In July 2001, NIOSH received a request for technical assistance from the Iowa Department of Public Health to evaluate the risk for lung disease in American Pop Corn Company workers exposed to butter flavorings. This request was made after NIOSH investigated severe fixed obstructive lung disease consistent with bronchiolitis obliterans in former workers of a microwave popcorn plant in Missouri and identified an association between occupational lung disease and exposure to butter flavoring vapors. NIOSH conducted a walk-through survey at the American Pop Corn Company plant in Sioux City, Iowa on September 26, 2001. Limited industrial hygiene air sampling on that day showed that air concentrations of diacetyl, a predominant butter flavoring chemical measured as a marker of butter flavoring exposure, were very low compared to levels that were associated with abnormal lung function at the Missouri (index) plant. The company reported that workers who handled flavorings had used full facepiece respirators with particulate filters and organic vapor cartridges since shortly after the microwave popcorn plant began operating in December 1988. In June 2002, the company informed NIOSH that one of its mixing room workers had been diagnosed with fixed obstructive lung disease consistent with bronchiolitis obliterans. NIOSH conducted a medical survey from July 22 through July 26, 2002, and an industrial hygiene survey from July 29 through August 1, 2002. The main findings from these surveys included: 1. The mixing room was isolated and ventilated separately from the packaging area; 2. All tanks of heated soybean oil and flavorings were located in the mixing room and had local exhaust ventilation; 3. Workers (mixers) measured flavorings in open containers in a separate area in the warehouse and then carried the flavorings into the mixing room and poured the flavorings into open tanks of heated soybean oil; 4. Area diacetyl air concentrations in the mixing room were 0.57 parts per million (ppm) parts air by volume when liquid and paste flavorings were in use on July 29 and were below the limit of detection of the sampling method when powder flavorings were in use on July 31 and August 1; 5. Personal diacetyl exposures for the mixers for the three days of sampling were 0.04, 0.004, and 0.005 ppm respectively; 6. Mixers' exposure to diacetyl increased to 80 to 120 ppm for several minutes while pouring liquid butter flavoring into a tank; 7. Six of 13 workers with experience as mixers had abnormal lung function (three with fixed obstruction and three with restriction); 8. The average percentage of time that workers reported using respirators when in the mixing room prior to June 2002 was 20 percent (workers usually did not wear respirators when handling flavorings in the warehouse); 9. The highest measured diacetyl air concentration in the packaging area was 0.03 ppm; 10. Apart from workers with mixing experience, the prevalence of abnormal lung function in microwave popcorn plant workers did not differ significantly from the prevalence in nonmicrowave popcorn plant workers, and was similar to the expected prevalence based on national general population surveys; and, 11. Unlike the index plant, risk for lung disease in workers who popped many dozens of bags of product in microwave ovens per shift was not identified in this plant (these workers popped fewer bags per shift than at the index plant). NIOSH conducted an industrial hygiene survey on September 17 and 18, 2003 to measure air concentrations of respirable and total dust during the handling of powdered flavorings. This survey showed that: 1. Air concentrations varied depending on the particular powdered flavoring being handled and the manner in which it was handled, and 2. Some of the airborne dust was of respirable size The findings from the NIOSH medical and industrial hygiene surveys at this plant show an apparent risk for lung disease in mixing room workers at much lower air concentrations of butter flavoring chemicals than those initially measured at the Missouri index plant. Lung disease risk was not identified in other areas of the plant apart from mixing. Recommendations for engineering controls, use of personal protective equipment, and medical surveillance for exposed workers are provided in this report.