Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxins, and Allergens

IndoorAir 1995. 5: 171--178 Printed i n Dminiark . all rights reserved

Copyright 0 Munksgaard 1995

Indoor Air ISSN 0905-6947

Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxins, and Allergens Richard L. Tyndall', Eva S. k h a n 2 , Elicia K. Bowman3, Donald K. Milton4, James M. Barbaree5

Abstract The propensity of various types of home humidzjiers w support and disseminate microbial contaminants inw indoor air was tested. Reservoir water and air discharged fiom humidifiers seeded in the laboratory or naturally contaminated in the home were analyzed by standard microbiological methods. Clin- ically insignificant as well as overt or potentially pathogenic microorganisms were found to colonize the reservoirs of all types of humidzjiers, but only cool mist and ultrasonic units readily aerosolized bacteria and endotoxin. Only cool mist units emitted hydrophobic fungal spores. Cool mist units discharged the greatest number of water particles in the inhalable size range (4-16 pm) while ultrasonic units were more likely to emit respirable-sized water particles (<O. 2-4 pm). Overt pathogens isolated fiom humidifiers in homes included Le- gionella and a pathogenic Acanthamoeba. Aerosolizing humidifiers should thus be avoided qfiequent, thorough cleaning of the units is not practical.

KEY WORDS: Aerosols, Bacteria, Endotoxin, LRgioneUa

Manuscript received: 14 October 1993 Accepted for publication: 24 October 1994 The views expressed in this article are strictly those of the authors and do not necessarily represent those of the Consumer Product Safety Commission. This study, conducted at the ex- pense of the U.S. Government, is considered in the public domain

Oak Ridge National Laboratory, P.O. Box 2008, Bdg. 9207 MS-8077, Oak Ridge, TN 37831-8077, USA Tel.: (6 19574-0686. Fax: (615)574-1274

USA A&G Technical Associates, Knoxville, TN, USA Harvard University, School of Public Health, Boston, MA, USA Centers for Disease Control, Atlanta, GA, USA

* Consumer Product Safety Commission, Washington, D.C.,

Introduction The U.S. Consumer Product Safety Commission initiated and supported a study to address concerns about airborne transmission of bioaerosols from a variety of humidifiers. Humidifiers, used by thou- sands of consumers, have been implicated in nu- merous published episodes of hypersensitivity pneumonitis, humidifier fever and infections in hospitals, industrial settings and homes (Kreiss and Hodgson, 1984). Klein and Kunze (1971) showed that Pseudomonads were aerosolized by cool mist humidifiers, and Covelli et al.. (1973) demonstrated that Pseudumonas could also be aerosolized by four other types of humidifiers. Solomon (1974) found a three-fold increase of airborne fungi in a majority of test homes during operation of cool mist humidifiers. Burge et al. (1 980) demonstrated the presence of bacteria and fungi in both furnance and evaporative console home humidifiers. Amoebic antigens and bacterial endotoxins isolated from humidifier fluids have been implicated in cases of hypersensitivity pneumonitis occurring in homes (Burke et al., 1977; Miller, 1976, Tourville et al., 1972; Shiue et al,, 1990) and in outbreaks of humidifier fever diag- nosed in industrial settings (Edwards, 1980; Ry- lander and Snella, 1983; Ashton et al., 1981). Free- living amoebae, such as Naegleria, were frequently detected in these studies (Edwards, 1980; Edwards, 1986). Gram-negative bacteria, such as Pseudomon- as species, have been implicated as a source of airborne bacterial endotoxins (Shiue et al., 1990; Ry- lander and Haglind 1984, American Conference of Governmental Industrial Hygienists 1990).

While these studies identified humidifiers as a source of indoor microbial contamination, they did not systematically compare types of humidifiers with the microbial populations in their reservoirs or

172 Tyndall et al.: Home Humidifiers os o Potential Source of Exposure to Microbial Pathogens, Endotoxions, and Allergens

their abilities to aerosolize microorganisms and endotoxins into the environment. The current study was undertaken to address these issues.

