The NORA document also presents critical new information regarding age correction and vulnerable populations that may make you think harder about what constitutes appropriate regulation and how workplace programs should be modified. For example, the document identifies diabetes, a common medical condition, as making workers more vulnerable to illness from noise exposure.  Different types of neurons are used in hearing. Thus, the location of damage matters. While the pure-tone audiogram—a measurement of a hearing threshold through observation of patient responses to sounds with a sinusoidal waveform—remains the gold-standard test of hearing ability, the hearing loss prevention community has also accepted that individuals with similar pure-tone audiograms cannot be expected to have the same functional hearing ability. Functional deficit can exist that is not evident on the pure-tone audiogram.  Between-frequency losses, known as “hidden hearing loss,” are a new focus for research. Having losses between tones could explain why some individuals report hearing trouble that hasn’t been identified on pure-tone audiometry testing. If new animal research can directly apply to humans, an important finding was made regarding the type of neurons tested in pure-tone audiometry; according to the NORA document, it is likely that “pure-tone threshold testing cannot evaluate the response of neurons most at risk for synaptic damage.”  It has been known for a while that individuals can experience a loss of hearing acuity, which reduces sensitivity, and a loss in clarity, which distorts what is heard and is not helped by amplification. The underlying factors that affect acuity and clarity are still not adequately understood; the lack of detailed exposure histories seems to be a major factor in the pace of progress. But the NORA document describes useful insights gained from research. Damage at the connection of auditory neurons to cochlear hair cells, called synaptopathy, is now considered “hidden” from the pure-tone audiogram. Researchers theorize that the symptoms of synaptopathy include problems understanding speech while in a noisy environment. Scientists are trying to understand the effects of the location of damage and effects from multiple types of damage on overall response. For example, it is thought that damage to outer hair cells may also result in difficulty understanding speech while in a noisy environment. Better understanding needs to be developed for the impact from each type of loss or combination.  Useful information provided in the NORA document is that not all neurons associated with hearing function respond the same. Some require more stimulus than others to be triggered. More sensitive neurons are relied upon for hearing low-level signals in quiet environments but can become overwhelmed in noisy environments. New evidence provides reason to suspect that the neurons relied upon to assist with hearing in noisy environments are not only more susceptible to damage but also cannot be evaluated by pure-tone testing.  New evidence also suggests that damage at the connection of the inner hair cell to the auditory neurons may be the “earliest site of noise-induced morphologic damage,” according to the NORA document.

New information from NIOSH regarding the age-related decline raises the question, how much decline is normal? Figure 1, from NIOSH, shows that the average 25-year-old carpenter has a similar audiogram to a non-noise-exposed 50-year-old, lacking an indication of a severe “notch” at the higher frequencies. In Section III, Chapter 5, of the , OSHA states that “[a]udiograms often display a 4,000-Hz ‘notch’ in patients who are developing the beginning stages of sensorineural hearing loss.” Figure 1 also shows that the average carpenter develops severe hearing loss, depicted via the development of severe notching by age 55, while no notching is seen in those who do not have noise exposure even by age 55. Figure 2 shows progression of hearing loss with age, to develop the notching for the average carpenter. Remember also that even before we had this new information, the said the notch at the 8,000 Hz frequency in the audiogram often indicates age-related loss as opposed to noise-induced loss. This indicates OSHA’s understanding that there needs to be an analysis of any loss to determine possible causes. Thus, the default interpretation even now cannot be to assume the loss is age-related. 

