T.R. Consulting, Inc. Monthly Safety Article


Hearing Conservation



October 2002

Compiled and Written by:  Tony Rieck

T.R. Consulting, Inc.



T.R. Consulting, Inc. publishes monthly safety articles.  For a list of archived articles go to www.trconsultinggroup.com/safety/archive.html




Noise-induced hearing loss is a common occupational problem.  Approximately 30 million workers are exposed to hazardous noise while performing their job duties and as many as ten million additional workers are at risk of hearing loss due to exposure to solvents, metals and other agents.  Noise-induced hearing loss is 100 percent preventable.  Yet, once the hearing loss has occurred, the hearing loss is permanent and irreversible.  Preventative measures include engineering controls (such as mufflers and acoustic barriers) and personal protective devices (ear plugs and muffs).  Ear plugs and muffs are chosen only when engineering controls are not feasible or inadequate.  All workplaces where hazardous levels of noise can be encountered should institute a hearing loss prevention program.  These hearing conservation programs should include noise assessments, engineering controls, employee education, guidelines for the selection and use of hearing protection devices, audiometric tests to monitor worker hearing/hearing loss, recordkeeping methods, and program evaluation.


The following topics are presented in this article:


*          Types of Occupational Noise-Induced Hearing Loss

*          Types and Levels of Sound

*          Acceptable Thresholds for Occupational Noise Exposures

*          Monitoring Noise in the Workplace

*          Elements of a Hearing Conservation Program

*          Monitoring Equipment and Methods

*          Noise Control Methods

*          Types of Hearing Protection Devices



Types of Occupational Noise-Induced Hearing Loss


There are two types of noise exposure that can lead to hearing loss: acute exposure, termed “occupational acoustic trauma”, and chronic exposure, termed “occupational noise-induced hearing loss”.  Occupational noise-induced hearing loss is a slowly developing hearing loss over a long period as the result of exposure to continuous or intermittent noise at hazardous levels.  Occupational acoustic trauma is a sudden loss of hearing resulting from a single exposure to a sudden burst of sound.  Once the exposure to hazardous levels of noise is discontinued, there is no significant progression of hearing loss.  Additionally, those having suffered from noise-induced hearing loss are not more susceptible to further hearing loss nor are they more sensitive to hazardous sound levels encountered after the time of impairment.  A diagnosis of noise-induced hearing loss needs to be made by a physician and should include a historical assessment of noise exposure.


Types and Levels of Sound


Sounds can be divided into two categories:  continuous and intermittent.  Intermittent sounds are sounds that have maxima (peaks) at intervals of one second or more.  Continuous sounds are sounds that are either uninterrupted or that have maxima at intervals of less than one second.  A hammer striking a nail every 1.5 seconds is an intermittent noise source.  The same hammer striking the same nail once every 0.5 seconds is a continuous noise source.  Exposure to continuous noise sources is considered more hazardous than exposure to intermittent noise sources of the same sound level because the intermittent exposure allows the ear to have a brief rest period between peaks.  Exposure to sound levels at or above 85 decibels has the potential to cause hearing loss.


Some common noise exposures are listed below (from NIOSH Sound Meter):


*        Whisper                                     30dB

*        Normal Conversation                  60dB

*        Ringing Telephone                      80dB

*        Hair Dryer                                  90dB

*        Power Lawn Mower                  90dB

*        Belt Sander                                93dB

*        Tractor                                       96dB

*        Hand Drill                                   98dB

*        Impact Wrench                         103dB

*        Bulldozer                                  105dB

*        Spray Paint System                   105dB

*        Gas Chain Saw                         110dB

*        Hammer Drill                            114dB

*        Pneumatic Percussion Drill        119dB


From the above, it becomes obvious that we are exposed to potentially hazardous levels of noise as a part of our every day lives and that many of us are exposed to these same potentially hazardous levels of noise as a part of our work as well.  Exposure to excessive noise in our daily lives tends to be more limited than the same exposures at our place of employment.  Mowing your own lawn and drying your hair using a hair dryer may expose an individual to hazardous levels of noise for a few hours a week.  However, the landscaping employee may be exposed to six or more hours of hazardous noise each day as would an employee working in a hair salon. 