Materials and Methods Five different types of humidifiers were tested in the laboratory. Three models each of cool mist and ultrasonic units and two models each of warm mist, console, and steam units were tested. Test models represented seven different manufacturers. Humidi- fication by steam vaporizers and warm mist units is effected by heating the water in the reservoir and discharging the water as steam. Cool mist units discharge various sized droplets of unheated water directly from the reservoir by impeller action. Ultra- sonic units discharge water from the bottom of the reservoir via a vibrating nebulizer. Evaporative console units produce water vapor by fan-forcing air through a fabric belt wetted by moving through the water reservoir. In addition to the laboratory humidifiers, a variety of humidifiers in homes in the Oak Ridge, Tennessee area were also studied.

In order to produce scale and biofilm in laboratory humidifiers which in turn facilitated growth of microbes, the units were operated continuously for a period of five months and were filled as required with tap water. Maintenance was not performed on any unit during the five-month period. The reservoirs of the units were not emptied or rinsed during this period. After this “conditioning” period, scale formation was apparent on various internal surfaces of the humidifiers. The conditioned units were then sterilized by exposure to ethylene oxide. New un- conditioned humidifiers were used in endotoxin studies.

For various laboratory tests, the conditioned, sterilized humidifiers were filled with sterile tap water, inoculated with microbes, and operated in two room-sized chambers (2.4 mX2.4 mX2.4 m). Morning and afternoon samples were taken from the reservoir or from the air in the chamber. Air samples were collected approximately one meter from the humidifier discharge. The air exchange rates within the chambers were monitored on an intermittent basis by releasing CO as a tracer gas. The data from the CO monitor and the relative hu- midityhemperature probe were recorded on a data logger, and the information from the data logger was regularly downloaded onto magnetic media for processing. The air exchange rate in the test chambers was 1.2-1.5 air changes per hour. During

operation, relative humidity in the test chambers generally ranged from 4675% with the steam-gen- erating models producing the lowest humidity. Temperatures ranged from 18” to 24°C.

A Portable Continous Aerosol Monitor (PPM, Inc., Knoxville, Tennessee) was used to determine droplet size emitted from various types of humidifiers without added microbes operating within the test chambers. The laser-scattering-based system determined the size range and mass concentrations of emitted aerosols. Samples were taken for five- minute periods, and the data reported are the average of at least two separate five-minute periods.

A spectrum of microbial genera was used to seed the laboratory humidifiers. The test microbes were Bacillus, Micrococcus, Pseudomonas, Flavobacter, As- pergillus spores, and Acanthamoebae ruyreba. The bacteria were originally isolated from either laboratory or home humidifiers. The bacterial cultures were grown on trypticase soy agar (TSA). Bacterial lawns were harvested from the TSA plates, sus- pended in sterile, distilled water, centrifuged, and washed in additional sterile distilled water. The re- sultant pellets were frozen in aliquots at -80°C until needed. The laboratory reservoirs were seeded with 104-105 total bacteria per ml. Fungal spores were supplied by Optima Company (New Haven, Con- necticut) as washed preparations from fungal cultures (295% pure with <3% contamination of the fungal mycelia) and were grown on malt extract (ME) media. Acanthamoebae ruyreba cultures were grown in tissue culture flasks (75 cm2) in a liquid casitone-based media (CGV). The protozoa were concentrated and washed by centrifugation in a 10% solution of Dulbecco-balanced salts solution. The amoebae were harvested immediately prior to their introduction into the test humidifiers.

Microbial concentrations in the laboratory and home humidifiers’ reservoir water were determined as follows: 10 ml samples were removed with sterile pipettes, and aliquots were titrated on TSA , ME, or on non-nutrient agar plates spread with a lawn of live E. coli (”AE). Plating of ten-fold dilutions of the water samples on TSA and ME at 23-25°C and NNAE at 35°C determined the concentrations of the bacterial, fungal, and Acanthamoebae titers, respectively. For detection of Legionella, samples were plated on BCYE agar either directly or after a 10-14 day incubation of the water sample at 35°C. In addition, iodine was added to aliquots of water from the home humidifiers to effect a 1% solution. These preparations were observed microscopically

Tyndall et a1 , Home Humidifiers as a Potential Source of Ex

for the presence of protozoa. Pathogenicity of some Acanthamoebae isolated on NNAE from home humidifier water was tested by inoculating approximately 1 O5 trophozoites intranasally into weanling BABUc mice. Three to six weeks after inoculation, mice were sacrificed and brain and lung tissues were plated on NNAE plates to determine the presence of amobae in the test tissue.

