Physical and Chemical Characterization of Indoor Aerosols Resulting from the Use of Tap Water in Portable Home Humidifiers V. Ross Highsmith"
Atmospheric Research and Exposure Assessment Laboratory, US. Envlronmental Protectlon Agency, Research Triangle Park, North Carolina 2771 1 Richard J. Hardy
Morrison-Knudsen Corporation, Environmental Services Group, P.O. Box 79, Boise, Idaho 83707 Daniel L. Costa
Health Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711 Mark S. German1
McCrone Associates, 850 Pasquineili Drive, Westmont, Illinois 60559 ~~~~~~~~
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Indoor PMlo concentrations exceeding 150 pg/m3 were observed when ultrasonic and impeller humidifiers, charged with tap water, were operated under whole-house conditions. Microscopic analysis confirmed that the resulting particles were predominantly submicron spheres with mineral composition reflecting that of the charging water. Elements measured in high concentrations were uniformly present as soluble salts in both the fine and coarse particles, with small differences observed between the ultrasonic and impeller humidifier-generated aerosols. Nearly 50% of the 39 commercial bottled water samples collected across the United States had dissolved mineral concentrations exceeding 10 mg/L, with 4 samples exceeding 100 mg/L. The results of this limited-scale study suggest that personal exposures to ultrasonic and impeller humidifier-generated aerosols can be minimized through the use of waters containing low concentrations of impurities.
Background Investigators recently reported (1)PMlo concentrations (particles lo0 pg/m3 inside a home with no known indoor combustion sources. (Except where noted, particle sizes refer to aerodynamic diameter.) After reviewing the homeowners' activity diaries, the researchers were able to correlate the increased PMlo with the documented use of a portable ultrasonic humidifier charged with local tap water. A series of limited-scale, range-finding experiments were conducted (2) to confirm that the ultrasonic humidifier was the source for the increased particle concentrations. These experiments were also designed to evaluate the potential for selected portable humidifiers to readily disperse dissolved impurities as a respirable aerosol as well as to examine the relationships among the charging water quality, the humidifier type, and the resulting aerosol concentrations. Selected portable humidifiers were charged with tap waters covering a range of total dissolved solids (TDS) 0013-936X/92/0926-0673$03.00/0
concentrations and individually operated under varying conditions to estimate potential indoor PMlo concentrations. PMloconcentrations exceeding 400 and 7000 pg/m3 were observed when an ultrasonic humidifier, charged with 303 mg/L TDS water, was operated under whole-house and single-room conditions, respectively. The resulting indoor aerosol concentrations were highly correlated with the TDS concentrations of the humidifier charging waters. These results were published (2) to advise indoor air researchers of the potential influence this previously unidentified source may have on future indoor air quality investigations. Indoor air quality standards similar to the National Ambient Air Quality Standards (NAAQS) and other occupational standards do not exist (3-7). Nonetheless, the elevated particle concentrations observed in the rangefinding study led to comparisons with the EPA's 150 pg/m3 PMlo primary standard for respirable particles (3) and the 5 mg/m3 Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for respirable nuisance particulates (5). The intention was to consider these standards as possible guides that signal aerosol concentrations of concern, especially when there are little data available on their potential impact on human health. However, it is important to emphasize that these standards were developed using health effects data associated with ambient and industrial particulate aerosol exposures. Consequently, direct comparisons should be viewed with caution. The report was widely distributed throughout the scientific community as well as the general populace. Several scientific questions were immediately raised. First, how do humidifier-generated aerosols compare both physically and chemically with the charging waters as well as outdoor PMIo aerosols? Second, how does the bottled water used in this experiment (24 mg/L) relate to the US. bottled water supplies and the United States Pharmacopoeia (USP) 10 mg/L standard for "purified" water (8)? And
0 1992 American Chemical Society
Envlron. Sci. Technol., Vol. 26, No. 4, 1992 873
Table I. Indoor PM,,Concentrations Resulting from Operating Portable Home Humidifiers under Whole-House and Single-Room Conditions
operating conditions"
humidifier type
ID
whole house whole house whole house whole house whole house whole house whole house single room
ultrasonic ultrasonic impeller impeller steam ultrasonic none ultrasonic
UH-1 UH-2 IM-1 IM-2 ST-1 UH-1 N/A UH-1
water consumption rate, L/h
charging water TDS, mg/L
0.48
303 303 303 303 303 24 N/A 303
0.35
0.17 0.09
0.34 0.48 N/A 0.48
aerosol concn, rg/m3 fine coarse total 593 378 146 33 16 41 11
65 4% 4% 23 25 13 8
6307 771 a Whole house: volume, 392 m3; air-exchange rate, 0.44 h-l. Single room: volume, 25 m3; air-exchange rate, 1.16 h-'.
most importantly, what, if any, are the health-related issues that may result from humidifier-generated aerosol exposures? These issues warranted conducting a series of follow-up analyses of the samples previously collected (2) and a reanalysis of the associated data. Selected humidifiergenerated aerosol samples were subjected to thorough physical and chemical analysis. A random survey of bottled waters, which are regulated by the US.Food and Drug Administration, was conducted to examine the TDS concentrations of selected commercial bottled waters available to the U.S. consumers. After careful review of these results, other available aerometric, particle, and health effects data were compared to the humidifier-generated aerosols in an effort to qualitatively assess potential health-related issues which could result from personal exposures to humidifier-generated aerosols. This report outlines the results of the follow-up analyses on the previously collected humidifier-generated aerosol samples, the results of the U S . bottled water supply survey, and a preliminary health assessment of humidifier-generated aerosols. The previously reported study findings are also summarized. Experimental Section Sample Collection. Selected commercially available ultrasonic, impeller, and steam portable home humidifiers were charged with waters containing 24-303 mg/L TDS concentrations and independently operated under wholehouse conditions. A series of environmental monitors were simultaneously operated in a lower level bedroom during each whole-house experiment. A PMIodichotomous sampler collected fine (1pm in diameter corroborates this conclusion and suggests that the collision-type droplet formation mechanism generates a broader distribution with more large-diameter droplets. The EAA and automated-SEM PSDs were combined to develop a representative cumulative number distribution (Figure 4). An aerodynamic cumulative volume distribution was calculated (23) from the number distribution (shape factor, 1.4 and constant density, 2.5 g/cm3). The resulting volume median aerodynamic diameter is 1.2 pm. For a constant particle density, the cumulative volume distribution should be equivalent to the dichotomous sampler cumulative mass distribution at the 2.5-pm virtual impactor cutoff. The cumulative volume distribution becomes nonlinear above 1 pm and intersects the 2.5-pm cut point a t 72%, a point well below the fairly constant experimental ratio of 92 f 4.5% (95% confidence level). Extrapolating the linear submicrometer portion of the aerodynamic volume distribution to the 2.5-pm dichotomous sampler cut point results in a fine/total ratio of 93%, which is nearly identical to the experimental value. The discrepancy between the observed dichotomous mass fraction and the computed aerodynamic distribution remains unresolved. However, the formation of less dense, possibly hollow spheres as observed by Leong (10) for >1-pm lithium carbonate particles is suspected as a plausible cause. Bottled Water Survey. Thirty-nine bottled water samples of six classifications (Table 111) were purchased and forwarded to a central laboratory for analysis, with the majority being “distilled” (n = 22) and “deionized (n = 7). The results of analysis on the 39 commercially available bottled water samples received from nine regional EPA offices, Boise, ID, and Raleigh, NC, showed substantial variation in bottled water TDS levels, ranging from 10 mg/L 22
