jersey. They found systematically and substantially higher chloroform exhaled from subjects who had elevated water levels of THMs. Nuckols et al. noted particularly that “epidemiology studies concerning THMs need to consider hot water use activities as important exposure events.” Wallace provided a thorough review of the state of knowledge regarding human exposure to chloroform in the US, concluding that “the major source of exposure to chloroform is chlorination of water supplies.” He also concluded that each of the three main exposure routes — ingestion, inhalation, and dermal absorption — “appear to be potentially substantial contributors to total exposure.” Disinfection byproducts other than THMs can be formed in drinking water treatment. Another category of regulatory concern is the haloacetic acids , including chloroacetic acid , dichloroacetic acid , and trichloroacetic acid . These HAAs have very high Henry’s law constants, so any inhalation exposure associated with indoor water use would likely be associated with inhaled particles rather than with gaseous species. Xu and Weisel261 conducted experiments to assess the rate of shower-generated particulate HAA and associated exposure. They reported that “the dose from inhalation exposure of disinfection byproducts in the particulate phase [would] represent less than 1% of the ingestion dose.” There also is a substantial literature on reactive chemistry and associated exposures and health risks from the use of hypochlorous acid for swimming pool disinfection. A highlighted concern in this case is the formation of chloramines, trimming tray arising from reactions of hypochlorous acid with ammonia and related reduced-nitrogen compounds produced by the swimmers.
Among the chloramines, the greatest attention focuses on nitrogen trichloride , also known as trichloramine, which is considerably more volatile in the presence of water than the other chloramines. Measured concentrations of gas-phase NCl3 in the air of indoor swimming pool facilities are high, with reported mean values of ~ 0.5 mg/m3 .The chemistry of chloramine formation in swimming pool environments is nicely summarized by Schmalz et al.;more generally, Zwiener et al. provide a thorough overview of the issues associated with disinfection byproduct formation in chlorinated swimming pools.Epidemiologically, clear evidence has emerged to document associations between time spent in indoor environments of chlorinated swimming pools and asthma risk. Bernard et al. stated that “regular attendance at chlorinated pools by young children is associated with an … increase in the risk of developing asthma.” Bernard et al. later reported that “use of indoor chlorinated pools especially by young children interacts with atopic status to promote the development of childhood asthma.” Jacobs et al. found “an excess risk for respiratory symptoms indicative of asthma … in swimming pool employees.” Organic acids are a vast class. Even considering only the species that are potentially relevant to indoor environmental concerns leaves a daunting challenge. On the other hand, only a few of the many organic acid species have been extensively studied indoors. The most prominent of these are formic acid and acetic acid, the simplest homologues of n-alkanoic carboxylic acids. This section reviews the state of knowledge regarding formic acid and acetic acid, in particular, plus other noteworthy examples from the broader class of carboxylic acids.The carbon in the functional group is double bonded to an oxygen atom and single bonded to the hydroxy moiety . For the broadest interpretation of carboxylic acids, R can represent any organic component. We will restrict our attention here to n-alkanoic carboxylic acids, for which R is a saturated, straight chain hydrocarbon .
Table 14 presents information on some of the more prominent of these acids. The compounds with carbon number C1-C6 will be substantially gaseous when airborne. Among these species, there is an overall tendency for the gaseous abundance to decrease with increasing carbon number. The tendency is not monotonic, however: acetic acid is typically more abundant than formic acid in indoor air. The three highest MW species in Table 14 are likely to be substantially in the particle phase if airborne. value is about 11, where Koa is the octanol-air partition coefficient. Interest in carboxylic acids indoors arises from several considerations. Acetic and formic acid are among the more abundant organic compounds found indoors, and so, provided there are no analytical barriers in sampling and analysis, broad surveys of indoor volatile organic compounds will include these species.If present at sufficiently high levels, carboxylic acids can contribute to odor and irritancy.Formic and acetic acids are among the most prominent and potent corrosive agents in air, so their abundance poses preservation threats for cultural artifacts.Conservation challenges are amplified because hardwoods, such as oak, which might otherwise be favored for storage cabinets and display cases, can be strong emission sources.Formic acid is an important oxidation product of atmospheric chemistry; comparing modeled to measured concentrations can help test and refine understanding of oxidative transformation processes.Particle-phase carboxylic acids are noteworthy tracers for cooking as an air-pollutant emission source.Considering measured indoor concentrations, even at the low ends of the reported ranges, formic and acetic acids could have substantial influence on the pH of indoor water. For example, the equilibrium pH of water exposed only to 800 ppm of CO2 would be 5.46. If that level of CO2 were combined with 1 ppb of formic acid and 4 ppb of acetic acid, the equilibrium pH of exposed water would decline by more than a full pH unit, to 4.36.