Microorganisms in air were collected and enu- merated by use of Andersen samplers using the sixth-stage plate. Air was sampled at a rate of 28.3 Vmin for two minutes. Vacuum pumps used in con- junction with the Anderson samplers were tested periodically with a rotameter to ensure the proper airflow rate. The colonies on the test plates were counted by the “positive hole” method.

For the endotoxin studies, Pseudomonas endotoxin (Sigma Chemical Coy St. Louis MO) was selected because this organism is frequently found as a contaminant of home humidifiers (Rylander, 1986). Background aerosol samples were obtained from indoor (chamber) and outdoor air and from humidi-- fier emissions when reservoirs were filled with pyrogen-free water before endotoxin addition. Endo- toxin levels in water samples were measured directly by the LAL test. Aerosol samples were collected from humidifier emissions at approximately 0.5 m and 1.5 m from the aerosol discharge by passage through polyflow filters and into condensate traps. The filter extracts and condensates were then analyzed according to recently published methods (Mil- ton et al., 1992).

45.000

40,000

35,000

5 30,000 6 25,000 z (r 20,000 W a 2 15,000 :: 10,000 2 2,000 0

W 5 1,500

1,000

500

COOL-MIST

CONSOLE I STEAM

II ULTRASONIC

I- =- 0 WARM-MIST

rl

BACTERIA FUNGI

Fig. 1 Aerosolizotion of mixed bacterial flora and fungi by various humidifer types in laboratory studies. The data shown are the average of six tests with each humidifier

:posure to Microbial Pathogens, Endotoxions, and Allergens 1 7 3

lL

z ULTRASONIC

W

25,000

(r 20,000 a

m

15,000 LL ; 10,000

2 2,000 0

W 2 1,500

1,000

500

0 BACTERIA FUNGI

Fig. 2 Airborne bacterial and fungal concentration in discharge of vorious humidifier types in area homes. The doto ore the overage of duplicate tests carried out on four ultrasonic units, four cool mist units, two console units, three steam units and nine furnonce humidifiers

To relate the laboratory studies with operation of humidifiers in the home environment, the microbial content and aerosolization by home humidifiers were investigated. On two separate occasions, Feb- ruary-April in 1987 and 1988, samples from reservoirs of home humidifiers were collected in sterile 250 ml containers. A total of 88 samples were obtained and analyzed for microorganisms as previously described. In addition, volunteers re- sponded to questions on humidifier use. Air samples were also taken in some of the homes as described in the laboratory studies. After the humidifiers had been turned off overnight, background air samples were collected using duplicate TSA, ME, and NNAE plates in the Andersen sam- pler. Water samples were collected from the reservoirs of the humidifiers. The units were then turned on and after running for 5 min, air samples were taken again using duplicate TSA, ME, and NNAE plates. Sampling took place in the air stream, generally at a distance of 0.9 to 1.8 m from the humidifier. Additional samples were taken 30 minutes later.

Resu I ts Laboratory Studies Experiments to evaluate the potential of selected humidifiers to disperse bioaerosols under laboratory conditions showed marked differences in the various humidifiers’ ability to aerosolize fungal spores

174 Tyndall et al.: Home Humidifiers as a Potential Source of Exposure to Microbial Pathoqens, Endotoxions. and Alleraens

Table 1 Air particle percentages in humidifier plumes*

Humidifier Concentration Particle percentages type (mg/m3) <0.2 pm 0.24.0 pm >4.0-15.9 pm

Steam 16 49 6 43 Steam 19 35 8 57

Ultrasonic Ultrasonic Ultrasonic

5 33 24 44 5 27 48 22 5 13 10 76

Cool mist 14 1 13 86 Cool mist 4 4 8 87 Cool mist 18 1 5 93

* Measured concentrations and sizes at the aerosol monitor sensor; sizes and concentrations may be slightly larger due to possible evaporation during transit in tubing Console humidifier units did not discharge detectable particles

and bacteria (Figure 1). The cool mist humidifiers discharged high numbers of both fungi and bacteria. The ultrasonic humidifiers showed marked discharge of bacteria but not of Aspergillus niger spores. Console units showed a slightly higher discharge of microbes than steam vaporizers and warm mist units but much less than cool mist or ultrasonic units.

The aerosolization of both bacteria and fungal spores by cool mist units and bacteria by the ultrasonic units was seen in both models of each type. The lack of aerosolization by steam vaporizers, warm mist, and console units was also consistent between models. Airborne amoebae were seen in only three samples: two from cool mist units and one from a console unit. Background and other air samples were negative for amoebae.