Acceptable Thresholds for Occupational Noise Exposures


Much like the allowable exposure to chemicals, allowable exposures to sound are calculated based on a time weighted average exposure (TWA).  The following chart provides information on the duration per day (HOURS) of allowable employee exposure to noise in the workplace based upon the OSHA Permissible Noise Exposure as listed in Table G-16 of 29 CFR Part 1910.95 (OSHA) and values provided in the CDC hearing conservation program for CDC employees (NIOSH/ACGIH).  Note that the OSHA values are the maximum acceptable under regulation.  The CDC program limits are more protective than the limits established by law and were derived from the ACGIH Threshold Limit Values (TLV) for noise exposure.  Sound levels are measured in dBA using the slow response mode.


HOURS                                  OSHA TWA                           NIOSH/ACGIH

16                                            85                                            80

8                                              90                                            85

4                                              95                                            90

2                                              100                                          95

1                                              105                                          100

½                                             110                                          105

¼                                             115                                          110

1/8                                           120                                          115


Note:  Additional values are provided in Table G-16 of 29 CFR Part 1910.95 for exposure values between below and above those provide in this chart.


Calculations of variable exposures throughout the day can be calculated as the sum (S) of the actual hours of exposure at a noise level (T) divided by the allowable hours of exposure at that level (H) for each of the different exposures.  If the value of S is greater than 1.0, the TWA has been exceeded.  The formula for the calculation is provided below:


T1                     T2                     T3

---        +          ---        +          ---        +   .  .  .  .            =          S

H1                           H2                           H3


Using this formula, it is possible to check the compliance of an employee exposure.  For example, if an employee is exposed to 95 dB for 1 hour of an 8 hour workday, 90 dB for 4 hours of the workday and 85 dB for the remaining three hours of the workday, the calculation would be as follows:


1                      4                      3

--(95dB) +       --(90dB) +       --(85dB)          =          0.9375

4                      8                      16


Since the result (S) is less than (or equal to) 1.0, the employee exposure is compliant with the standard.  Because the standard is based on an 8 hour workday, a longer workday would require that employees be exposed to somewhat lower levels of noise throughout the workday.


Monitoring Noise in the Workplace


Not all employers are required to measure the level of employee exposure to noise in the workplace.  Noise monitoring or measuring is only a requirement when employee exposures are at or above 85dB (TWA).  The employer would be wise to begin measuring the noise levels in the workplace if there have been employee complaints, normal conversation is difficult, specific equipment is provided by the manufacturer with information indicating that the 85dB threshold may be exceeded, or there are any indications that employees are losing their hearing.  If there is a question as to the need for a noise monitoring program, actual workplace noise measurements are a useful diagnostic tool.  If employees are exposed to a TWA noise exposure of 85dB or more, a hearing conservation program needs to be implemented.


Elements of a Hearing Conservation Program


A hearing conservation program begins with a written plan that tells how the employer is going to monitor the exposure of workers to noise, reduce employee exposures to noise, train and equip employees, and verify the effectiveness of the hearing conservation program.  The typical elements of the program are outlined below.




This section of the plan tells employees and other interested parties about the reasons that the employer has instituted a hearing conservation program, the goals of the program and how the program is organized.  This section should also describe how the policy is accessible to interested parties.




Administrators, trainers, supervisors and employees all have responsibilities necessary for the successful implementation of the hearing conservation program.  This section should also be used to designate employees and job duties affected by the implementation of the hearing conservation program.




In order for the hearing conservation program to be effective, several topics need to be addressed.  Areas where hazardous noises are generated need to be identified.  Appropriate measurements of noise levels need to be obtained.  Personnel monitoring may be required for employees that move from one area to another as a part of their activities.  Initial measurements of noise levels need to be periodically repeated and verified, and changes in equipment or acoustic controls may require reevaluation.




This section of the plan should spell out the engineering controls that are in place, standard operating procedures that reduce noise exposure, administrative controls and the types of hearing protection devices that are available to employees.  Administrative controls should include information on signage that is provided to alert employees to the need for hearing protective devices in areas where hazardous noise levels might be present.  Information on the types of hearing protection devices available to employees should include policies for issuance, use and maintenance of hearing protective devices.




