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

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Transcript of 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 dis- charged 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 (

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

    their abilities to aerosolize microorganisms and en- dotoxins 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 dis- charge various sized droplets of unheated water di- rectly from the reservoir by impeller action. Ultra- sonic units discharge water from the bottom of the reservoir via a vibrating nebulizer. Evaporative con- sole units produce water vapor by fan-forcing air through a fabric belt wetted by moving through the water reservoir. In addition to the laboratory hu- midifiers, a variety of humidifiers in homes in the Oak Ridge, Tennessee area were also studied.

    In order to produce scale and biofilm in labora- tory 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 reser- voirs 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 24C.

    A Portable Continous Aerosol Monitor (PPM, Inc., Knoxville, Tennessee) was used to determine droplet size emitted from various types of humidi- fiers 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 aver- age 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 labora- tory 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 -80C 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 cul- tures (295% pure with

  • 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 hu- midifier water was tested by inoculating approxi- mately 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 endo- toxin (Sigma Chemical Coy St. Louis MO) was se- lected because this organism is frequently found as a contaminant of home humidifiers (Rylander, 1986). Background aerosol samples were obtained from in- door (chamber) and outdoor air and from humidi-- fier emissions when reservoirs were filled with pyro- gen-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 ana- lyzed according to recently published methods (Mil- ton et al., 1992).




    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






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

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




    (r 20,000 a


    15,000 LL ; 10,000 2 2,000 0 W 2 1,500




    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 humidi- fiers

    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 reser- voirs of home humidifiers were collected in sterile 250 ml containers. A total of 88 samples were ob- tained and analyzed for microorganisms as pre- viously 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 hu- midifiers had been turned off overnight, back- ground air samples were collected using duplicate TSA, ME, and NNAE plates in the Andersen sam- pler. Water samples were collected from the reser- voirs 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, gener- ally at a distance of 0.9 to 1.8 m from the humidi- fier. 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 vari- ous 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) 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 bac- teria. The ultrasonic humidifiers showed marked discharge of bacteria but not of Aspergillus niger spores. Cons