Total Exposure Hearing Health Preservation
Jeremy Slagley
Wright-Patterson Air Force Base
Introduction
Total exposure health (TEH) incorporates occupational and non-occupational (environmental and recreational) exposures, as well as individual genetic makeup and health habits, to assess possible risk of negative health outcomes. The long-used paradigm of assessing occupational risk by individual chemical, physical, and biological agents may have worked well in years past. Preventive medicine practitioners would assess worker exposure to a single chemical agent and estimate risk levels. Then we could consider similar target organs among several chemical exposures and combine estimated exposures. While this is a simplistic view of possible chemical interactions within the human receiver, it would be difficult to produce appropriate toxicological and epidemiological studies on the near infinite possible multiple chemical interaction scenarios in industry to address the possible risk levels. Implicit in these risk assessments is also the underlying bias of epidemiological studies and exposure levels drawn largely from American industry, with some limits on w'ork cycles, some controls on worker exposure, and a worker population of a particular genetic background generally receiving healthcare, clean water and sufficient nutrition for vigorous life. As scientific research probes further into the varied exposures which may impact an organ system or the entire human person, we find more links
FIGURE 10.1 Total exposure health exposures that could affect hearing health.
between chemical, physical, and biological exposures, and more links between the exposure and the individual person’s response to the exposure when considering that person’s diet, health history, habits, and genetics. If we can estimate risk better, perhaps we can control risk better with better health outcomes for the individual as well as the herd.
Hearing health is especially well suited for TEH analysis as the hearing organ is sensitive to noise (continuous and impulse), other industrial chemicals (ototoxic lead, styrene, xylene, etc.), and pharmaceuticals (see Figure 10.1). These exposures can impact the person’s body from the workplace, in recreation, at rest, via airborne, ingestion, and sometimes skin absorption, and make up the previously elucidated internal and external “exposome” from conception in the mother’s womb until natural death (Wild 2012). There may also be individual genetic predisposition to hearing health damage. In 2018, Captain Bill Murphy of the National Institute for Occupational Safety and Health (NIOSH) presented these aspects and the many years of NIOSH research contribution to hearing health preservation, entitled Total Hearing Health (Murphy 2018).
Overview of Hearing Health
Human hearing health is a vital sensory function that affects quality of life and safety. Some practicing preventive medicine professionals accept a level of typical hearing loss in a population where they would become much more concerned with a different injury or illness, such as amputation or cancer. Noise exposure and hearing loss are so prevalent, almost ubiquitous, in our society, that we become accustomed to accepting a level of damage. Almost all Americans know someone or have a dear relative who suffers from hearing loss. There is a significant social isolation and emotional suffering component among victims of hearing loss since humans are designed as social creatures.
The scale of hearing loss makes it a very significant problem. The World Health Organization (WHO) estimates 466 million people (greater than 5% of the world’s population) suffer from disabling hearing loss and estimates the financial loss at US $750 billion annually (WHO 2019). Within the United States (US), The National Institutes of Health, National Institute on Deafness and Other Communication Disorders (NIDCD) reports 15% (37.5 million) of American adults have some trouble hearing (NIDCD 2019). Hearing loss is associated with exposure to high levels of noise, some infectious diseases, exposure to ototoxic chemicals in the workplace, ambient environment, and some pharmaceuticals (Sliwinska-Kowalska et al. 2003, Sliwinska-Kowalska and Davis 2012). Noise exposure may also affect developing humans in their mother’s womb (Selander et al. 2019, ACGIH 2019). Noise-induced hearing loss may also be affected by genetic factors (Konings et al. 2009, Sliwinska-Kowalska and Pawelczyk 2013). Hearing loss is the most common occupational illness in the US with some 22 million workers exposed to high noise (Masterson et al. 2016). Within the US Department of Defense (USDOD), auditory illness is the second most prevalent disability, accounting for 13.4% of veterans’ disability claims - 3.3 million of a total 25 million disabilities (Department of Veterans Affairs 2019). This is an enormous problem affecting health, stress, social interaction, and quality of life. To reduce the global burden of hearing loss, we must understand all the exposure and individual genetic factors that affect human hearing. Then we must develop measures to estimate risk from the factors and develop methods to control those risks to acceptable levels. Individuals do not always adhere to control measures, so we must also understand and account for factors that affect individuals’ risk control behaviors. Many researchers have dedicated their careers to improving and protecting hearing health. Recent review articles are excellent summaries of published work (Lie et al. 2016, Kerr et al. 2017, Suter 2017, Murphy 2018).
