R&D Portfolio - Research Goal 4.4:
Improve detection and prevention of occupational hearing loss by understanding the aging component
The U.S. workforce is aging (Figure 4.2). As people age, their hearing deteriorates. The hearing loss progresses in a characteristic way, generally affecting the high frequencies first then progressing to the low. The loss of high frequencies causes the person to miss speech components which allow them to discriminate between words. Loss of lower frequencies can lead to missing safety signals in the workplace.
Figure 4-2. Demonstration of the aging workforce in construction, manufacturing and mining, 1994 and 2004. Bureau of Labor Statistics.
The rate of hearing loss differs from individual to individual. The differences may be related to genetics, environment and perhaps even nutrition. Hearing loss also seems to differ by gender, with males losing hearing sooner and faster than females. It is not clear if the gender difference is due to physiological differences or life-style differences. It is currently unknown if noise exposure increases the rate at which age-related hearing loss progresses. It is also unknown whether or not the hearing loss seen in aging is related to cumulative mini-noise traumas occurring during a person’s life.
OSHA offers the option of collecting age-corrected audiograms for a workplace hearing loss prevention program. Age-corrected audiograms attribute a portion of detected hearing loss to aging. Use of this option can determine if a worker is sent to an audiologist for further examination or declared “normal” for his or her age and returned to work. Age-correcting audiograms can cause workers experiencing an early, minor hearing loss to be missed.
An important question is whether aging is additive with the noise-induced hearing loss, or if the noise-induced hearing loss produces a loss which remains stable until the aging component “catches up” to the noise threshold shift. There is conflicting evidence.
About ten years ago, the HLR program began a partnership with the University of Cincinnati, Department of Biological Sciences to conduct a series of studies looking at the genetic basis of noise susceptibility in a mouse model. The mouse strain chosen also showed early age-related hearing loss. Mice serve as a good model for the effects of aging in humans because their genetic backgrounds are well characterized and their ears are similar to humans except they respond to much higher frequencies. Their auditory function can be easily measured using the auditory brainstem response, and mice live only two years, enabling researchers to study age-related changes in a short period of time.
The HLR program/University of Cincinnati team exposed a certain strain of mouse which develops early age-related hearing loss to noise. When exposed to a moderate intensity noise, a permanent threshold shift was generated. After a few months, a threshold shift due to aging could be detected. At about the same time, the unexposed control animals showed approximately the same amount of hearing loss. Based on this model, one would conclude that the shifts due to age and noise were not additive; as the unexposed control mice “caught up” to the noise-exposed mice.
In a more recent set of experiments, the HLR program tested the question of noise-induced hearing loss susceptibility over the mouse life time. In a separate experiment, they looked at the effect of introducing an antioxidant into the mouse’s diet on age-related hearing loss. Although both studies are still in progress, initial results indicate that aging does not change the susceptibility of the ear to noise, and that the dietary antioxidant does protect the ear from early hearing loss due to aging.
In 2003, the HLR program co-organized a four-day conference, “The Mouse as an Instrument for Hearing Research,” that was held at the Jackson Laboratory, Bar Harbor, Maine. The conference brought together about 75 researchers in mouse genetics and hearing to present research and to provide information about cutting-edge techniques. This cross-fertilization between the molecular genetic researchers and the hearing researchers has lead to a number of informal and formal collaborations. A second meeting was held in October, 2005, also at Jackson Laboratory.
In an analysis of a number of occupational hearing conservation databases, HLR researchers showed that if aging effects are subtracted from the audiograms with hearing threshold shifts (as currently allowed by OSHA regulations), the majority of the workers who are adversely affected by workplace noise will not be detected in a standard hearing conservation program. This study was presented at a Consensus Meeting of the Medical Research Council, Institute of Hearing Research and the Hearing Research Trust in Great Britain held in Nottingham, England.
The HLR program has also been an important partner in the current NHANES, a representative survey of the current U.S. population. These data allow for direct measurement of hearing loss due to aging in the population.
 Franks, J.R. and Davis, R.R. Longitudinal Studies of Age and Noise Effects: ANSI Databases. Consensus Meeting Noise and Ageing. MRC Institute of Hearing Research and the Hearing Research Trust, November 4-6, Nottingham, England.