On December 7, 1999, the National Institute for Occupational Safety and Health (NIOSH) received a request from the Occupational Safety and Health Administration (OSHA) for assistance in the evaluation of potential microbial contamination in the Southwest Airlines Dallas Reservations Center, Grand Prairie, Texas. A similar request had been previously received by NIOSH from employees of the reservations center. Health concerns included upper respiratory infections, fatigue, asthma, headaches, dry cough, and ear/nose bleeds. On December 27-29, 1999, NIOSH investigators conducted a site visit at the reservations center. A walkthrough inspection was made of the building interior and exterior. Bulk material and sticky tape samples of surfaces were collected from the interior of the air handling units (AHUs) to assess these areas for microbial contamination. Measurements to detect moisture incursion and general indoor air quality comfort parameters were also collected. Confidential medical interviews were conducted to assess health concerns. Fungal concentrations from bulk material samples of interior insulation in the AHU units ranged from nondetectable (ND) to 1.8x107 colony forming units per gram of material (CFU/g). Exophiala, Rhodotorula glutinis, Acremonium, Cladosporium sphaerospermum, and Cladosporium herbarum were the predominant fungi identified. Four of the six bulk samples revealed high fungal concentrations and/or were identified with significant genera known to have irritant effects. Bacterial concentrations from the bulk insulation samples ranged from ND to 1.9x107 CFU/g. Gram negative bacteria were the most prevalent bacterial type detected and were found in the highest concentrations. Gram negative bacteria are commonly found in association with moisture. A tape sample taken from the laminated covering on the interior insulation within one of the AHUs revealed mostly dust, skin flakes, and glass fibers. No fungal growth was observed. Although temperatures measured in the reservations center were within the range recommended by the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE), different temperature zones and fluctuating temperature patterns were observed on the first and second floors. Relative humidity measurements were generally lower than the range recommended by ASHRAE. Several carbon dioxide measurements exceeded 800 parts per million (ppm), a level indicating an inadequate amount of supplied outdoor air. Based on medical interviews most of the cases of asthma began during childhood or were non-allergic in origin. Many workers reported symptoms suggestive of allergic upper respiratory diseases, however the characteristics of the symptoms suggest that a large proportion of the symptoms were non-allergic. Few workers had lower respiratory symptoms (cough, wheezing, shortness of breath) and did not have asthma. The diseases and symptoms reported are common in the population. There were no unique factors in this workplace that may cause worsening of allergic symptoms only in the workplace. Several workers reported increased number of infections (primarily viral infections) since they began work at Southwest Airlines (SWA). This may be due to exposure to infectious agents in the workplace. Few workers complained of headache and fatigue. Among the nonsmokers (never smokers and former smokers), 82% stated that smoke in the workplace caused annoying irritant symptoms. Based on the information and measurements obtained during this health hazard evaluation, NIOSH investigators conclude there is limited evidence of microbial contamination in the Dallas Reservations Center. Most of the health symptoms reported appear to be non-allergic in origin. There does not appear to be allergic diseases (including asthma) that can be attributed only to workplace exposures. There is evidence, however, that the heating, ventilating, and air-conditioning (HVAC) system may not be able to adequately service the building. Recommendations addressing the HVAC system, general ventilation concerns, cleaning procedures, and future water incursion incidents are included in the report.