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In spaces defined by walls, however, sound fields are more complex. When sound-reflecting objects such as walls or machinery are introduced into the sound field, the wave picture changes completely. Sound reverberates, reflecting back into the room rather than continuing to spread away from the source. Most industrial operations and many construction tasks occur under these conditions. Figure 5 diagrams sound radiating from a sound source and shows how reflected sound (dashed lines) complicates the situation.

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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.

Presbycusis is a gradual sensorineural hearing loss associated with aging. The onset and the degree of hearing loss can vary considerably and is related to genetics, other impacts such as an accumulation of diseases, medications, and the cumulative effect of noise in the modern environment. Presbycusis and noise induced hearing loss appear to be additive and both can contribute to hearing loss in older people. Both types of hearing loss affect the upper range of an audiogram. A sloping audiogram with tapering to the lowest levels at 8,000 Hz often indicates that the hearing loss is aged-related, but after years of exposure, noise-induced hearing loss can have the same pattern. As humans begin losing their hearing, they often first lose the ability to detect quiet sounds in the high frequency range. This progresses to difficulty understanding conversations in noisy environments, even when amplified by hearing aids.

The primary effects of workplace noise exposure include noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus. A noise-induced temporary threshold shift is a short-term decrease in hearing sensitivity that displays as a downward shift in the audiogram output. It returns to the pre-exposed level in a matter of hours or days, assuming there is not continued exposure to excessive noise.

If noise exposure continues, the shift can become a noise-induced permanent threshold shift, which is a decrease in hearing sensitivity that is not expected to improve over time. A standard threshold shift (STS), as defined by OSHA, is a change in hearing thresholds of an average of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear when compared to a baseline audiogram. Employers can conduct a follow-up audiogram within 30 days to confirm whether the STS is permanent. Under 29 CFR 1910.95(g)(8), if workers experience an STS, employers are required to fit or refit the workers with hearing protectors, train them in the use of the hearing protectors, and require the workers to use them. Recording criteria for cases involving occupational hearing loss can be found in 29 CFR 1904.10; also see information and examples in Appendix I.

In 1998 NIOSH published Criteria for a Recommended Standard: Occupational Noise Exposure (DHHS 98-126). That publication recommends a 3 dB rather than a 5 dB exchange rate. Although OSHA enforces its own standard, the Council for Accreditation in Occupational Hearing Conservation (CAOHC), which trains and certifies clinicians in managing audiometric programs, expects clinicians to understand how the different exchange rates influence estimates of noise dose and therefore affect attribution to hearing loss.

The basic elements of an HCP include monitoring, training, use of hearing protectors, and audiometric evaluations for noise-exposed workers (those above 85 dBA) required to be enrolled in the program (see 29 CFR 1910.95(c)). Training requirements for an HCP are provided in the previous Section M. For more information about determining whether the attenuation of a HPD is sufficient, see Appendix F; for evaluating benefits versus costs of HCPs, see Appendix H; and for more information on reviewing audiograms, see Appendix I.

In some workplaces your visit will be the first time a thorough investigation has been performed; frequently, however, at least some aspects of noise investigations will have been completed previously through the employer's workplace health and safety measures or sometimes as part of seemingly unrelated activities, such as expanding operations or upgrading equipment. To conduct an investigation, you will need to determine what information is already available through employer or industry records, and then confirm it and fill in the gaps. To ensure that the investigation is efficient, however, you must be prepared to accomplish both these steps simultaneously, which requires some advance planning.

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.

Your completed sketch will show a series of contours around the noise source(s) (Figure 25). Expect the contours for adjacent noise sources to overlap. Workers operating entirely outside the contour are not exposed to noise in excess of the AL. Workers whose tasks take them closer to the equipment might experience exposures between the AL and the PEL, or even in excess of the PEL. Take photographs to document the type of equipment or process.

When monitoring is complete at the end of the day, follow standard procedures for recording results from the instruments. If necessary, consult the instrument user's manual or contact CTC for assistance. Dosimeter output usually includes the TWA (normalized to 8 hours), the LAVG or LEQ representing the average dose for the period monitored, the percent dose, and the maximum or peak reading. Do not neglect to perform the post-use calibration check on each instrument.

The economic feasibility of lowering noise levels with engineering controls is an important factor in deciding whether to implement specific controls. In addition to the direct costs of design, materials, construction or installation, and maintenance of engineering controls, these controls can have indirect costs and benefits, such as decreasing worker absenteeism, increasing or decreasing worker productivity, and increasing or decreasing the life of process equipment. Furthermore, if an engineering control reduces worker TWAs below 85 dBA, the need for a hearing conservation program is eliminated, along with the associated costs. These costs include expenses for audiometry, training, HPDs, recordkeeping, and program administration.

For additional information on ultrasound exposure levels, ceiling values, and 8-hour TWAs that apply to other frequencies, as well as ceiling values measured underwater, refer to the complete ACGIH TLV for ultrasound (see ACGIH, 2020, Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices).

A longitudinal study (Schäper et al., 2003; Schäper et al., 2008) on the relationship between hearing impairment measured by pure tone audiometry and occupational exposure to toluene and noise has not found ototoxic effects in workers exposed to a concentration of toluene lower than 50 ppm. The observed hearing loss was associated only with noise intensity. However, the use of hearing protection was not taken into account in the conclusions relative to the potential interaction between noise and toluene on hearing.


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