Likely a bit further off is research that will expand the science of auditory brainstem response. This method has been applied to research of high-level firearm discharge, but the science needs to be further expanded for application to populations with chronic exposures at frequencies where progressive accumulation of damage is the concern. Many IHs will probably attest to witnessing situations that seem to have different risk but whose final calculated values for OSHA compliance purposes are similar because of the equal-energy model OSHA uses. This simplistic model, in widespread use, is appropriate for steady-state noise but does not adequately quantify risk for complex noise profiles with impulse or impact noise. For those curious about the possibilities of improved technology, information about use of “kurtosis” may be of interest. According to the NORA document, kurtosis “describes the distribution of amplitudes of noise exposure.” Adjustment with the concept of kurtosis is used to correct measurements of sound pressure levels to account for the more harmful impulse noise embedded in steady-state noise. Kurtosis-corrected measurement of sound pressure levels is not a new concept, but it has not gained widespread use. Including the concept of kurtosis would better predict hearing loss. In 1998, NIOSH revised the criteria for a recommended standard for occupational noise exposure. One of the agency’s revisions was that it no longer recommended age correction. The document informs that age correction currently in use does not represent the ethnicity and gender of the current United States population, was based on a small sample size, and lacked audiometric data for workers older than 60 years.  While the NORA document indicates that more research is needed, it provides new information related to ethnicity and gender: non-Hispanic blacks have better hearing than non-Hispanic whites, and women have better hearing than men. Different methods for age correction were recommended as research topics. Additional protective action, for extended durations, is recommended for vulnerable workers who have certain medical conditions and are exposed to hazardous noise. Key information reported in the NORA document is that “increased vulnerability to noise may last long beyond the end of the therapeutic treatment.” The document states that medical conditions such as “diabetes, hypertension, renal disease, compromised immune systems, or conditions that require use of pharmaceuticals, can increase the vulnerability of the audio-vestibular system.” Training for those at risk is advised. The NORA document suggests that recommended exposure limits may need to be lowered to account for effects on hearing from multiple hazards at once. Of the 22 million noise-exposed workers, it is reported that between 5 and 10 million are also exposed to the ototoxic organic solvents toluene, xylene, styrene, trichloroethylene, and carbon disulfide. Heavy metals and asphyxiants are ototoxic chemicals, which have synergistic interactions with noise. Using information from three studies published between 2007 and 2016, the NORA document informs that about half of U.S. workers—11 million—“achieve less than 5 to 15 dB of attenuation without training.” Incorporation of fit testing for individual hearing protection into hearing loss prevention programs is a need NIOSH identified decades ago. The NORA document recognizes that fit testing of individual hearing protection is “[t]he only way to more precisely identify the specific at-risk population.” The result of the fit test is a personal attenuation rating, or PAR, which describes how well an individual can fit hearing protection. PAR is not referenced much on the OSHA website—the only current mention is a published from 2011 on preventing occupational hearing loss—but the NORA document suggests that the term may make a comeback.  Also on the OSHA website is a from OSHA to 3M regarding a personal fit-testing system. In its interpretation, OSHA indicates that the personal fit-test system is considered appropriate to determine initial fitting, as required by 29 CFR 1910.95(i)(5), but reiterates that the regulation codified methods for determining adequacy of attenuation in Appendix B. Thus, it appears the regulation would need to be changed to allow for new technology like the PAR to be used to determine adequacy of attenuation. OSHA currently approves these new technologies to train workers. The fact that more than half of hearing protection users achieve less than half of the labelled attenuation is a serious issue when considering that OSHA does not force employers to follow the hierarchy of controls to protect workers from noise. In an , the agency states that “29 CFR 1910.95(b)(1) allows employers to rely on personal protective equipment and a hearing conservation program, rather than engineering and/or administrative controls, when hearing protectors will effectively attenuate the noise to which employees are exposed to acceptable levels.” According to the NORA document, “Preliminary research suggests that the attenuation of a hearing protector may underestimate the level of protection provided.” The document recommends acoustic standards for in-ear dosimetry, recognizes in-the-ear noise monitoring as feasible, and acknowledges problems with the accuracy of traditional dosimetry.  Information in the document supports a case for expanding dosimetry to those at the margins of the definition for inclusion in hearing conservation programs. Consider the following quotes:

Currently, OSHA regulations mandate only baseline and annual audiograms. A standard threshold shift, or STS, is a loss determined by comparing values measured at the baseline with subsequent (that is, annual) audiograms at defined frequencies only. Specifically, defines an STS as “an average shift in either ear of 10 dB or more at 2,000, 3,000, and 4,000 hertz.”

Most octave frequencies tend to be tested for work- related audiograms, with some inter-octave frequencies tested at frequencies of higher risk. For example, the inter-octave frequencies 3,000 and 6,000 Hz are also often tested. As explained in a CDC manual for audiometry procedures, the shape of the human ear causes increased amplification in the 2,000–3,000 Hz range, resulting in increased risk around these ranges. 

Where do we go with these new insights?

OSHA already recognizes that most noise is not pure tone. While we may have to wait for changes to OSHA regulation until better quantification is possible, the NORA document provides IHs with actionable information. For example, we are now aware of hidden losses that cannot yet be measured but can at least be acknowledged. We also have information about effects on sensitive populations that can be put into practical use in policies and programs. It’s true that we wish we had more details, but that’s usually the case in our field. Since now we know that an audiogram may not be the complete picture, perhaps we should put more stock into qualitative information such as reports and observations from workers. We need to admit that when insufficient tools exist to quantify what workers are experiencing, we must supplement with qualitative information. When, and to what extent, we rely on qualitative information is a subject for further exploration. Let us hope for speedy advancement on this important topic. Science will continue to provide finer details as technology improves the ability to investigate the microscopic functions of the human body.    Send feedback to The Synergist.