This section of the plan details the way that employees will be notified, and explain the program for baseline and annual audiometric tests.




The training needs to include:  symptoms of overexposure to hazardous noises, the recognition of probable sources or conditions indicative of hazardous noise, information about the progression of noise-induced hearing loss, employer procedures related to hearing protection devices, advantages and limitations of hearing protection devices, noise measurement procedures, and the requirements of the hearing conservation program.




This section provides information on how employees can provide input about the program, how audiometric testing will be used as a part of an evaluation process, and whether the employer will use an outside third party to verify the effectiveness of the program.




This section states where records are to be kept and length of time that the records will be maintained.  This section will include any safeguards to be implemented to prevent unauthorized access and to assure access to those entitled to review the records.


The above outline represents a very basic approach to the written hearing conservation plan.  It is also helpful to include as appendices a copy of the regulations of 29 CFR Part 1910.95 and a copy of the most current training materials related to the program.  Once the plan is written and implemented, the hearing conservation program is in place.


Monitoring Equipment and Methods


Two instruments used to measure noise exposure are the sound level meter and the dosimeter. 


The sound level meter provides a “snap shot” of the noise at a given instant in time.  Multiple measurements taken at different times throughout the day can provide an estimation of the noise exposure associated with the area where the meter is placed.  Where there is significant fluctuation of noise levels (such as a noise source that regularly cycles on and off), the length of time that the noise remains at each level is determined.  This procedure is repeated at different locations within the workplace allowing for the creation of a sound level map.  Estimates of individual exposure are made by taking information on typical employee movement throughout the workplace.  This method of noise level monitoring is called area monitoring.


Dosimeters are typically used for personal monitoring.  The dosimeter is attached to the employees outer garment and a microphone is placed in the hearing zone of the employee.  The dosimeter records the sound levels during the test period, providing an average noise exposure for the test period.


Noise Control Methods


There are three methods available for controlling worker exposure to noise: engineering controls, administrative controls and personal hearing protective devices.  Engineering controls, if feasible, are the first option.  Where engineering controls are not feasible or are inadequate to reduce employee noise exposure to acceptable levels, administrative controls are the next preferred method.  Personal hearing protection devices are used as a permanent means of noise exposure reduction only after engineering and administrative controls have proven inadequate or unfeasible. 


Engineering Noise Control – any modification or replacement of equipment or related physical changes either at the noise source or along the noise transmission pathway that results in a reduction in the noise level at the employee’s ear.  Mufflers and acoustic barriers are examples of engineered noise controls.


Administrative Noise Control – any change in work schedules or operations that result in a reduction in employee noise exposure.  Increasing the distance between a noise source and a workstation, and rotation of employees between areas of greater and lesser noise exposure are examples of administrative noise controls.


Types of Hearing Protection Devices


Earplugs and earmuffs are the two categories of personal hearing protection devices.  Hearing protective devices have NRR (Noise Reduction Ratio) ratings.  Employees must be trained in the proper use, selection and maintenance of hearing protective devices that they might use.  In all instances, the NRR rating of the selected hearing protection device must be sufficient to reduce the noise at the ear drum to 85 dBA or less.  While the use of both earplugs and earmuffs may reduce the level of sound exposure beyond that achieved with either earplugs or earmuffs by themselves, the protection is not additive (NRR 29 earplugs + NRR 23 earmuffs does not equal NRR 52 protection).  It is also important to understand that the maximum sound attenuation achieved when wearing any hearing protection device is limited due to the ability of noise surrounding the head and body to be transmitted to the inner ear through tissue and bone pathways.


Premolded Earplugs – usually designed as either single or triple flange inserts, premolded earplugs are pliable devices of a fixed size.


Formable Earplugs – usually cylindrical or torpedo shaped, formable earplugs are rolled between the fingers, inserted into the ear canal and held until they expand to provide a snug fit.  Some formable earplugs are equipped with cords.


Custom Molded Earplugs – an option available to those who cannot be properly fitted with either premolded or formable earplugs.


Earmuff – a device worn around the exterior of the ear to reduce the level of noise that reaches the ear.  An air tight seal between the cushions of the earmuff and the wearer’s head is essential for proper protection when wearing ear muffs.


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