R&D Portfolio - Improve understanding of occupational hearing loss through surveillance
and investigation of risk factors
Since its inception, the HLR program has investigated risk factors for hearing loss through extramural and intramural research. Understanding basic mechanisms of hearing loss allows the HLR program to improve prevention methods and technology. There are many things that we do not yet understand. For example, there are a variety of agents that can cause hearing loss, including noise, pharmaceuticals, aging and chemicals. Epidemiologic and population studies guide the targeting of research to better understand the biological basis of the effect of these agents on hearing.
The lack of surveillance data for noise exposure or occupational hearing loss is one of the fundamental knowledge gaps. There is no national surveillance system. The HLR program has worked to address this through carefully selected research efforts to develop surveillance methods, and collect and evaluate data from appropriate sources to partially fill the gap. Partnerships are a key part of this effort, including participation in the National Health and Nutrition Examination Survey (NHANES), and other more narrowly targeted surveys.
A second fundamental gap the HLR program has been working to fill is research to understand the risk criteria associated with impulsive noise and its effect on the auditory system. Much of this work has been conducted through extramural investigators. Intramural research on the damage resulting from impulsive noise focused on animal studies prior to 2000. Since that time, field studies, primarily carried out under the HHE program, have examined the limitations of standards and instruments to adequately characterize impulsive noise exposures in the workplace. These field studies have led to the development of new instruments and recommendations for improved evaluation of exposure to impulsive sounds. By correctly characterizing and measuring impulsive noise, damage risk criteria can be developed to protect workers’ hearing. Laboratory and modeling work on the more basic physiological and mechanical (ototraumatic) damage to hearing inflicted by impulsive noise has continued the extramural researchers in the HLR program.
Aging factors are the third fundamental gap that the HLR program has been working to fill. Several aspects of the aging phenomenon in hearing have been observed, but are not well-understood. Existing data are unclear about whether or not the ear changes in susceptibility to noise-induced hearing loss with age. The extent of the possible interaction of noise exposure and aging is also unknown. Current research also shows gender and racial differences in the relationship between hearing and aging, but it is not clear if these differences are due to biological or life style differences.
The fourth fundamental knowledge gap is the genotype of noise susceptibility. Recent evidence, including that from the HLR program, indicates that there may be a genetic component to susceptibility to noise-induced hearing loss, but the nature of this genetic factor in hearing loss has not been fully characterized. The HLR program is contributing to the understanding of both aging and genetic factors in hearing loss through collaborative research with university partners, who are working primarily with animal models.
The fifth knowledge gap is hearing loss due to chemicals alone or in combination with noise. Many chemicals commonly found in industry, construction and agriculture can cause hearing loss, either alone or in conjunction with noise exposure. These ototoxic chemicals include solvents, heavy metals and asphyxiants. Ototoxicity can be thought of as a specialized form of aging. Research, including that supported by the HLR extramural program, has shown that a number of occupational ototoxic chemicals produce reactive oxygen species (ROS) in the ear similar to those observed to be related to noise-induced hearing loss, and thought to be involved in the aging process. ROS interact with cellular components of the hair cells to damage hearing. Yet we do not understand the mechanism by which these chemicals cause damage, and the characterization of dose response and levels of exposure in the workplace are inadequate to develop specific recommendations for exposure limits. The demonstrated interaction between combined exposures to noise and chemicals is also of interest. The HLR program has collaborated with international partners to conduct epidemiologic studies of workers exposed to noise and chemicals. The program has also supported laboratory studies with animals to better understand the effects of solvents on hearing. But a more extensive survey of chemicals is needed to determine which occupationally-related chemical exposures are ototoxic. While the research to understand the effect of these factors, whether alone or in combination with noise, is still in its early stages, the HLR program has already moved to transfer information about the risks of chemical damage to hearing so that in some cases, steps can be taken to protect workers now. Interim recommendations include enrolling workers who are exposed to chemicals in hearing conservation programs.