Analysis of reservoir populations showed a con- tinuing presence of the various microorganisms throughout the 72-hour test period except for loss of fungal spores and amoebae from one cool mist unit. The dispersal rate of the microbial population could not be determined because of lack of information concerning the percent of dead airborne microbes at sampling time, the rate of loss of viability when airborne, and the effects of concentrating and diluting the microbial population as water is lost and replaced in different amounts.

Results of a modest effort to characterize the physical nature of aerosols from humidifiers in the laboratory using the aerosol monitor are shown in Table 1 and indicate the particle size profiles and the measured output of the humidifiers.

The ultrasonic units expelled 13% to 33% of their output as particles below 0.2 pm i.e. diffusing particles. From 22% to 76% of their output was in

the 4.0 to 16 pm range and 10% to 48% in the 0.2- 4.0 pm range. The cool mist units ranged from 86% to 93% of their output in the 4.0 to 15.9 pm range. The highest percentage of particles emitted by steam vaporizers was in the <0.2 pm and the 4.0 to 15.9 pm range. The console humidifiers gave no detectable measurements with the aerosol monitor since water loss was due to evaporation.

The results of the endotoxin studies are shown in Tables 2 and 3. The outdoor samples measured 0.18 and 0.10 ng/m3. One indoor background sample was 0.02 ng/m3, and endotoxin was not detectable in the other indoor background sample. Endotoxin levels that exceed background levels by 1 00-fold may be associated with respiratory toxicity (American Conference of Governmental Industrial

Table 2 Endotoxin emitted by aerosolizing humidifiers during four hours of operation (ng/m3)

Humidifier Filte?

ULTRASONIC cleanb spikedc

clean spiked

outdoord indoord

COOL MIST

BACKGROUND

Close" Fa? (0.5 m) (1.5 m)

0.33 0.08 620 360

0.04 0.13

Sample 1 Sample 2 0.10 0.18 0.02 N.D.

1100 530

a - Distance from source b - Pyrogen free water c - Seeded with 1000 ng/ml of Pseudomonas endotoxin d - Before turning on humidifiers nd - None detected

Tyndall et 0 1 . : Home Humidifieis as a Potential Source of Exposure to Microbial Pathoaens. Endotoxions. and Alleraens 175

Table 3 tndotoxin emitted by evaporotive humidifiers during four hours o f ooeration (ng/m3)

Humidifier FilteP

Close" Far" (0.5 m) (1.5 m)

WARM MIST clean" spiked' clean spiked

clean spiked

clean spiked

STEAM- 1

STEAM-2

0.04 0.10 0.06 0.29

0.03 0.07

0.15 0.04

0.27 0.18 0.07 0.32

0.03 0.1 1

0.11 0.21

BACKGROUND Sample 1 Sample 2 outdoor 0.10 0.18 indoord 0.02 N.D.

a - Distance from source b - Pyrogen free water c - Seeded with 1000 ng/ml of Pseudomonas endotoxin d - Before turning on humidifiers nd - None detected

Hygienists, 1990). Therefore, the aerosol levels as measured on collecting filters in this study which exceeded 18.0 ng/m' are likely to be significant.

Air levels of endotoxin from ultrasonic and cool mist units, as shown by the close filter collections (Table 2), exceeded control background levels by about 15,000-fold for the cool mist and almost 8,000-fold for the ultrasonic unit. The largest emissions of endotoxin collected on the far filters also came from the cool mist (530 ng/m3) and the ultrasonic unit 360 ng/m3. Evaporative humidifiers such as the warm mist and steam vaporizers did not aerosolize endotoxin (Table 3).

Home Studies Table 4 summarizes the results obtained from the humidifier reservoir samples obtained from homes

in 1987 in the Oak Ridge, Tennessee area. Bacteria were recovered from 84% of the samples. The predominating genera were Pseudomonas, Flavobacter, and Bacillus. Micrococcus, Diphtheroides, and Staphy- lococcus were found in a few units. Only 33% of humidifiers contained recoverable fungi which were predominantly Cladosporium, Penkllium, and Asper- gillus. Candida, Rhodowrula, Fusarium, and Rhi- zobium were isolated from a few units.

