NIHL can be prevented by reducing noise at the source through engineering controls, limiting exposure by administrative controls, and employing effective hearing protection practices for exposures that cannot or are not avoided. The key component to all prevention efforts is education. Workers exposed to hazardous workplace noise need to understand that hearing can only be protected and preserved if efforts to reduce all hazardous exposures, not just those in the workplace, are undertaken.
Regulation of Occupational Noise Exposure
Noise exposure associated with the workplace has been known for centuries to produce hearing loss. In fact, “boilermakers' deafness” was the term coined to describe the now-familiar bilateral sensorineural hearing loss associated with excessive exposure to occupational noise. Largely on the basis of knowledge gained through field studies of hearing loss in industrial workers and military personnel, the U.S. Department of Labor promulgated regulations in the 1970s and 1980s designed to protect the hearing of employees who work in noisy environments. The principal regulation is the Occupational Noise Standard, promulgated by the Occupational Health and Safety Administration in 1972 and amended in 1983. This regulation covers workers in industry governed by the Department of Labor; other federal agencies (Federal Railroad Administration, Mine Safety and Health Administration, etc) have regulations that share key features with the OSHA rule.
The maximum exposure levels permitted by OSHA for various durations are shown in Table 58–1. The levels stated in the table represent the maximum allowable daily noise exposure, or the “permissible exposure limit” (PEL), as specified by OSHA and other federal agencies. The PEL for an 8 hours exposure is referred to as the “criterion;” it reflects the sound level in dBA (decibels measured with the A-weighting filter network in place, which reaches the PEL after 8 hours of exposure. Note that for exposures that differ from 8 hours the allowable daily exposure level is increased or decreased by 5 dB for each halving or doubling of exposure duration: ninety decibels are allowed for 8 hours daily, 95 dB for 4 hours daily, etc. Each of the exposures listed in the table represents an equivalent TWA exposure of 90 dBA for 8 hours. By definition an 8 hours TWA of 90 dB represents 100% of the allowable “dose.”
Table 58–1. OSHA Noise Standard Permissible Noise Exposure. ||Download (.pdf)
Table 58–1. OSHA Noise Standard Permissible Noise Exposure.
|Daily Duration (h)||Sound Level (dBA)|
|0.25 or less||115|
When the daily noise exposure is composed of two or more periods of exposure at different levels, their effects are combined by the following rule:
C1/T1 + C2/T2 + … + Cn/Tn
where C is the exposure duration at a given level; and T the allowable duration at that level.
“Percent allowable dose” is then calculated by multiplying the result by 100%. That is,
D = 100(C1/T1 + C2/T2 + … + Cn/Tn)
All exposures between 80 and 130 dBA are required to be integrated into the dose calculation. Exposures below the so-called threshold are not counted in the calculation of daily exposure; threshold values range from 80 to 90 dBA among various regulations. For example, consider the following daily noise exposure for a sheet metal worker:
|Activity||Level (dBA)||Duration (h)|
Calculation of Daily Dose is Made as Follows.
|Lunch, breaks||1/ infinity||0|
Therefore, this employee's exposure would not exceed OSHA's PEL. The dose could also be expressed as a TWA level in decibels by calculating the 8 hours exposure level that would result in the same dose:
- TWA = 16.61log10(dose/100) + 90
- = 16.61 Xlog (56.25/100) + 90 = 85.9dBA
It is important to remember that “dose” and “TWA” really refer to the same measurement: the 8-hours equivalent exposure for any measured duration or combination of levels and durations, expressed as percent or decibels.
As amended in 1983, the current U.S. occupational noise exposure standard identifies a TWA of 85 dBA, or 50% dose, as an “action level.” Workers covered by the standard who are exposed above the action level must be provided an effective hearing conservation program, including annual audiometric evaluations, personal hearing protection if desired, and education programs. With a daily noise exposure of 56.25%, or a TWA above 85 dBA, the sheet metal worker described should be in a company hearing conservation program.
