Noise, Under Control

Most exposures vary from minute to minute, depending upon factors ranging from the employee's positioning to what equipment is operating at the time.

• By Jeffrey Birkner
• Jun 01, 2005

NOISE is a particularly insidious hazard. It can be enjoyable and hazardous at the same time. The effect that it has on our hearing generally depends on how long we have been exposed to the noise and how loud the sound is.

The sound of a pin dropping on a hard surface is about 20 decibels (dB). Hearing a sound this low does not cause problems. In contrast, a jet engine is about 160 dB and can be very painful. OSHA has determined that noises exceeding 90 dB for extended periods of time are generally considered to be hazardous.

Unfortunately, hearing loss is usually painless, progressive, and permanent. Even though noise levels above 90 dB may be hazardous, they are usually not painful.

Hearing
How do we hear sound? Sound, which is in the form of acoustical energy, enters the ear through sound pressure waves. These sound waves enter the outer ear and travel down the ear canal to the eardrum. When the sound waves hit the eardrum, they cause it to vibrate. The eardrum then transmits the vibrations through three small bones in the middle ear: the malleus, the incus, and the stapes.

These vibrations are further transmitted to the oval window and the fluid-filled cochlea of the inner ear. The cochlea contains thousands of tiny hair cells. These tiny hair cells convert vibrations to nerve impulses that are then transmitted to the brain for interpretation. The hair cells are very sensitive and lose their resilience when they are repeatedly exposed to excessive noise. When this occurs, it can result in noise induced hearing loss.

If you are exposed to excessive noise for short periods of time, you may experience muffled hearing or tinnitus, a ringing in the ears, for a few hours. Repeated exposure to excessive levels of noise for long periods of time may result in permanent hearing loss. At first, you may have trouble hearing high-frequency sounds. Then, as time progresses, you may begin to have trouble hearing speech--first consonants, then vowels, and finally, all speech.

How Can Hearing Loss Be Prevented at Work?
Most of us spend a large portion of our lives at work. During that time, we are exposed to any number of hazards, including slippery surfaces, falling objects, noise exposures, and dusty environments. Most of these hazards, such as excessive noise exposures, can be avoided. If the employer and the employee take the proper steps, hearing loss from work-related exposures could easily be prevented. The following steps can be used as a guide to prevent work-related hearing loss.

Monitoring
To properly evaluate the situation and limit exposure to noise, the employer must be able to quantify how much noise is present. OSHA requires that evaluations be performed by a qualified health and safety professional. It also requires that a comprehensive hearing protection program be instituted when exposure levels exceed 85 dBA (decibels estimated using OSHA A weighting) as an eight-hour Time Weighted Average (TWA).

Noise monitoring can be done using various devices. The two main devices used to monitor noise are a sound level meter and a noise dosimeter. The sound level meter will give the noise levels in various areas. Readings can be taken relatively quickly, and they will give enough information to determine whether a more in-depth evaluation using noise dosimetry is required. However, unless the noise levels are constant throughout the day in the particular work environment being evaluated, the sound level meter will not give a realistic evaluation of the actual and complete exposure of the employee throughout their work shift. A sound level meter can help the employer get a general idea of noise at various operations and also help when employers decide whether their operations require a more sophisticated evaluation.

Most exposures vary from minute to minute, depending upon any number of factors. These factors can be anything from where the employee is positioned to what equipment is being operated at the time. Carpentry is a good example of an occupation with many varied noise levels. The carpenter can be hammering, using and electric saw, or quietly doing measurements. A sound level meter would have to be used for each of these various activities and somehow averaged.

A noise dosimeter solves this problem. A noise dosimeter has a microphone that can be positioned near the ear, measuring the exposure of the individual throughout the day. It measures continuously and will provide an eight-hour TWA, which is how OSHA requires that employers assess overall noise exposure. There are unique situations when a more sophisticated device is required, such as an Octave Band Analyzer; however, these situations are rare.

Engineering Controls
Once employee exposure has been characterized and it has been determined that noise levels must be controlled, the employer should investigate how the equipment or the surrounding areas might be changed or modified to reduce the amount of noise. This may be done by modifying the equipment, building enclosures with sound-dampening materials, reducing vibrations, and installing sound-absorbing walls and carpeting.

Unfortunately, retrofitting a work environment to reduce noise can be expensive and requires an in-depth knowledge of acoustics. It also may require the help of an acoustical engineer to do the job right, but the expense will be well worth it if it can reduce hearing loss.

Administrative Controls
When engineering controls cannot completely alleviate the hazard, are not feasible, or are in the process of being implemented, administrative controls can be considered. Actions such as operating noisy equipment on shifts when there are fewer employees present or rotating employees out of noisy areas to limit the overall exposure to each individual can work in the short term.

Personal Protective Equipment
If exposures exceed 90 dBA as an eight-hour TWA and other controls are unable to reduce exposures below 90 dBA, OSHA requires the use of hearing protection devices. Although OSHA considers this the least desirable method of reducing noise exposure, many times it is the only practical way to limit noise exposure.

Ear plugs, ear muffs, and semi-aural devices are the most common types of hearing protection devices. The employer needs to decide which device is the most appropriate for their particular situation. Employers must consider whether the employee will be wearing the device all day or intermittently, the noise levels, the comfort factors, the cleanliness of the environment, and cost effectiveness.

In a very dirty environment, for instance, an employee may want to remove the device when there are no noise exposures. In this instance, a banded product may be the most appropriate device to wear. The banded product, such as a muff or semi-aural device, is easy to keep clean. When the device must be comfortable because it has to be worn all day and removed only during meals and breaks, a foam product may be best.

Complying with 29 CFR 1910.95
Regardless of the approach(es) taken, employers must be able to properly justify that they have, in some way, reduced noise exposures adequately. Usually, this will involve further monitoring and/or documentation of the actions the employers have undertaken, which may require the services of a health professional such as an industrial hygienist. But it will be money well spent: A good hearing program can reduce hearing loss in employees, possibly reduce worker's compensation claims, and potentially reduce or eliminate fines.

One special note: Assuming an employer has determined some of the areas in its facility exceed 85 dBA as an eight-hour TWA, it will have to implement a Comprehensive Hearing Protection Program in accordance with 29 CFR 1910.95. This standard covers all of the elements required to institute a minimally effective hearing protection program. It includes, but is not limited to, defining when a program is to be implemented, defining excessive noise level, monitoring, employee notification, observation of monitoring, audiometric testing program requirements, training, and recordkeeping.

The employer also should determine whether there are state or local regulations. The employer may be required to comply with these additional regulations, and they are often more stringent than those imposed by federal OSHA.

This article appears in the June 2005 issue of Occupational Health & Safety.

This article originally appeared in the June 2005 issue of Occupational Health & Safety.