By Joene Hendry

NEW YORK (Reuters Health) - Employees involved in workplace hearing conservation programs may still be at increased risk for noise-induced hearing loss, especially from cumulative exposure to high levels of noise, a study indicates.

"Noise-induced hearing loss is an insidious illness that has profound consequences on a person's ability to communicate," Dr. Hugh Davies, of the University of British Columbia, Vancouver, Canada, told Reuters Health.

Workplace hearing conservation programs may be widely considered a solution to workplace-induced hearing loss, yet very little work has been done to evaluate the efficacy of these programs, Davies explained.

He and colleagues investigated hearing loss among 22,376 male lumber mill workers who were 31 years old on average when they had their first hearing test between 1979 and 1996. The researchers followed the workers for an average of 7 years to assess hearing acuity over time and workplace noise exposures.

A worker's cumulative noise exposure was estimated in decibel-years as the sum of the products of noise level and duration for all occupations reported up to and including the current hearing test.

According to a report in the American Journal of Industrial Medicine, hearing protection devices were associated with 30 percent reduced risk of a standard hearing threshold shift -- an indicator of worsening hearing, defined as a shift in hearing threshold of 10 decibels or more from one hearing test to the next.

Recent involvement in a workplace hearing conservation program was associated with an additional 30 percent reduction in risk of worsening hearing.

However, these benefits, Davies told Reuters Health, "were outweighed by much higher risks for those exposed at high noise levels."

The risk of hearing loss increased more than two times for workers cumulatively exposed to the lowest noise levels (80 to 85 decibel-years), and more than six times for those exposed to the highest noise levels (greater than 100 decibel-years), compared with noise exposures lower than 80 decibel-years.

These risks held regardless of the worker's age, baseline hearing ability, hearing protection use, or history of trap, skeet, or target shooting.

Workers in hearing protection programs may assume their hearing loss is age-related rather than work-related. Moreover, "affected workers who become deaf may not report it as they assume that they were protected," Davies said.

Davies and colleagues therefore suggest thorough examinations of the short and long term efficacy of hearing conservation programs.

SOURCE: American Journal of Industrial Medicine, December 2008

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