Amoebae were detected in 71 of 88 units. Thirty amoebic isolates were tested for pathogenicity by in- tranasal inoculation of weanling mice. Twelve of these were later isolated from mouse lungs and one from brain tissue. Other protozoa including En- terobius were found in 35% of the units. In a subset of 27 homes with either ultrasonic or cool mist humidifiers, the water was also analyzed specifically for Legwnella. Two of the water samples yielded viable Legionella. Serotyping showed that one of the isolates was L. pneumophila serogroup one. This isolate had the antibody typing profile indicative of virulent Legionella most frequently associated with Legion- naires' disease outbreaks (Barbaree, 1993). The other LRgwneUa isolate is as yet unidentified.

Data obtained in response to questions addressed to humidifier owners indicated no obvious relation- ship between the type of water used, the thermostat setting, or the room where the units were used (Tyndall et al., 1989, 1991). All units receiving no decontamination treatment had microbial reservoir concentrations from lo5 - lo7 viable colony form- ing units (CFU) per ml. Units treated infrequently with chlorine bleach or vinegar had similar counts. Units with low microbial concentrations (< 1 05/ml) had been treated routinely with chlorine and/or hy- drogen peroxide by owners (Tyndall et al., 1989, 1991).

Air, reservoir, and scale samples collected from humidifiers in 22 homes during the spring of 1988 showed that the predominating bacteria and fungi in the reservoirs and scale were akin to those pre-

Table 4 Types of miciobial contaminants found in 88 reservoir water samples in different types of home humidifier

Type of Number Number and (%) of units containing humidifier evaluated Bacteria Fungi Amoebae Protozoa

Steam 6 6 (100) 2 (33) 4 (67) 2 (33) Ultrasonic 15 10 (66) 2 (13) 7 (47) 3 (20) Cool-Mist 15 12 (80) 8 (53) 15 (100) 5 (33) Console 16 15 (93) 2 (12) 13 (81) 10 (63) Furnace 36 31 (86) 15 (42) 32 (87) 11 (31)

Total 88 74 (84%) 29 (33%) 71 (81%) 31 (35%)

176 Tyndall et 01.: Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxions, and Allergens

viously encountered in the 1987 sampling of the home humidifier reservoirs. Even though the bacterial concentrations in reservoirs of furnace and console units were similar to those in cool mist and ultrasonic units, only the latter units generally showed a marked propensity for aerosolization of the bacterial flora (Figure 2). The wide range, but high average concentration, of airborne bacteria in the cool mist and ultrasonic units is indicative of the few units that were treated routinely and had low bacterial reservoir concentrations. Airborne amoebae were detected in only one home where a cool mist humidifier was operated.

Discussion In these laboratory studies, mechanically produced aerosols from microbially seeded cool mist and ultrasonic humidifiers contained microbes and endotoxins. Consistent with these studies, ultrasonic and cool mist humidifiers from homes also aerosolized microbes while console, steam and fb-nace humidifiers showed little propensity for aerosolization of their bacterial contents. Whether in a clinical or home environment, aerosolization of potentially pathogenic microorganisms, allergens and associated endotoxins may present a respiratory health hazard.

The isolation of Legionella and pathogenic amoebae from humidifiers in homes in eastern Tennessee underscores the need to minimize or eliminate this potential source of exposure to microbial pathogens.

Legionella, the causative agent of the flu-like Ponti- ac fever and pneumonic Legionnaires’ disease, has previously been isolated from humidifiers and respiratory equipment (Zuravleff et al., 1983; Arnow et al., 1982; Woo et al., 1986; Fields et al., 1989) in hospitals and from both hospital and home hot water heaters (Fields et al., 1989; Alary and Joly, 199 1; Bol- lin et al., 1985). This report confirms the isolation of L. pneumophila from home humidifier reservoirs and their potential for aerosolization.

Since amoebic dispersion from humidifiers is probably infrequent due to the relatively large size of the microorganism, the importance of free-living amoebae in humidifier reservoirs relates to their interac- tions with Legionella. Initially laboratory studies (Rowbotham, 1980; Tyndall and Dominque, 1982; Anand et al., 1984, Barbaree, 1986) showed that free-living amoebae such as Acanthamoebae and Nae- gleria could be infected by and markedly amplify Le- gwnella. Subsequently, the presence of Hartmannella in tap water and hot water heaters was also linked to

the amplification and consequent aerosolization, via shower heads, of Legionella (Bollin et al., 1985; Wa- dowsky et al., 1991). Thus, the isolation of both Le- gionella and free-living amoebae from home humidifer reservoirs suggests that the concomitant presence of these two microbes could result in amplification and dissemination of Legwnella.

