Changing the Frequency of Occupational Noise Exposure

Carlton W. Jackson

September 1, 1997

Noise-induced hearing loss from unwanted or undesirable sound was first documented centuries ago in a study by Ramazzini titled “De Morbis Artificium Diatriba,” and published in the year 1700. This study described how workers hammering copper “have their ears so injured by that perpetual din that workers of this class became hard of hearing and, if they grow old at this work, [they would become] completely deaf.”

Before the Industrial Revolution, very few people were exposed to high levels of noise in the workplace. The approach of the 20th century brought the widespread use of steam power as the primary energy source to fuel machinery of the hungry Industrial Revolution. This changed what, how and where the people would work. Along with all of the obvious benefits of the Industrial Revolution, there were many changes and “side effects” with which to deal. Among the side effects was the elevation of excessive noise from a mere annoyance to a serious occupational health hazard.

The medical world’s attention to noise as an occupational hazard began when the workers who fabricated the new steam boilers were found to develop serious hearing loss in such large numbers that the phenomenon became known as “Boilermaker’s Disease.” It was soon apparent that the increased mechanization of industry would proliferate the noise problem. The medical community had to understand what the occupational health hazards of excessive noise were and how they could be prevented or reduced. Although the number one effect of excessive noise exposure is hearing loss, other adverse non-auditory effects have been documented in the workplace including, but not limited to, psychological stress, poor job performance, hypertension and industrial accidents.


The ear is a non-selective organ, meaning you cannot select what to hear, so the ear is exposed to all the sounds within ear-shot” including continuous, loud and potentially dangerous noise. The ear is especially adapted and most responsive to the pressure changes in sound production. When the ear is exposed to excessive noise levels (above 85dBA) for an extended period of time, inner ear damage can result.

The ear is divided into three subdivisions-the outer, middle and inner ear. The outer and middle ear collect and transmit all sound pressure to the inner ear where the receptors for hearing are located. The receptors (hair cells) are arranged in several rows along the length of the basilar membrane, one of the two partitions which spiral around the bony cochlea. In the cochlea, mechanical energy from sound pressure is transformed into electrical energy that is carried by the auditory nerve to the brain.

Excessive noise exposure will injure the hair cells along the basilar membrane and result in noise-induced hearing loss (NIHL). At first, excessive exposure to high levels of noise causes only a temporary threshold shift (TTS), which is measured by testing a person’s hearing before and after noise exposure. This shift is temporary because pre-exposure hearing levels normally return within a few hours after exposure. But repeated exposure over the years can result in a permanent threshold shift (PTS), which is an irreversible sensorineural hearing loss.

Additional research has shown that a combination of noise exposure and certain physical or chemical agents (e.g. vibration, organic solvents, carbon monoxide, ototoxic drugs, and certain metals) may have negative effects on hearing. Although there are other causes of hearing loss in addition to excessive occupational noise, such as normal aging, disease, loud music, gun shooting, lawn-mowing, and so forth, excessive industrial noise remains the number one hearing problem in the American workplace.


Over a three-year period in the early 1980s, the National Institute of Safety and Health (NIOSH) conducted the National Occupational Exposure Study (NOES). This study was designed to provide benchmark data on the occupational safety and health conditions in the American workplace.

For the purpose of the study all workers exposed to noise levels of 85dBA or greater were considered to be noise-exposed, regardless of the duration. Using the data collected from the different economic sectors and applying these percentages to the 1993 Bureau of Census Report, NIOSH estimated that the number of “at risk” noise-exposed workers in the United States to be approximately 30 million workers. Special Trade Contractors were rated number four on the NIOSH list with an estimated 2,113,347 noise-exposed workers. The large number of potentially exposed workers led to the development of current occupational regulation regulations.


The military (U.S. Air Force) was the first organization to establish occupational noise regulations for members of the armed forces in 1956. Although safety and health standards had been issued by Walsh-Healey Public Contracts Act of 1936, the act did not establish excessive noise limits and did not acknowledge the existence of an occupational hearing-loss problem.

