Methods
The study took place at three company locations of Alcoa Inc., a worldwide producer of aluminum and other industrial products.
Ongoing research collaboration agreements between Alcoa, the Yale University School of Medicine, and Stanford University School of Medicine have resulted in the analysis of audiometric data collected for hearing conservation purposes. As part of this research collaboration, an intervention study of voluntary use of daily noise exposure monitoring was initiated in 2007 at two aluminum smelters with funding from the National Institute for Occupational Safety and Health. The two smelters are located in geographical proximity to each other, but differ somewhat in production process, with one employing a prebake process and the other a Söderberg smelting process. In 2010, additional volunteers were recruited at a smaller company location with a different production process (a turbine component factory).