The exposure limits set by OSHA were empirically determined from epidemiological and laboratory data concerning hearing damage from noise exposure and were designed to protect employees against sustaining a material impairment in hearing after a working lifetime. They were derived by subtracting the percent of workers sustaining a material impairment in hearing as a function of exposure level from a control population without occupational exposure. The resultant percentage is the “percent risk” or “percent additional risk” of a material impairment in hearing after, say, 40 year of exposure, above that expected from presbycusis alone. Estimates of percent risk vary depending upon which criteria and databases are used; the estimates provided in the original standard have been revised downward on the basis or more modern statistical fitting methods by the National Institute for Occupational Safety and Health (NIOSH) and support a conclusion that that the PEL of 90 dBA, with the 85 dBA action level, if enforced, would protect 93–96% of the working population from sustaining occupational noise-induced hearing loss.
In 1998 NIOSH revised its original 1972 recommendation that the permissible exposure limit for occupational noise be set to a time-weighted exposure of 85 dBA, and added, further, that the exposures be calculated with a 3 dB exchange rate. In other words, daily exposure at 85 dBA represents a 100% dose and the dose is doubled or halved for every 3 dB increase or decrease (ie, 88 dBA= 200%, 82 dBA= 50%, etc). NIOSH's recommended exposure limit (REL) is much more conservative than the OSHA standard, particularly for workers exposed at high levels for relatively short durations. For instance, the sheet metal worker's OSHA PEL 56% exposure (TWA = 85.8 dBA) described above would be 292% of the REL (TWA = 89.7 dBA) using the NIOSH limits. Although various groups have recommended the NIOSH REL as the appropriate federal standard for many years, it has not been adopted by any federal agency as a national standard.
The American Conference of Government Industrial Hygienists has also listed guidelines for occupational exposure to noise. These limits are specified as “threshold limit values” (TLVs), and they use the same metrics as the NIOSH REL (85 dBA criterion; 3 dB exchange rate). However, the TLV is specified by ACGIH as the minimum exposure at which one should consider implementing a hearing conservation program; it does not imply an upper limit of tolerable exposure, as does the OSHA PEL. Viewed in this way, there is no conflict between the ACGIH TLV and the OSHA PEL; hearing conservation programs should be implemented if the exposure exceeds the ACGIH TLV, and workers should not be exposed above the OSHA PEL.
The OSHA noise standards are useful to the physician in arriving at a diagnosis, and in determining whether a recommendation about hearing protection should be made. First, because workers exposed to excessive occupational noise should be in a hearing conservation program, evidence of exposure history and prior company-obtained audiograms may be available for consideration (Table 58-2). If the worker is not in a hearing conservation program, he or she may not work in significant occupational noise. Unfortunately, because not all workers are covered by OSHA standards and because enforcement has been weak, lack of participation in a hearing conservation program by a worker does not guarantee he or she has not been exposed to excessive occupational noise. However, in evaluating a patient, if it is determined that exposure to occupational noise did not exceed a TWA of 85 dBA, or a dose of 50%, then exposure to occupational noise should be ruled out of the etiology.
Table 58–2. Permissible Noise Exposure in the Workplace. ||Download (.pdf)
Table 58–2. Permissible Noise Exposure in the Workplace.
|Hours Per Day||Sound Levels dBA (Slow Response)|
Hearing Conservation Program
OSHA requires workers exposed above the action level (8 hours TWA of 85 dBA, or 50% dose) to be enrolled in a continuing, effective hearing conservation program. The major components of the program are summarized below.
Two general types of exposure assessment are commonly used in industry: area noise surveys and personal noise dosimetry. Area surveys are used to identify job locations where the TWA exposure may exceed 85 dBA; they are conducted by placing a sound level meter in a specific location and sampling the noise field. Exposures are then calculated based upon the amount of time an employee works in that specific location. Area surveys are also useful in determining the sources of exposure in an industrial environment and for planning noise control engineering strategies for reducing exposure (Table 58-3).