Auditory and Non-Auditory Health Effects
As noted previously, noise exposure can result in both auditory and non-auditory health effects. Noise-induced hearing loss (NIHL) and tinnitus are auditory effects associated with excessive exposure to noise. NIHL presents as a loss of hearing acuity as well as a loss of speech perception with background noise. At this point, it is permanent and not ameliorated by medical treatment. Tinnitus is a subjective report of “ringing in the ears,” not directly detectable by the clinician, but still maddening and permanent for the suffering patient. The effects of hearing loss and tinnitus for those continuing to work may also result in unsafe acts when workers do not detect warning signals or cross-traffic from forklifts, for example.
Non-auditory effects include social isolation, withdrawal, depression, hypertension, heart disease, and others. When considering impacts to a baby in Шею, excessive noise exposure to expectant mothers has been associated with low birth weights (Belojevic et al. 2008, Berglund et al. 1999, Stansfield and Matheson 2003). Risk estimates should include all effects on hearing health, as well as other non-auditory effects from noise.
Exposure Lifetime
TEH should also consider the exposure lifetime. Hearing health begins at conception and ends with (hopefully natural) death. Studies have shown that noise passes through the abdominal wall, reaching the developing fetal hearing organs, primarily in the lower frequencies (Abrams and Gerhardt 2000). We also know that the hearing organ changes during gestation so that low-frequency damage in the first trimester may result in higher-frequency hearing loss at birth (Harris and Dallos 1984, Muller 1996). There are standards limiting fetal exposure to noise, but some have argued for a specific frequency weighting curve for fetal exposures (Eninger and Slagley 2017). Ambient noise exposures are termed “environmental noise,” and US Environmental Protection Agency (USEPA) and WHO guidelines exist to limit those exposures from aircraft, traffic noise, and the general environment (USEPA 1974, Berglund et al. 1999). Both ambient and industrial noise exposures contain an important risk consideration. It is assumed that the receiver is not voluntarily accepting the excess risk, and therefore the entity creating the noise risk is responsible to mitigate the risk or compensate the victim. However, children can voluntarily receive important noise exposures in school, musical bands, sporting events, and recreationally. Generations have had the opportunity for almost constant noise input from voluntary music-listening with headphones and ear buds. All of these other exposures may still affect the same ears which employers have a responsibility to protect.
Noise Type: Continuous and Impulse
Another important distinction emerges when considering the type of noise exposure. Gaussian, or continuous, noise has been shown to acceptably follow an equal energy exchange damage risk. The well-known 3-dB exchange rate works acceptably well to predict hearing loss of a cohort of workers. An average working lifetime of exposure kept below 85 А-weighted decibels (dBA) would result in an average of 5-dB NIHL for the group according to the International Organization for Standardization (ISO 2013). Non-Gaussian, or impulse noise, consists of a rapid, often transient, rise in pressure. Note that practitioners often differentiate impulse from impact noise based on the causative agent. Impulse noise arises from the rapid release of pressure into the fluid medium. Impact noise arises from striking two solids and often has an accompanying “ringing.” Peak levels can be very high in level, far exceeding the equal energy assumption limits. Current research is inconclusive on the appropriate levels of risk from impulse noise, and even how to best measure the risk. Measurement of noise signal kurtosis, or “peakinessshows some promise (Fuente et al. 2018) but is beyond the capabilities of most practicing occupational hygienists. Some have been working on better noise dose measurement systems (Kardous and Willson 2004, Kardous et al. 2005, Davis et al. 2019). Researchers active in the field of impulse noise suggest that the potential damage from impulse noise may be worse than current risk models indicate (Chan et al. 2016, Suter 2017). This is even more important for TEH when considering the popularity of recreational impulse noise exposures from firearms, fireworks, and other sources (Meinke et al. 2017).
Non-Noise Exposures
Two important non-noise exposures affecting hearing health are ototoxic chemicals and pharmaceuticals. They could be considered different ends of the same spectrum. Any chemical agent in the body, which affects the nervous system, may have some effect on the auditory nerve. Several solvents and heavy metals have substantial evidence in toxicological and epidemiological studies that they have an ototoxic property. Pharmaceutical preparations such as aminoglycosidic antibiotics, loop diuretics, certain analgesics, antipyretics, and antineoplastic agents also have warnings that they may induce hearing loss (NIOSH and OSH A 2018). A worker co-exposed to noise and ototoxic chemicals should be considered as “hypersensitive” for NIHL. Also, some agencies, such as the US Army, advise that airborne exposure to ototoxins above 50% of the occupational exposure limit requires workers to receive annual audiograms as a measure of possible ototoxic health effects (US Army 2015). Further, the combination of impulse noise and ototoxins may be more injurious than either alone or the assumed additive effects (Pons et al. 2017).
