Health hazard evaluation report: HETA-2004-0239-3014, MK Ballistic Systems, Hollister, California.
In May 2004, the owner of MK Ballistic Systems, Hollister, California, requested that NIOSH evaluate potential exposure to silver iodide during the manufacturing of cloud-seeding flares to determine if this exposure could be a cause of thyroid disease in two employees. Since 1991, the company has employed nine workers to manufacture flares; however, production was stopped in July 2003, due to concerns about exposure to silver iodide. During the NIOSH visit in August 2004, 28 flares were produced by two employees to enable NIOSH investigators to evaluate exposure to flare components. The flare composition is a pyrotechnic mixture of metal powders and oxidizers containing silver iodide, strontium nitrate, potassium perchlorate, aluminum powder, and magnesium powder. One worker ("blender") combines these powders with a polyester resin, and the other worker ("presser") packs the mixture into paper tubes to finish producing the flare. The blender had a personal breathing-zone airborne concentration of 0.38 mg/m3 of silver iodide, and the presser was exposed to 0.01 mg/m3. No occupational exposure limits have been developed for silver iodide. Silver iodide is a non-soluble compound; therefore, the more protective evaluation criteria for silver metal, soluble silver compounds, and iodine are not applicable. Exposures to aluminum, magnesium, and strontium in both workers were well below their evaluation criteria. Three of the nine employees were interviewed. Review of their medical records revealed no association between their diagnosed medical conditions and their exposures at MK Ballistics Systems. While there was concern that exposure to silver iodide might precipitate thyroid disorders, our evaluation did not find high exposures. In addition, there is no evidence in the medical literature that exposure to iodide compounds is related to the development of the specific thyroid disorders found in the workers. NIOSH investigators concluded that there were no health hazards from exposure to flare components among employees at MK Ballistics Systems. The thyroid disorders confirmed in two employees are not uncommon and could not be associated with occupational exposures at this facility.