The threshold of hearing is the quietest sound that can typically be heard by a young person with undamaged hearing. This varies somewhat among individuals but is typically in the micropascal range. The reference sound pressure is the standardized threshold of hearing and is defined as 20 micropascals (0.0002 microbars) at 1,000 Hz.
The threshold of pain, or the greatest sound pressure that can be perceived without pain, is approximately 10 million times greater than the threshold of hearing. It is, therefore, more convenient to use a relative (e.g., logarithmic) scale of sound pressure rather than an absolute scale (OTM/Driscoll).
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Conductive hearing loss results from any condition in the outer or middle ear that interferes with sound passing to the inner ear. Excessive wax in the auditory canal, a ruptured eardrum, and other conditions of the outer or middle ear can produce conductive hearing loss. Although work-related conductive hearing loss is not common, it can occur when an accident results in a head injury or penetration of the eardrum by a sharp object, or by any event that ruptures the eardrum or breaks the ossicular chain formed by the small bones in the middle ear (e.g., impulsive noise caused by explosions or firearms). Conductive hearing loss may be reversible through medical interventions such as hearing amplification (e.g. hearing aids) or surgical treatment. It is characterized by relatively uniformly reduced hearing across all frequencies in audiometric tests of the ear, with no reduction using hearing tests that transmit sound through bone conduction.
Sensorineural hearing loss tends to be a permanent condition that is often associated with irreversible damage to the inner ear. The normal aging process and excessive noise exposure are both notable causes of sensorineural hearing loss. Studies show that exposure to noise damages the sensory cilia that line the cochlea. Even moderate noise can cause twisting and swelling of the cilia and biochemical changes that reduce cilia sensitivity to mechanical motion, resulting in auditory fatigue. As the severity of the noise exposure increases or if the noise exposure is chronic, the cilia and supporting cells disintegrate and the associated nerve fibers eventually disappear. Occupational noise exposure is a significant cause of sensorineural hearing loss, which appears on sequential audiograms as declining sensitivity to sound, typically first at high frequencies (4,000 Hz), and then lower frequencies as damage continues. Often the audiogram of a person with sensorineural hearing loss will show a "Notch" between 3,000 Hz and 6,000 Hz, and most commonly at 4,000 Hz. This is a dip in the person's hearing level at 4,000 Hz and is an early indicator of sensorineural hearing loss due to noise. Results are the same for audiometric hearing tests and bone conduction testing. Sensorineural hearing loss can also result from other causes, such as viruses (e.g., mumps), congenital defects, and some medications. Modern hearing aids, though expensive, are able to adjust background sounds, changing signal-to-noise ratios, and support hearing and speech discrimination despite the diffuse nature of sensorineural hearing loss. The role of cochlear implants remains unclear.
In 1979, the U.S. Environmental Protection Agency (EPA) developed labeling requirements for hearing protectors, which required hearing protector manufacturers to measure the ability of their products to reduce noise exposure--called the noise reduction rating (NRR). OSHA adopted the NRR but later recognized that the NRR listed on hearing protectors often did not reflect the actual level of protection. The actual level of protection is likely lower than indicated on the label because most workers are not provided with fit-testing, and donning methods in a controlled laboratory setting are not representative of the donning methods that workers used in the field. EPA is considering options for updating this rule. See Appendix F for current information on NRRs and hearing protection labeling requirements. In special cases, noise exposure originates from noise-generating headsets. See Appendix G for a discussion of the techniques used to evaluate the noise exposure levels of these workers.
HPDs are rated to indicate the extent to which they reduce worker noise exposure. New technologies have been developed to test the effectiveness of earplugs and could eventually change the way hearing protection is rated. Although not required by the OSHA noise standard, several tools are available to employers that allow fit testing of HPDs, similar to respirator fit testing. HPD fit-testing enables employers to determine how well individual HPDs actually protect, and is also especially useful when workers have experienced an STS. See Appendix F for information on NRR methods, ratings, and requirements.