Table 58–3. Examples of Noise in Industry and Noise in the Environment. ||Download (.pdf)
Table 58–3. Examples of Noise in Industry and Noise in the Environment.
|Jet Engines-Flight Line|
- FA-18E engine at 80% (rear) < 50 ft
- FA-18E engine at idle (rear) < 50 ft
- FA-18 after burner test (rear) < 50 ft
- F104 Engine at idle from 200 ft
- Diesel hydraulic jenny
- 130 dBA
- 105 dBA
- 139 dBA
- 91 dBA
- 107 dBA
|Heavy Mobile Equipment|
- Road graders
|Tool Operations (Metal)|
- Pneumatic grinders aluminum
- Chipping weld on large aluminum structure
- Cut-off grinder cutting aluminum pipe
- Cut-off grinder cutting galvanized pipe
- Needle gun on Â¼-in.steel plate
- Punch Press 3/8-in. flat bar steel
- 100–102 dBA
- 120 dBA
- 100 dBA
- 96–98 dBA
- 108 dBA
- 118 dBA
|Tool Operations (Woodworking)|
- Cut-off saw
- Radial arm saw
- 112 dBA
- 98 dBA
- 93 dBA
- 106 dBA
- Normal conversation
- Hunting weapons
- 50–60 dBA
- 74–114 dBA
- up to 110 dBA
- 85–109 dBA
- up to 96 dBA
- 143–173 dBA
Occupational exposures can also be measured directly by using personal noise dosimeters. These devices are small, computerized integrating sound level meters that can record the minute-by-minute sound exposure throughout the workday. They are worn on a belt or in a pocket, and the microphone is positioned on the shoulder, approximately 5 in. lateral to the ear. Technological advances in microprocessor design, incorporating low power consumption and component miniaturization, have led to sophisticated, small, lightweight dosimeters that can be worn unobtrusively. A distinct advantage to using dosimetry over other methods is that the measurement instrument travels with the worker and can therefore provide a more accurate assessment of exposure as he or she moves among different noise environments during the work day. Exposure assessment using dosimetry does have some disadvantages. Because the instrument is mounted on the shoulder, reflection of sound off the body adds about 2 dB to the exposure assessment. In addition, it is nearly impossible to prevent bumping or touching the surface of the microphone during a normal workday. Contact with the microphone, even with a windscreen in place, will always increase the dose measure, and for short-duration, high-level impacts, the exposure can be inflated dramatically. Finally, because the dosimeter travels with the worker rather than staying under the control of the professional assessing the exposure, errors of commission and omission can occur. For example, a dosimeter attached to a jacket, which is then deposited in a locker before the workshift begins, will record the exposure of the jacket, rather than that of the worker.
Can be implemented to reduce employee exposure in many cases. Designers' conceptualize possible engineering solutions in terms of (1) the source (what is generating the noise), (2) the path (the routes the generated noise may travel, and (3) the receivers (the noise-exposed workers). To reduce noise exposure to workers such controls may involve (1) enclosures to isolate the sources or receivers, (2) barriers to reduce energy transmission along the path, and (3) distance to increase the path and ultimately to reduce acoustic energy at the receiver. Additional important engineering controls include the design of quieter manufacturing processing (low-acoustic emission saw blades).
Include (1) reducing the amount of time a given worker might be exposed to a noise source to prevent a TWA of noise exposure from reaching 85 dBA and (2) establishing purchasing guidelines to prevent the introduction of equipment that would increase the dose of noise to which workers are subjected. Though simple in principle, the implementation of administrative control requires management commitment and constant supervision, particularly in the absence of engineering or personal protection controls. In general, administrative controls are used as an adjunct to existing noise control strategies within a hearing conservation program rather than as an exclusive approach for controlling noise exposure.
Workers and management must understand the potentially harmful effects of noise in order to satisfy OSHA requirements and–most important–to ensure that the hearing conservation program is successful in preventing NIHL. A good worker education program describes (1) program objectives, (2) existing noise hazards, (3) how hearing loss occurs, (4) the purpose of audiometric testing, and (5) how workers can protect themselves. In addition, the roles and responsibilities of the employer and the workers should be clearly stated. Training is required to be provided annually to all workers included in the hearing conservation program. Opportunities for maintaining awareness occur during periodic safety meetings, as well as during audiometric testing appointments, when testing results are explained.
