On May 6, 1998, the National Institute for Occupational Safety and Health (NIOSH) received a confidential employee request for a Health Hazard Evaluation (HHE) at the Sergio Cuevas Bustamante Filtration Plant of the Puerto Rico Aqueduct and Sewer Authority. The requesters expressed concern regarding respiratory problems and dermatitis possibly associated with exposures to hydrated lime [calcium hydroxide, Ca(OH)2], chlorine gas [Cl2], and two coagulants [GC 850 (basic aluminum chloride solution) and PRP 4440 (polydimethydiallylammonium chloride)]. A site visit was conducted at this potable water filtration plant on June 29-30, 1998. Of the 25 people employed at the filtration plant, 19 (76%) worked the morning shift (6 a.m. to 2 p.m.), when most of the activities at the filtration plant were performed. On the day of the site visit, 17 employees were present and all were interviewed. Two of the interviewed workers reported that they were diagnosed by physicians as having asthma, which had developed since working at this plant. Both workers reported noticeable improvements of this health condition during vacations, sick leave, or lay-offs. Five of the workers (including the two asthmatics) reported at least two respiratory symptoms in the ten days preceding the interview. These symptoms included difficulty breathing, phlegm production, chest tightness, wheezing, and shortness of breath. All symptomatic workers reported that they believed their symptoms were related to long-term exposure to hydrated lime and chlorine gas. Other reported health problems included sinusitis, itchy eyes, skin rash, and dermatitis. Area air sampling for hydrated lime, chlorine, and particulates was conducted at locations and times of anticipated elevated exposures. Personal breathing zone (PBZ) air sampling for hydrated lime also was performed; however, the method used to analyze the hydrated lime failed. A count of particles in air during hydrated lime loading operations, a procedure reported by workers to often result in elevated dust levels, revealed only moderate losses of hydrated lime during loading when compared to the period after loading. No detectable levels of chlorine were measured in the area where chlorine cylinders were stored or in the room where chlorine was added to the water supply. The two coagulants of concern were confined to an area where little human exposure should occur, so monitoring was not conducted. Management and employees reported that there had been problems associated with the handling of hydrated lime in the past. A crack, still visible but repaired by the time of the NIOSH site visit, had developed in the side of a silo which contained about 130,000 pounds of hydrated lime, allowing hydrated lime to be released into the workplace. The conveyor feeding hydrated lime into the water supply was designed to have barriers around it but they had been removed and hydrated lime was observed on surfaces throughout the room. Workers reported that hydrated lime periodically escaped into the environment from the top of the silo during loading, despite a filtration system to prevent its escape. The connection between the supply truck and the pipes carrying the hydrated lime to the top of the silo was difficult to seal properly and reportedly hydrated lime was spilled. On the day preceding the NIOSH site visit, a worker received a dose of hydrated lime in the face during a maintenance task at the silo. Several other safety concerns were identified at the plant, including absence of appropriate guarding of roof openings and lack of a complete confined space entry program, which indicate that more rigorous administrative controls should be implemented.