On February 1, 2005, the National Institute for Occupational Safety and Health (NIOSH) received a confidential employee request for a health hazard evaluation (HHE) at the Taft Elementary School in the Santa Ana Unified School District (SAUSD) in Santa Ana, California. The requesters were concerned about exposure to "toxic mold" in the school. There were reports that teachers had been diagnosed with toxic encephalopathy (brain damage) while other complaints included migraine headaches, sinusitis, asthma and other health conditions related to toxic mold exposure in the school. A site visit was made on April 5-6, 2005, which included an opening conference, an inspection of the school, and confidential employee interviews. A closing conference was held in which preliminary findings and recommendations were presented. Measurements of indoor environmental quality were made including temperature, relative humidity, and carbon dioxide; and a moisture meter was used to detect moisture in walls and ceilings or wherever evidence of water staining was visible in the buildings. Visual inspections were made on each of the five 30-ton multizone heating, ventilation, and air conditioning (HVAC) units and on 6 of the 12 Trane (3-ton) air handling units (AHUs). To investigate the nature of fine black particulates on filters installed downstream of the central AHUs, samples of air filters were collected and evaluated using scanning electron microscopy. Indoor temperature, relative humidity (RH), and carbon dioxide (CO2) in buildings A and B ranged from 70 degrees-77 degrees, 25%-38% and 569-1060 ppm, respectively. Indoor temperature, RH and CO2 in bungalows B-3, B-4, and B-9 ranged from 72 degrees-79 degrees, 41%-58% and 1374-1477 ppm, respectively. Outside measurements were 76 degrees-77 degrees, 23% and 443-450 ppm. The presence of moisture was not detected on or in walls, ceiling tiles or wood framing members where evidence of previous water staining was visible. Microscopy results from air filters revealed various mineral fragments, skin cells, pollen and mold spores, and diesel particulates. Other reported particulates appeared physically similar to rubber dust. Minor deficiencies were found in several AHUs. Fifteen of the 37 (41%) staff members interviewed reported no symptoms related to work. The most common work-related symptoms were nasal symptoms such as runny or stuffy nose (8/37 or 22%). Work-related eye irritation was reported by four persons (11%). Headache, throat irritation, and cough were each reported by three (8%). Two persons described sinus pressure that got better when away from work. Nobody reported shortness of breath, chest tightness, or wheezing. Eight of 37 (22%) reported constant sinus problems or recurrent sinusitis. Three persons reported memory problems and being diagnosed by the same physician with toxic encephalopathy from mold exposure in the school. Medical records were also reviewed for seven persons. One person had evidence of sinusitis and rhinitis and one had rhinitis. None had evidence of toxic encephalopathy. NIOSH investigators did not identify an occupational health hazard at the Taft Elementary School. Inspection of the HVAC system revealed minor problems such as evidence of dust intrusion, and presence of standing water and biofilm in some of the condensate drain pans. All water leaks had been repaired, and there was no evidence of mold or fungal growth. Taft employees had rates of work-related nasal problems similar to those reported in studies of buildings with indoor environmental quality complaints; however, we were unable to directly relate any symptom to the work environment. No exposure was identified in the school that would cause toxic encephalopathy, and there was sufficient evidence to conclude that none of the Taft Elementary School employees interviewed had toxic encephalopathy. Recommendations are included in this report to address HVAC deficiencies in all units that were inspected.