Workers in environments that exceed the OSHA PEL must wear hearing protectors that will reduce exposures to below the action level (85 dBA TWA). Hearing protectors must be offered to employees exposed at or above the action level but below the PEL, and hearing protector use is mandated if the employee exhibits a standard threshold shift (defined below). It is important that workers not be forced to wear hearing protectors with attenuation values significantly exceeding the attenuation needed to reduce the exposure to safe levels (ie, below the action level). There is no need to reduce exposures below the safe limit; doing so decreases workers compliance and decreases the ability of workers to communicate and hear warning sounds, which increases the probability of injury from accidental causes.
For many years OSHA has required employers to use the “noise reduction rating” (NRR), a method for rating hearing protectors approved by the Environmental Protection Agency in 1979. Because the NRR was developed as a laboratory standard to provide a benchmark comparison value among earplugs, it has long been recognized as an inadequate method for determining the real-world attenuation values that can be expected for a worker in a real-world environment. Recognizing the problems with the original NRR, OSHA provides, in a complicated appendix to the noise standard, three methods for derating the NRR values in order to arrive at a more “realistic” estimate of the actual attenuation that might be expected for a worker using the earplug. Discussion has been ongoing about a possible new rule, and a new one appears to be around the corner at the writing of this chapter.
The new rule, EPA Noise Labeling Standards for Hearing Protection Devices (40CFR 211 subpart B), is expected to be issued early in 2010. The rule provides a new method that gives NRRs determined by subjects who fit themselves without assistance. Further, a range of NRRs is provided rather than a single number. The range is designed to account for differences in individual subject fitting and also will address inherent differences in hearing protector type, as well as addressing modern hearing protectors that include active noise reduction circuitry. The low number in the range is the attenuation achieved by 80% of the subjects; the higher number reflects the attenuation achieved by 20% of the subjects. (Note in editing: there is considerable sentiment that the upper number should be 10%.) The new rule, when enacted, will form the basis of OSHAs hearing protector fitting methods.
Provides the only quantitative means of assessing the overall effectiveness of a hearing conservation program. A properly managed audiometric testing program supervised by either a certified audiologist, physician, or other personnel trained and experienced in occupational hearing conservation can detect changes in hearing over time that might otherwise be overlooked. The results of audiometric testing must be shared with employees to ensure effectiveness. The overall results or trends noted in an audiometric testing program can be used to fine-tune the hearing conservation program, including determining what types of hearing protection devices to offer to employees and the location where additional employee training is needed.
Employers are required to obtain an initial audiogram, usually obtained within the first 6 months of employment to be used as a baseline to which subsequent audiograms are compared. If a “standard threshold shift” defined as a change in hearing averaging 10 dB or more at the audiometric test frequencies of 2, 3, and 4 kHz occurs in either ear, the individual employee is notified and further action is required that may necessitate both modifying the hearing conservation program and notifying the appropriate authorities (eg, the employer or the appropriate government agency). In some cases, a referral to an otologist is indicated to determine the work-relatedness of the shift and to evaluate other potential medical causes.
American Conference of Governmental Industrial Hygienists (2006). Threshold Limit Values (TLVs) for Chemical Substances and Physical Agents and Biological Exposure Indices (BEIs). Cincinnati, OH. (Reference source for toxic exposures.)
American National Standards Institute, Inc. [ANSI]. (1996). American National Standard: Determination of occupational noise exposure and estimation of noise-induced hearing impairment (ANSI S3.44-1996). New York: American National Standards Institute, Inc.
National Aeronautics and Space Administration. National Auditory Demonstration Laboratory TWA Calculator. Website: http://adl.grc.nasa.gov/340/twa-calculator
. Accessed March 31, 2010. (Resource for determination of noise exposure parameters.)
National Institute for Occupational Safety and Health [NIOSH]. (1998). Criteria for a recommended standard. Occupational exposure to noise. Revised criteria. U. S. (DHHS Publication No. 98–126). Cincinnati, OH: NIOSH. (Proposal for new standard for noise exposure.)
Occupational Safety and Health Administration [OSHA]. (1983). Occupational noise exposure: Hearing conservation amendment; final rule. Occupational Safety and Health Administration, 29 C.F.R. 1910.95; 48 Fed. Reg. 9738–9785. (Federal rule for occupational noise exposure.)