In September 1999, an authorized representative of Local 3523, United Steelworkers of America, asked NIOSH to evaluate an apparent cluster of hypoplastic left heart syndrome among offspring of three male workers employed at Thomas Steel Strip (TSS) Corporation, Warren, Ohio. Two of the three male employees worked in the Finishing Department. Workers in this department were exposed to nickel, copper, iron, and zinc dusts, as well as to a dust of another metal designated by TSS as a trade secret (heretofore referred to as "TS metal.") TSS advised NIOSH as to the identity of this metal. Workers in this department were also exposed to a rust inhibitor that was applied to the electroplated strip steel. Medical records of heart defect cases born in 1998 were obtained and reviewed by NIOSH. To calculate a rate of birth defects among all TSS employees, insurance claims information was collected and analyzed. Rates were calculated separately for employees who worked in the Finishing Department during the prenatal period and those who did not. Airborne concentrations of the following substances were measured in the Finishing Department: (1) nickel, zinc, copper, iron, and a trade secret metal (TS Metal); (2) the active ingredient in the rust inhibitor, 2,6-di-t-butyl-4-methylphenol (2 BHT); and (3) organic solvents. Workers in the Finishing Department had reported that they periodically pass through the Old Plating Department en route to their jobs. Therefore, airborne concentrations were obtained in the Old Plating Department for hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, hexavalent chromium, ammonia, and cyanide. In addition, hand wipe samples, shoe wipe samples, and cotton pad samples (affixed to the workers' shirt and trousers) were obtained and analyzed for the five metals. The intent of this sampling effort was to assess the potential for metals to be transported from the workplace to the worker's car and home. Wipe samples for metals analyses were also obtained in defined locations within the automobiles of several workers employed in the Finishing Department. Control samples from the automobiles of workers employed outside of the Finishing Department were also obtained. Ionizing radiation measurements were obtained in and around Finishing Department workstations to evaluate potential doses from radioactive sources used in connection with the steel strip gauging systems. Water samples were obtained at drinking fountains near the Finishing and Old Plating Departments. Levels of selected substances found in samples from these locations were compared with levels found in samples taken from two off-site locations, one approximately 10 miles from TSS, another from Cincinnati. Three major heart defects in children born in 1998 to TSS employees were confirmed, and one major heart defect in a child born in 1993 was confirmed. Four additional, less severe, heart defects were identified in children born in 1996, 1998, and 1999. The rate of severe heart defects among validated TSS births was 21.4%, and the rate of all identified heart defects among validated TSS births for 1998 was 27.8%. These rates are higher than other regional data, which show approximately 1% of live births to have major heart defects. Airborne levels for four of the five metals of interest, as well as airborne levels of four organic solvents, were below the NIOSH recommended exposure limit (REL). One personal exposure measurement to nickel dust was above the NIOSH REL. Patch and wipe samples for several metals showed higher levels in TSS employees compared to NIOSH control samples. Automobile wipe samples obtained from the floor and seat locations on the driver's side indicated measurable levels of copper, iron, nickel, and zinc. These levels were higher in automobiles of production employees than in automobiles of administrative employees. Thus, there is a potential for metals to be taken into the homes of workers, which may pose a risk to family members. However, the health significance of these metal levels is not known. Results of ionizing radiation measurements indicated that radiation doses are at or near background levels at the operator position of the Finishing Department workstations. Comparison of levels of selected analytes found in production area drinking water samples with those obtained at offsite locations indicated no major differences in level or content. Levels that were found were well below maximum contaminant levels set by the Environmental Protection Agency. Levels of environmental contaminants were unremarkable. In consideration of the current state of knowledge regarding known reproductive toxicants, it is concluded that the cluster cannot be clearly linked with these exposures. Recommendations regarding personal hygiene practices among TSS employees are in the Recommendations section of this report. In addition, temperature control of the cyanide-containing plating baths should be kept at process design levels.