In addition to overt pathogens such as Legionella and Acanthamoebae, discharge of Pseudomonads from humidifiers is of potential concern since they are opportunistic pathogens, can release endotoxins and cause infection in compromised individuals (Lennette et al., 1985). Similarly, allergies to fungal antigens are not uncommon, and amoebic antigens have been associated with allergic responses such as hypersensitivity pneumonitis and humidifier fever. Considering the size of water droplets dispersed by cool mist and ultrasonic humidifiers, it is not surprising that Legionella and other bacteria can be aerosolized by such units (Table 1). Laser analysis of droplet size showed that cool mist units generate and expel1 the greatest proportion of large water droplets of all tested units. The most likely expla- nation for the dissemination of bacteria but not of hydrophobic Asper~llus spores by ultrasonic units, is that aerosolized water was drawn from the bottom of the unit while the hydrophobic spores float on the surface of the water. Hydrophilic yeast cells were aerosolized by ultrasonic units (Tyndall et al., 1989, 199 1). While a greater percentage of smaller water droplets were aerosolized from ultrasonic units relative to those emitted from cool mist units, a significant percentage of the drops was large enough to contain viable microorganisms. The lower concentration of microbial aerosolization by the console units in this study and as previously noted by Schul- ze et al. (1 976) probably relates to the evaporation of water from the belt as the means of humidification. It was not surprising that the steam vaporizers and warm mist units showed little dissemination of viable microbes since thermally induced evaporation of water is unlikely to produce aerosols containing viable microorganisms.

A recent report by Highsmith and Rodes (1 988) also compared the particulates discharged by various humidifier types. As in the studies reported here, these researchers showed that cool mist units discharged larger-sized particles compared to ultrasonic and steam units. These authors also pointed out that the concentration of particulates discharged by ultrasonic and cool mist humidifiers exceeded the EPA standards for outdoor ambient air. Thus,

Tyndall et 0 1 . : Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxions, and Allergens 177

these types of humidifiers may discharge not only high levels of particulate matter, but also copious amounts of microorganisms and associated endotoxin which may potentially affect the health of exposed individuals.

As was seen with bacteria, endotoxin was also readily aerosolized from the reservoirs of some home humidifiers. While both cool mist and ultrasonic humidifiers aerosolized endotoxin from their reservoirs, warm mist and steam vaporizers did not. Seeding the reservoirs with 1000 ng/ml of endotoxin was based on studies (Rylander, 1986) where concentrations of 160 to 5000 ng/ml were associated with illness. Rylander reported that inhalation chal- lenges with endotoxin from Gram-negative bacteria produce symptoms typical of acute episodes of humidifier fever (Rylander, 1986). Endotoxins from Gram-negative bacteria have been identified in reservoir water and in aerosols of humidifiers where outbreaks of humidifier fever have occurred (Flah- erty, 1984a; Flaherty, 1984b; Burrell, 1990; Ry- lander and Snella, 1983). In 32 out of 39 water samples from humidifiers associated with clinical cases of humidifier fever, the endotoxin level was in excess of 100 ng/ml (Rylander and Haglind, 1984).

The air levels of endotoxin obtained in the experimental chamber in the current study were above the levels that have been associated with the humidifier fever syndrome. According to recent guidelines from the American Conference of Governmental Industrial Hygienists, significant health effects may occur at inhalation exposures of 100 to 1000 times background levels and as such indicate that remedial action should be taken. Endotoxin aerosolization by the cool mist and ultrasonic units in this study exceeded these criteria.

Both physicians and the public should be aware of the potential health hazard associated with humidifiers which aerosolize reservoir contents since they may contain overt pathogens, opportunistic pathogens, endotoxins, and allergens. To reduce this hazard, effective cleaning and maintenance practices as described elsewhere are essential (United States Consumer Product Safety Com- mission, 1988; United States Environmental Pro- tection Agency, 199 1).

Acknowledgements The authors would like to thank Kevin S. Ironside, Rebecca A. Jernigan, and Diana S. Katz for their expert technical assistance.

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Home Humidifiers as a Potential Source of Exposure to Microbial Pathogens, Endotoxins, and Allergens

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