A more recent study suggested that garbage burning “is the main global source of HCl.”Garbage often contains polyvinyl chloride, which emits HCl when burned. The presence of HCl in outdoor air means that ventilation contributes HCl to indoor environments. Indoor HCl also can be generated, in principle, by unvented indoor combustion processes. For example, there is some evidence, detailed below, that cigarettes can be a meaningful indoor emission source where smoking occurs. The relevant thermodynamic parameters are a Henry’s law constant of KH = 1500 M/atm142 and an aciddissociation constant recently reported as pKa = -5.9. 241 Hydrochloric acid is so strong, one can safely assume that HCl will be fully dissociated in water, regardless of the pH of the solution. This combination of thermodynamic parameters, along with the reasonable expectation of a negligible vapor pressure of the Cl- ion, suggests that there should be no gaseous HCl in the presence of an aqueous phase, at least under equilibrium conditions. It also suggests that one could look for elemental Cl in fine particulate matter as an indicator of the influence of HCl on the composition of air. Specifically, if there is an aqueous phase associated with particulate matter, then HCl will dissolve into that water and dissociate to form H+ and Cl- . Gaseous HCl might also react with ammonia to form the ammonium chloride salt, NH4Cl, which would tend to condense onto preexisting particles. In either case,vertical grow condensation onto fine-mode particles would be favored because of mass transport considerations. In brief, and to simplify a more complex story, the condensation onto fine mode particles would be favored because the majority of particle-associated surface area is on particles in this size range.
By contrast, coarse-mode aerosol chlorine would probably be dominated by primary production, e.g. by sea spray processes associated with breaking waves. Only a few studies have reported measured indoor air concentrations of HCl and/or fineparticle chloride. The concentrations tend to be low, typically less than about 1 µg/m3. Li and Harrison measured indoor and outdoor concentrations of HCl and total aerosol Cl- during repeated 24-h sampling events at the University of Essex, UK. They reported average indoor levels of 0.44 µg/m3 for HCl and 0.74 µg/m3 for total aerosol Cl- . Allen and Miguel measured indoor and outdoor levels of HCl and fine-particle chloride at six sites in southeastern Brazil. These sites were characterized as hotel/restaurant , restaurant , and office . Each site was sampled once during normal occupancy over periods ranging from two to six hours. The reported mean HCl level was 0.28 µg/m3 indoors versus 0.52 µg/m3 outdoors. Interestingly, the average fine particle chloride levels were substantially higher indoors than outside . In combination, these results suggest the possibility of an indoor source of HCl with reactions indoors that convert gaseous HCl to aerosol Cl- . In the United States, the PTEAM study conducted personal sampling of respirable particles along with simultaneous residential indoor and outdoor measurements of respirable and fine particles for 178 subjects selected to be statistically representative of the population of nonsmoking residents of Riverside, California. Chemical elements were measured using x-ray fluorescence. As reported by Wallace, chlorine was predominantly attributable to indoor emission sources in both the fine and inhalable size ranges.
The two noted indoor chlorine sources were cigarette smoking and cooking. Estimated mean emission factors for fine-particle Cl- were 103 µg/cig from smoking and 5.9 µg/min from cooking.In an earlier source apportionment study based on week-long samples in 394 homes in New York state, a fine-particle Cl- emission factor for smoking was determined to be 69 µg/cig. As previously noted, Sinclair et al. reported measurements of the ionic composition of airborne fine particles and surface accumulations in telephone switching offices in four US cities. Average indoor concentrations of Cl- were in the range 2-11 ng/m3 , lower than the corresponding outdoor range of 11-121 ng/m3 . In an earlier report of data from two of the study sites, Sinclair et al. indicated that the indoor/outdoor ratios of fine particle Cl- ions were elevated compared with other ions; they suggested that cigarette smoking might have been a contributing source. By comparing surface accumulations of soluble Cl- ions on aluminum and zinc surfaces, Sinclair et al.indicate a role for the deposition of Cl-containing gases : “chloride accumulation on aluminum surfaces was shown to occur almost entirely by particle deposition , while chloride accumulation on zinc surfaces occurred by both particle deposition and reaction with chlorine-containing gases.” The emergence of advanced instrumentation for atmospheric science studies has permitted real-time measurements of chlorine associated with sub-micron particles and gaseous HCl. Johnson et al. reported the first application of aerosol mass spectrometry to “analyze real time indoor aerosol composition and outdoor-to-indoor transformation.” They found, though, that “chloride was rarely above detection [limit].” Dawe et al. reported the first time resolved measurements of HCl indoors using cavity ring-down spectroscopy. Their study entailed residential measurements during three common types of household activities.