When utilizing HPDs, consideration must also be given for potential interference with communication requirements at the worksite, as they may make it difficult to hear warning alarms such as equipment alarms, emergency notifications, or backup alarms on mobile equipment. In these situations, communication headsets with integrated hearing protection may be a feasible solution, as they would provide hearing protection while allowing workers to hear and communicate with others. In addition, HPD fit-testing could be utilized to determine the appropriate attenuation for a given environment, by identifying an HPD that would provide necessary attenuation to protect the hearing, but not so much that it would interfere with the ability to hear warning alarms. Special consideration may be needed for workers with pre-existing hearing loss, as use of HPDs may further interfere with their ability to hear warning signals. However, it should be noted that such workers still must be protected from exposure according to requirements under the general industry and construction noise standards, as applicable, as there is no exception for employees who have diminished capacity to hear or who have been diagnosed as deaf (see letter of interpretation, August 3, 2004).
One study characterized overall and specific costs associated with HCPs at US metal manufacturing sites, and examined the association between these costs and several noise-induced hearing loss (NIHL) outcomes [Sayler et al., 2018]. Hearing impairment prevalence was about 15% overall. Higher expenditures for training and hearing protector fit-testing aspects of the HCPs were significantly associated with reduced STS prevalence. Higher training expenditures were also related to lower hearing impairment prevalence and high-frequency hearing loss rates. The study concluded that HCP costs were substantial and variable and that increased workplace spending on training and HPD fit-testing may help minimize NIHL.
Most sounds are not a pure tone but rather a mix of several frequencies. The frequency of a sound influences the extent to which different materials attenuate that sound. Knowing the component frequencies of the sound can help determine the materials and designs that will provide the greatest noise reduction. Therefore, octave band analyzers can be used to help determine the feasibility of controls for individual noise sources for abatement purposes and to evaluate whether hearing protectors provide adequate protection.
Review employer records to determine whether hazardous noise levels have been found in the past and to evaluate the employer's hearing conservation and recordkeeping programs. The records can also indicate what steps the employer has taken to reduce any excessive noise exposure and whether there is evidence that workers are experiencing noise-induced hearing loss. Audiometric testing records should be requested and reviewed as well as the OSHA 300 Log required under 1904.10 to determine if work-related hearing loss cases have been recorded. Also, ask the employer for noise questionnaires that may be in use. Refer to CPL 02-02-072, Rules of Agency Practice and Procedure Concerning OSHA Access to Employee Medical Records (8/22/07), for guidance on appropriately requesting, reviewing, documenting, and retaining worker audiogram records.
Ototoxic substances came gradually to the attention of occupational health and safety professionals in the 1970s, when the ototoxicity of several industrial chemicals, including solvents, was recognized. The possibility of noise/solvent interaction was raised more recently, when Bergström and Nyström (1986) published the results of a 20-year epidemiological follow-up study in Sweden, started in 1958 and involving regular hearing tests in workers. Interestingly, a large proportion of workers employed in the chemicals divisions of companies suffered from hearing impairment, although noise levels were significantly lower than those in sawmills and paper pulp production. The authors suspected that industrial solvents were an additional causative factor in hearing loss.
OSHA requires employers conduct baseline and annual audiometry testing on workers who have daily TWA noise exposures of 85 dBA or more. Audiogram review is an important tool for assessing the effectiveness of the hearing conservation program. OSHA requires employers to establish a program to monitor annual audiograms to determine if employees exposed to noise hazards are losing their hearing over the course of employment. OSHA defines a Standard Threshold Shift (STS) as a change in the hearing threshold relative to the baseline audiogram of an average of 10 or more dB in 2,000, 3,000, and 4,000 Hz in either ear. OSHA requires employers to record hearing loss in the OSHA 300 log if there is an STS and the employee meets the definition of mild hearing impairment/loss defined as an average hearing loss in 2,000, 3,000 and 4,000 Hz greater than 25 dB from audiometric zero.
Mean values and standard deviations (ms) of absolute latencies and interpeak intervals on BAEP tests of young adult subjects with normal hearing according to biologic calibration done at a university hospital. 2ff7e9595c
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