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The sump of an evaporative air cooler (EAC) artificially was contaminated with ≥ 106 colony-forming units (CFU) mL−1 of Micrococcus luteus, a pigmented, gram-positive, coccoid bacterium. Air in the room served by the EAC was sampled during EAC operation to detect water-to-air transfer of the test bacterium. Median indoor air concentrations of the tracer bacterium were 2.4 and 1.2 CFU m−3 at the low and high EAC fan speeds (respective air flow rates and air velocities through the EAC pads, 23 and 44 m3 min−1 and 0.47 and 0.89 m s−1), for an estimated source strength, or transfer rate, of approximately 55 CFU min−1. The rate at which tracer bacteria circulated through the EAC pads during these tests was ≥ 9 × 109 CFU min−1. Tracer bacteria were detected indoors during operation of a highly contaminated EAC, but the relative tracer transfer rate and resultant air concentration were low. Proper EAC operation and maintenance likely would ensure minimal human exposure to potentially harmful microorganisms and other materials that could multiply or collect in EAC sump waters and be transferred to indoor air as a result of sump water dripping or splashing.
