Envi,o,.,,.,,t lateraational, Vol.16, pp. 203-211, 1990 0160-4120/9053.00 +.00 Printed in the U.S.A. All rights reserved. Copyright @1990 Pergamon Press plc MULTIPLICATION OF MICROORGANISMS IN AN EVAPORATIVE AIR COOLER AND POSSIBLE INDOOR AIR CONTAMINATION J.M. Macher and J.R. Girman Indoor Air Quality Program, Air and Industrial Hygiene Laboratory, California Department of Health Services, Berkeley, CA 94704, USA E1 8905-185M (Received 30 May 1989; accepted 6 January 1990) Evaporative air coolers (EACs) cool and humidify hot, dry, outdoor air by pulling it through pads that are wetted continuously by recirculated water. An EAC in a single-family house was examined for three summer months, during which the amount of dissolved solids in the sump water rose to ten times the level in the chlorinated tap water supply. Although the concentration of bacteria in the tap water was always <10 colony-forming units per milliliter (cfu/mL), the concentration in the cooler water reached 1 x 10 s cfu/mL in the ninth week. Indoors and outdoors, the concentrations of airborne microorganisms were similar on individual days, but throughout the study they varied from 102 to 10 4 cfu/m 3 for bacteria, and from 102 to >10 3 cfu/m 3 for fungi. When the EAC was not operating, the concentration of airborne microorganisms tended to be higher indoors than outdoors. When operating, the cooler introduced large volumes of outdoor air into the house and there was less difference between the indoor and the outdoor microbiolog- ical air quality. Although the pads of the EAC appeared to filter microorganisms from the outdoor air, the types of bacteria that predominated in the sump water were more abundant in the indoor air, and several types of bacteria and fungi were found only in the sump water and in the indoor air. INTRODUCTION Airborne biological materials (e.g., plant pollen, bacteria, fungi, insect fragments, and soluble bacte- rial byproducts such as endotoxins) cause a variety of health problems, including allergy, respiratory in- fection, allergic asthma, and hypersensitivity pneu- monitis. Biological material in the indoor air often originates from contaminated heating, ventilating, and air conditioning equipment (Mercy 1984; Mercy et al. 1984). The use of wetted-media air coolers, generally known as evaporative air coolers (EACs) or swamp coolers, has been associated with hyper- sensitivity alveolitis or pneumonitis, and with in- creased indoor levels of airborne fungi (AI-Zubaidy 1987; Marinkovick and Hill 1975; Sneller and Pinnas 1987). A statewid¢, random survey found that 13% of 332 California households used EACs (Liu et al. 1989). In certain areas of California, more than 40% of the surveyed households had EACs. An EAC consists of evaporative pads, usually com- posed of wood shavings or straw, a water tank (or sump), a water-circulating pump, and a motor-driven blower (see Figure 1). Water is pumped from the tank to a distribution system from which it runs over the pads and back into the sump. The blower fan pulls outdoor air through the pads and discharges it to the indoor environment. The sump is provided with a make-up water connection (usually to a drinking water supply), a float valve, an overflow pipe and a drain. Water lost through evaporation is replaced with make- up water and any excess water in the tank is removed through the overflow drain. When the unit is not operating, the water remains stagnant in the tank. Water usage depends on the air flow, the effective- ness of the evaporator pads, the temperature and humidity of the incoming air, and on how much of the water is bled off to assure dilution and flushing of the sump water (ASHRAE 1988; Watt 1986). 203