Larger, but realistic concentrations of carboxylic acids could cause substantial further pH decline. Specifically, a combination of 800 ppm CO2, 30 ppb formic acid, and 70 ppb acetic acid would yield an equilibrium pH for exposed water of 3.64. Starting with 800 ppm of CO2 and 20 ppb of NH3, the equilibrium pH of exposed water, in the absence of carboxylic acids, would be 7.12. Adding the lower levels of 1 ppb formic acid plus 4 ppb acetic acid to this mix would decrease the equilibrium pH to 6.02. At the higher carboxylic acid levels of 30 ppb formic acid plus 70 ppb acetic acid, the equilibrium pH would further decline to 5.30. In sum, ordinarily encountered levels of these carboxylic acids in indoor environments have the potential to contribute to notable shifts in the acidity of exposed water. In several studies, formic and acetic acid have been measured in special types of indoor environments or under special conditions. In museums and archives, these acids pose an unusual concern that arises, in part, because degradation of cellulosic and lignin materials may contribute substantially to indoor emissions, and, in part, because these acids pose corrosive damage risks to certain artifact materials. An extended monitoring campaign was undertaken in the Baroque Library Hall of the National Library, Prague. Over a nine-month period,trim tray pollen the median monthly average indoor concentration of acetic acid was 215 µg/m3 . The peak, which occurred during summer, was 417 µg/m3 . The corresponding monthly median and monthly peak values for formic acid were 24 µg/m3 and 102 µg/m3 . Gibson et al. reported on the concentrations of acetic acid and formic acid measured by passive sampling over 28-day exposure periods for three locations in each of eight museums and archives in the UK. The average ± standard deviation results for the 24 reported measurements for each species are 145 ± 91 µg/m3 for acetic acid and 63 ± 61 µg/m3 for formic acid. Hodgson et al. measured acetic acid concentrations in manufactured houses and site built houses in the eastern and southeastern United States. These were sampled shortly after construction under unoccupied and unfurnished conditions. The geometric mean of the measured concentrations were 117 ppb for the manufactured houses and 54 ppb for the site-built homes. Maddalena et al. sampled acetic acid concentrations in trailers intended to provide emergency shelter in the aftermath of hurricanes in the southern United States. The trailers were sampled for two 1-h periods under unoccupied and closed conditions. The average ± standard deviation acetic acid concentration for the four trailers was 1090 ± 340 µg/m3 . Formic and acetic acid have been measured in a simulated aircraft cabin. Here, in addition to primary emissions from furnishing materials and from the passengers, there is the possibility of secondary production of the acids as byproducts of ozone reaction with skin oils and other unsaturated organic molecules.
These experiments utilized a 2 ´ 2 matrix design, with low and high ventilation rates combined with low and high ozone levels . For formic acid, cabin air concentrations ranged from a low of 0.8 ppb for the low ozone – high ventilation condition to 5.3 ppb when the high ozone level was combined with the lower ventilation rate. The analogous results for acetic acid were 3.1 ppb for low ozone – high ventilation and 10.6 ppb for high ozone – low ventilation. In the past few years, instruments that can measure carboxylic acids with high sensitivity and fast response times have begun to be employed in indoor air studies. The first few of these studies have already revealed important new information about factors influencing the abundance and dynamic behavior of carboxylic acids indoors. Tang et al. used proton-transfer-reaction time-of-flight mass spectrometry to make time-resolved measurements of a broad suite of volatile organic compounds in a university classroom during normal use. From the data generated, they apportioned the source of individual VOCs among three major categories: outdoor air, indoor building materials and furnishings, and the occupants. They determined occupant associated emission rates to be 48.5 µg h-1 person-1 for formic acid and 329 µg h-1 person-1 for acetic acid. On a mass-weighted basis, among the quantified occupant-associated VOC emissions, acetic acid ranked 3rd and formic acid 10th. Both compounds were among those “whose source was ~ 1/3 or more from human occupants.” Liu et al.13 also studied the organic gas composition of a university classroom, applying a high resolution time-of-flight chemical ionization mass spectrometer. Carboxylic acids were prominently featured in their study. Overall, the average indoor concentration of total carboxylic gases was 6.8 ppb whereas the average outdoor level was only 1.0 ppb. The timeaveraged indoor concentrations of n-alkanoic carboxylic acids reported in this study were 1.2 ppb for formic acid, 38 ppt for propionic acid, 110 ppt for butyric acid, and 54 ppt for valeric acid. Acetic acid could not be measured with their analytical method. Duncan et al.32 used iodide reagent ion chemistry in high-resolution time-of-flight chemical ionization mass spectrometry to study time-resolved concentrations of water-soluble organic gases, including acetic and formic acid, in a North Carolina residence over several days. Measured concentrations were in the range 30-130 µg/m3 for acetic acid and 15-53 µg/m3 for formic acid. A striking feature was the rhythmic and substantial decline of indoor acetic and formic acid concentrations associated with air conditioner cycling. The authors suggested that “these highly water-soluble compounds … are taken up by water condensed on the AC surfaces and/or in the AC condensate.” Liu et al. conducted an extensive monitoring campaign in a single-family house in northern California. They utilized PTR-ToF-MS to analyze indoor air VOC composition with high time resolution over two multi-week sampling campaigns. Among the species quantified were the series of n-alkanoic carboxylic acids extending from formic acid through undecanoic acid9COOH. Table 16 shows that the time-averaged concentrations tended to decrease with increasing carbon number . The average indoor air concentrations and the effective emission rates were also consistently higher in the summer than in the winter. Considering the sum of continuously emitted compounds that they were able to measure with PTR-ToF-MS, the authors reported that “acetic acid alone accounted for half of the summed VOC emission rate.” They also observed a systematic temperature dependence of emissions, stating that “comparing 23 °C to 16 °C, an overall doubling of building-associated VOC emission rate was observed.” Another important inference was that “high abundance of acetic acid and furfural in both the attic and in the living zone … is consistent with the hypothesis of wood decomposition being their major source.” It is worthwhile to highlight a comparison of emissions data from the classroom study of Tang et al. and the residence study of Liu et al.