Ozone gas concentrations (OGC) and population densities of airborne bacteria and fungi on plant leaves were investigated in a naturally ventilated greenhouse cooled by a fogging system with ozonated water [OW; dissolved ozone concentration (DOC) of 10 mg/L at a generator outlet] or tap water (TW; DOC of 0 mg/L). The fog cooling occurred for 30 min with a combination of 20 s on and 100, 80, 60, 40, or 20 s off using 5 μm (catalog-specified fog size) nozzles and of 16 s on and 44 or 74 s off using 50 μm (catalog-specified fog size) nozzles. The population densities of airborne bacteria and fungi attached on the leaves of poinsettias (Euphorbia pulcherrima) and pothos (Epipremnum aureum) were determined using an agar stamp method before and after the fog cooling. There was no large difference in the greenhouse cooling efficiency between the TW and OW treatments. The highest OGC in the greenhouse never exceeded 0.04 mg/L with all treatments, even with a combination of fogging 20 s on and 20 s off. This result indicates that the OGC in the greenhouse cannot accumulate to harmful levels (over 0.05 mg/L during 8 ordinary working hours) because of the natural ventilation required for fog cooling. Although population densities of airborne bacteria and fungi on the plant leaves showed a slight decrease during OW-fogging, these levels were not significantly different from population densities during TW-fogging. However, it is still unknown whether repetitive OW-fog cooling for more than 30 min and/or several days of OW-fog cooling will reduce the population densities of airborne bacteria and fungi on plant leaves.