the free flow of dirt. The US Centers for Disease Control and Prevention (CDC) does not recommend that health-care facilities regularly test for the Legionella pneumophila bacteria. Scheduled microbiologic monitoring for Legionella remains controversial because its presence is not necessarily evidence of a potential for causing disease. The CDC recommends aggressive disinfection measures for cleaning and maintaining devices known to transmit Legionella, but does not recommend regularly-scheduled microbiologic assays for the bacteria. However, scheduled monitoring of potable water within a hospital might be considered in certain settings where persons are highly susceptible to illness and mortality from Legionella infection (e.g. hematopoietic stem cell transplantation units, or solid organ transplant units). Also, after an outbreak of legionellosis, health officials agree that monitoring is necessary to identify the source and to evaluate the efficacy of biocides or other prevention measures. Studies have found Legionella in 40% to 60% of cooling towers. Terminology Windage or Drift — Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same concentration of impurities as the water entering the tower. The drift rate is typically reduced by employing baffle-like devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower. Drift can also be reduced by using warmer entering cooling tower temperatures. Blow-out — Water droplets blown out of the cooling tower by wind, generally at the air inlet openings. Water may also be lost, in the absence of wind, through splashing or misting. Devices such as wind screens, louvers, splash deflectors and water diverters are used to limit these losses. Plume — The stream of saturated exhaust air leaving the cooling tower. The plume is visible when water vapor it contains condenses in contact with cooler ambient air, like the saturated air in one's breath fogs on a cold day. Under certain conditions, a cooling tower plume may present fogging or icing hazards to its surroundings. Note that the water evaporated in the cooling process is "pure" water, in contrast to the very small percentage of drift droplets or water blown out of the air inlets. Draw-off or Blow-down — The portion of the circulating water flow that is removed (usually discharged to a drain) in order to maintain the amount of Total Dissolved Solids (TDS) and other impurities at an acceptably low level. Higher TDS concentration in