They found that “surface application of bleach resulted in a reproducible increase of 0.1 ppbv”; “emissions of HCl from automated dishwashers were observed only when chlorinated detergents were used”; and that indoor HCl levels increased “during meal preparation on an electric element stovetop.”Arguably, the most important chlorinated acid for indoor environmental quality is hypochlorous . This chemical and its conjugate base, the hypochlorite ion , are associated with the widespread application of chlorine for treating municipal drinking water, for maintaining the hygiene of swimming pools and spas, and as an active ingredient in bleach and other cleaning products used indoors. The impact of HOCl and OCl- on indoor environmental quality emerges largely indirectly, only partly in relation to their acid-base properties. A key fundamental factor is the oxidative potential of the chlorine atom: in hypochlorite, Cl is in oxidation state +I, whereas in the chloride ion and in many common chemically combined forms, Cl is in oxidation state -I.Hypochlorous acid is weak with moderately high water solubility .Commercial chlorine bleach for household use commonly is an aqueous solution of 3-6% NaOCl along with a strong base, such as NaOH, to establish a basic pH of approximately 12. The corresponding molarity of hypochlorite, representing the sum of [HOCl] and [OCl- ], would be in the range 0.4-0.8 M. At pH 12, about 4.5 units above the pKa value,indoor growers the ratio of [OCl- ] to [HOCl] in solution would be about 104.5 or 32,000:1. The expected aqueous concentration of [HOCl] would be 13-26 µM and the corresponding equilibrium mole fraction of gaseous HOCl in the headspace of a bleach bottle would be 19-39 ppb. Decreasing the pH by 1 unit would increase the equilibrium abundance of gaseous HOCl in the headspace by 10´. In common cleaning applications, household bleach is diluted by factors in the approximate range 10-100, which correspondingly reduces the equilibrium partial pressure. However, in application, the pH of the cleaning solution may shift, and any shift in the direction of lower pH would tend to increase the relative abundance of HOCl as a fraction of the total hypochlorite. In turn, this shift would tend to promote increased vapor pressure and enhancements of the gas-phase abundance of HOCl. This point is illustrated in Figure 10, which presents the equilibrium gas-phase concentration of HOCl versus solution pH for different fixed levels of the total concentration of hypochlorite in the aqueous solution, Ct = [HOCl] + [OCl- ]. The upper three traces in the figure correspond to undiluted consumer bleach , and dilution of this product into water at respective ratios of 1:10 and 1:100. Hypochlorous acid is a key chemical ingredient when chlorination is used for drinking water disinfection.
Chlorine is also commonly used to maintain microbial hygiene in swimming pools. In US municipal drinking water applications, a typical target level of residual free chlorine is 0.2 ppm, which corresponds to Ct = 2.8 µM. In swimming pool applications, the common target level of free chlorine would be 1-3 ppm, corresponding to Ct = 14-42 µM. A common pH in these treated waters is 7.5. The lower three traces of Figure 10 show that for Ct values in the range 1-100 µM at pH = 7.5, the equilibrium gas-phase concentration of HOCl would be in the range 0.8-80 ppb. There are few published measurements of airborne HOCl. Lawler et al. reported “the first measurements of tropospheric HOCl” based on a June 2009 atmospheric sampling campaign conducted in the eastern tropical Atlantic. They found that “in air with trajectories originating over Europe … HOCl maxima [exceeded] 100 ppt [0.1 ppb] each day.” Wong et al. reported indoor air concentrations of HOCl and related compounds in response to applying a bleach solution to the floor of a well-ventilated room . They utilized online chemical ionization mass spectrometry along with aerosol mass spectrometry and reported peak HOCl levels of 100s of ppb. They also reported measuring gaseous chlorine , nitryl chloride , dichlorine monoxide , and chloramines . Peak observed submicron particle Cl in these experiments was about 0.1 µg/m3 . There are several circumstances by which the presence and reactivity of hypochlorous acid in aqueous solutions generates human health risk concerns related to indoor air pollution. The following paragraphs highlight some key points. In these descriptions, we’ll use the term “hypochlorous acid” to include both HOCl and OCl- , recognizing that the aqueous partitioning between these species is controlled by pH , and that only HOCl has a meaningful gas-phase presence. There are three primary species in this system: gaseous HOCl, aqueous HOCl, and aqueous OCl- . Bleach and certain other consumer cleaning products contain hypochlorous acid. Acute adverse respiratory outcomes have been documented owing to the improper mixing of such products with other chemicals. For example, combining hypochlorous acid with ammonia based cleaners can lead to the formation of chloramines, including nitrogen trichloride , a volatile respiratory irritant. On the other hand, combining bleach with acids that lower the solution pH increases the release of gaseous HOCl and can also produce chlorine gas ; both of these species are potentially health harmful if inhaled. The risks of adverse chemistry initiated by hypochlorous acid is not limited to improper mixing of cleaning products. For example, Odabasi and coworkers have documented that halogenated VOCs including chloroform and carbon tetrachloride can be formed in product bottles that combine fragrances or surfactants with hypochlorous acid. Schwartz-Narbonne et al. studied the reactive chemistry of gaseous HOCl with surface-bound squalene and oleic acid, compounds prevalent on indoor surfaces. They documented that chlorohydrins are formed in this process at rapid rates, concluding that “chlorination of skin oil, which contains substantial carbon unsaturation, is likely to occur rapidly under common cleaning conditions, potentially leading to the irritation associated with chlorinated bleach.” Epidemiologic studies show an elevated risk of work-related asthma for professional and domestic cleaning. 253 Specific causes for the elevated risk aren’t known, but the use of cleaning products containing hypochlorous acid is considered to be one possible contributor. Interestingly, a study by Nickmilder et al. found that “children living in a house regularly cleaned with bleach were less likely to have asthma … eczema … and of being sensitized to indoor aeroallergens, … especially house dust mite.” The underlying process may be associated with chlorine-based damage of allergenic proteins. On the other hand, there is also evidence of increased risk of respiratory irritation associated with regular use of bleach indoors. Beyond respiratory irritation, allergenicity, and asthma, the use of hypochlorous acid as a cleaning product ingredient raises other health-risk concerns. Such risks can arise from exposure to disinfection byproducts such as trihalomethanes . The most prominent THM from hypochlorous acid chemistry is chloroform , which is among the higher ranking causes of cancer for organic hazardous air pollutants. The use of hypochlorous acid as a cleaning ingredient has been shown to contribute to indoor emissions of chloroform in the case of residential washing machines and automatic dishwashers. In particular, Shepherd et al. reported that “washing machine environments are very conducive to chloroform formation.” They suggested that similar annual emissions of chloroform indoors would occur from bleaching laundry as from showering.