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 S ö derberg smelting
process. In 2010, additional volunteers were recruited at a smaller
company location with a different production process (a turbine
component factory).