  • New regulations were promulgated under the Walsh-Healey Act in 1969 (41 CFR 50-204.10) for the purpose of hearing conservation and defined noise limits for occupational noise exposure. Only firms with supply contracts valued at $10,000 or more with the U.S. Government were covered by the new law. Additional public laws were passed under the Service Contract Act and the Coal Mine Health and Safety Act of 1969 (Public Law 91-173).
  • In 1970 Public Law 95-164, known as the Occupational Safety and Health Act, was enacted creating the Occupational Safety and Health Administration (OSHA) within the Department of Labor. OSHA was now the law enforcement agency responsible for protecting the health and safety of the majority of the private sector American workforce.
  • It presented a recommended exposure limit (REL) of 85 dBA for an 8-hour time weighted average (TWA) and methods for measuring and calculating noise, as well as providing a hearing conservation program. This document also acknowledged that NIOSH could not determine the technical feasibility of obtaining the REL, and 15 percent of those exposed to the REL over the course of a lifetime would develop noise-induced hearing loss (NIHL).
  • Since NIOSH was unable to determine the technological feasibility of the REL established in the criteria document, OSHA adopted the existing Walsh-Healey exposure limit of 90 dBA, 8-hour TWA, with a permissible exposure limit (PEL) of 5 dBA for general industry (29 CFR 1910.95).
  • For the next nine years, NIOSH studied the problem and continued to research existing data, until finally OSHA amended the noise control standards in 1981 (46 Fed. Reg. 4, 078) and again in 1983 (48 Fed. Reg. 9,738) to include specific requirements for a hearing conservation program to limit exposure to occupational noise at 85 dBA and above.
  • Not all industries are covered by the amended OSHA standards, specifically the Hearing Conservation Amendment does not cover the transportation, oil and gas well drilling/servicing, agriculture, construction or mining industries. For example, the construction industry is covered by another OSHA noise standard (29 CFR 1926.52), and the mining industry is covered by four separate standards (30 CFR 56; 30 CFR 57; 30 CFR 70; 30 CFR 71) which are enforced by the Mine Safety and HealthAdministration(MSHA).

Most recently NIOSH has issued “Criteria for a Recommended Standard Occupational Noise Exposure Revised Criteria 1996,” August 12, 1996. This document is available through NIOSH. (Editor’s note: For more information on the Construction Occupational Noise Standard (29 CFR 1926.52) and the General Industry Occupational Noise Standard (29 CFR 1910.95), see the article entitled “Hearing Protection: It’s the Law!” beginning on Page 24 in this edition of Insulation Outlook.)


When vibration or turbulence causes pressure changes in the air or other medium our perception of sound begins. Pressure changes in the air create waves that are transmitted away from the vibrating or turbulent source toward the receiver (your ears) in the form of compression and rarefaction of molecules. Your ears in turn translate these waves into what we call “sound.” How the sound will effect the receiver depends on three physical principles: amplitude, frequency and duration.


Sound pressure level (SPL), is expressed as decibels (dB), and is a measurement of the amplitude of the pressure change that produces sound. We perceive this amplitude as loudness. Sound measuring instruments use “weighted networks” to modify the sound pressure level. When measuring the effect of sound on people the A-Weighted Network (dBA) is commonly used to express exposure limits. When ever you see “dB” used without the suffix “A,” this usually implies that no network has been used.


ISO (1990). International Organization for Standardization, “Acoustics Assessment of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment.” Geneva, Switzerland: ISQ-1999.

OSHA (1989). Industrial Hygiene Field Operation Manual. Washington, D.C.: U.S. Department of Labor, Occupational Safety And Health Administration, OSHA instruction CPL 2.45B.

NIOSH (1995) Preventing Occupational Hearing Loss-A Practical Guide. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 96-110.