lunes, 8 de febrero de 2016

Understanding Noise Exposure Limits: Occupational vs. General Environmental Noise

Understanding Noise Exposure Limits: Occupational vs. General Environmental Noise

Categories: Hearing Loss

Noise-induced hearing loss (NIHL) is 100% preventable; however, once acquired, it is permanent and irreversible [NIOSH 1998]. Understanding and minimizing the risks associated with noise exposures are the keys to preventing noise-related hearing loss.  NIOSH has a long history of leadership in conducting research, advancing control measures, and recommending noise-exposure limits to prevent job-related hearing loss.  Sometimes, observers ask whether our recommended limits for occupational exposure can be applied to exposures in the general environment from sources such as street noise, consumer appliances, and recreational pastimes. 
The answer, as we’ll explain below, is not exactly.

What is the NIOSH Recommended Exposure Limit?

NIOSH establishes recommended exposure limits (RELs) to protect workers against the health effects of exposure to hazardous substances and agents encountered in the workplace. These NIOSH limits are based on the best available science and practices.  In 1998, NIOSH established the REL for occupational noise exposures to be 85 decibels, A-weighted (dB[A]) as an 8-hour time-weighted average.  Exposures at or above this level are considered hazardous.  The REL is based on exposures at work 5 days per week and assumes that the individual spends the other 16 hours in the day, as well as weekends, in quieter conditions.  Importantly, the NIOSH REL is not a recommendation for noise exposures outside of the workplace in the general environment.
NIOSH also specifies a maximum allowable daily noise dose, expressed in percentages. For example, a person continuously exposed to 85 dB(A) over an 8-hour work shift will reach 100% of their daily noise dose.  This dose limit uses a 3-dB time-intensity tradeoff commonly referred to as the exchange rate or equal-energy rule: for every 3-dB increase in noise level, the allowable exposure time is reduced by half.  For example, if the exposure level increases to 88 dB(A), workers should only be exposed for four hours.  Alternatively, for every 3-dB decrease in noise level, the allowable exposure time is doubled, as shown in the table below.
Average Sound Exposure Levels Needed to Reach the
Maximum Allowable Daily Dose of 100%
Time to reach 100% noise doseExposure level per NIOSH REL
8 hours85 dB(A)
4 hours88 dB(A)
2 hours91 dB(A)
60 minutes94 dB(A)
30 minutes97 dB(A)
15 minutes100 dB(A)

When to Apply the NIOSH REL

The NIOSH REL is an occupational exposure limit, and was set to protect workers from developing hearing loss –substantial enough to make it difficult to hear or understand speech – over the course of a forty-year working career.  Risk of hearing loss from noise exposure is a complex issue.  Some single, brief intense exposures (such as a gunshot going off near your ear) can cause immediate hearing loss; however, these cases are rare.  Most noise-induced hearing loss is a result of accumulated damage from repeated exposures to hazardous noise.  In addition, the risk of noise damage depends on several factors: how loud the noise is, how long you listen to it, how much rest your ears get between exposures, and your individual susceptibility to noise.
Occupational noise exposure limits are established to simplify the complex question of risk and protect as many workers as possible from the effects of noise. The NIOSH REL is not designed to protect all workers from all hearing damage. When setting this limit, NIOSH acknowledged that approximately 8% of workers could still develop hearing loss.  In order to protect the most sensitive 8% of the population, NIOSH recommends that hearing protection be worn whenever noise levels exceed 85 dB(A) regardless of duration.

The Relationship between Occupational and General Environmental Noise Exposures

Noise can be found everywhere – restaurants, music and sporting venues, movie theaters, hospitals, and schools. Can the same occupational noise exposure guidelines that apply to workers also apply for assessment of risk to the general public?  The NIOSH REL is not meant to be used to protect against general environmental or recreational noise; it does not account for noisy activities or hobbies outside the workplace (such as hunting, power tool use, listening to music with ear buds, playing music, or attending sporting events, movies and concerts) which may increase the overall risk for hearing loss.
What noise recommendations exist for the general public? A 1974 U.S. Environmental Protection Agency report [EPA 1974] recommended a 70 dB(A) over 24-hour (75 dB(A) over 8-hour) average exposure limit for environmental noise (note that the 1974 report was explicit to state that it should not be constituted as a standard, specification, or regulation).  This limit was chosen to protect 96% of the general population from developing hearing loss as well as to protect “public health and welfare” (defined as personal comfort and well-being and absence of mental anguish and annoyance).
Both the NIOSH and EPA limits are based on the same scientific evidence and the equal-energy rule (i.e., 3-dB time-intensity tradeoff). However, the NIOSH REL and the EPA limit are designed to protect against different problems – the EPA limit is set to prevent noise that is annoying as well as hearing loss, whereas the NIOSH limit is set solely to protect against hearing loss.  The limit values (85 vs. 70) also differ because the EPA limit is averaged over 24 hours with no rest period while the NIOSH limit is averaged for just 8 hours and includes a rest period between exposures.  In addition, the EPA limit includes a 1.4 dB(A) allowance to protect against exposures for 365 days a year while the NIOSH REL is calculated to protect against work place exposures for 250 working days a year.  Finally, the EPA limit does not consider cost or feasibility of implementation as the Occupational Safety and Health Administration (OSHA), in accepting a NIOSH REL as the basis for a mandatory standard, is required to do under the Occupational Safety and Health Act of 1970External Web Site Icon.

Noise Level versus Time-Weighted Average Noise Exposure

It is important to differentiate between noise level and time-weighted average noise exposure.  While noise levels describe the intensity of sounds at a given point in time, the NIOSH and EPA exposure limits are set as time-weighted average exposures over periods of time.  While few people are able to measure their average noise exposures outside of work; , sound levels can be measured with a sound level meter or a smartphone sound measurement app.  Suppose you are at a restaurant, a concert hall, or a sporting event and you are able to measure the sound levels… how do you know whether your hearing is at risk?  The sound level at a given point in time can be higher than the exposure limit without creating risk, provided it is balanced out by enough time at lower levels during the day.  Even without knowing your time-weighted average, if the readout shows a level of 85 dB(A) or higher, NIOSH recommends that you take precautions to protect your hearing by reducing the noise when possible, limiting your exposure time, and/or using appropriate hearing protection.
Hopefully, the many considerations involved in setting and using noise exposure limits are clearer now. In a nutshell, while the NIOSH REL only applies to the workplace, protecting your hearing whenever sounds reach 85 dB(A) or more is a good health practice no matter where your ears are!

Chuck Kardous, MS, PE, is a research engineer with the NIOSH Division of Applied Research and Technology.
Christa L. Themann, MA, CCC-A, is an audiologist with the NIOSH Division of Applied Research and Technology.
Thais C. Morata, Ph.D., is a research audiologist with the NIOSH Division of Applied Research and Technology and the Coordinator of the NORA Manufacturing Sector Council.
W. Gregory Lotz, Ph.D., Captain, USPHS; is the Division Director of the Division of Applied Research and Technology (DART) and the manager of the NORA Manufacturing Sector Council.

For more information on about protecting your hearing and noise at work, including free materials, videos and tools, please visit the Noise and Hearing Loss Prevention Topic Page or send us your comments or questions in the comments section below.


EPA [1974]. Information on levels of environmental noise requisite to protect public health and welfare with adequate margin of safety. EPA/ONAC 550/9-74-004. Adobe PDF fileExternal Web Site Icon
NIOSH [1998]. Criteria for a recommended standard: occupational noise exposure. DHHS (NIOSH) Publication Number 98-126.

No hay comentarios:

Publicar un comentario