Occupational Noise Exposure

Noise, or unwanted sound, is one of the most common health problems in American workplaces. However, it is often ignored because there are no visible effects and it usually develops over a long period of time.

Human hearing is amazingly sensitive. Our ears can distinguish 400,000 different sounds and can detect sounds so quiet that they cause the eardrum to vibrate less than 1/80,000,000^th of an inch. But there's no guarantee that this remarkable sensitivity will last a lifetime — to maintain it, it must be protected.

The National Institute for Occupational Safety and Health (NIOSH) estimates that 30 million workers in the U.S. are exposed to hazardous noise. Exposure to high levels of noise can cause hearing loss, create physical and psychological stress through a progressive loss of communication, reduce productivity, interfere with communication and socialization, and contribute to accidents and injuries by making it difficult to hear warning signals.

In the work environment, OSHA requires employers to determine if their employees are exposed to excessive noise. If so, the employers must implement feasible engineering or administrative controls to eliminate or reduce hazardous levels of noise. Where controls are not sufficient to reduce noise to an acceptable level, an effective hearing conservation program must be established.

Sound and Noise

Sound is usually defined as what you hear; while noise is any sound you don't want to hear. The point at which sound becomes a problem is when it's so loud that it destroys the ability to hear the sounds we want to, or need to, hear.

Sound is the physical phenomenon that stimulates our sense of hearing. It is an acoustic wave that results when a vibrating source, such as machinery, disturbs an elastic medium, such as air. In air, sound is usually described as variations of pressure above and below atmospheric pressure. These fluctuations, commonly called sound pressure, develop when a vibrating surface forms areas of high and low pressure, which transmit from the source as sound.

Sound is measured in two ways: decibels and frequency. Decibels indicate the pressure of sound. Sound waves transfer that pressure from place to place and are expressed in units on a logarithmic scale. Frequency is related to a sound's pitch and is measured in units called hertz (Hz), or cycles per second.

The pitch of a sound — how high or low it seems — is how you perceive its frequency. The higher the pitch of a sound, the higher its frequency. High-frequency sounds are generally more annoying than low-frequency sounds and can be more harmful to hearing. Human hearing is most sensitive to frequencies between 3,000 to 4,000 Hz. That's why people with damaged hearing have difficulty understanding higher-pitched voices and other sounds in the 3,000 — 4,000 Hz range.

Anatomy of the Ear

The ear is the organ that makes hearing possible. It has three main parts:

1. External outer ear,
2. Air-filled middle ear, and
3. Fluid-filled inner ear.

The function of the ear is to gather, transmit, and perceive sounds from the environment. This involves three stages:

1. Modification of the acoustic wave by the outer ear, which receives the wave and directs it to the eardrum.
2. Conversion and amplification of the modified acoustic wave to a vibration of the eardrum (transmitted through the middle ear to the inner ear).
3. Transformation of the mechanical movement (vibrations) of the wave into nerve impulses. The vibrations pass the small bones of the middle ear, which transmit them to sensory cells, called hair cells, located in the cochlea. The vibrations become nerve impulses and go directly to the brain, which interprets the impulses as sound.

Effects of Excessive Exposure

People differ in their sensitivity to sound, and there's no way to determine who is most at risk for hearing damage. Factors such as sound pressure, frequency, and length of exposure all play roles in determining whether what a person hears is harmful or just annoying.

When noise is too loud, it can damage the sensitive hair cells of the inner ear. As the number of damaged hair cells increases, the brain receives fewer impulses to interpret as sound. While a single exposure to loud noise can damage hair cells, it probably won't destroy them. The person may experience ringing in the ears and some sounds may be muffled, but the hair cells will recover and so will hearing. This is called a temporary threshold shift. But, without proper hearing protection, repeated exposures to loud noise can damage hair cells to the point that they won't recover. Because the damage is severe, it results in a permanent threshold shift, and no treatment will restore it.

In its early stages, when hearing loss is above 2,000 Hz, it affects the ability to understand or discriminate speech. As it progresses to the lower frequencies, it begins to affect the ability to hear sounds in general. The three main types of hearing loss are conductive, sensorineural, or a combination of the two.

The effects of noise can be simplified into three general categories:

• Primary effects: Noise-induced temporary threshold shift, noise-induced permanent threshold shift, acoustic trauma, and tinnitus.
• Effects on communication and performance: Isolation, annoyance, difficulty concentrating, absenteeism, and accidents.
• Other effects: Stress, muscle tension, ulcers, increased blood pressure, and hypertension.

Ultrasonics

Ultrasound is high-frequency sound that is inaudible, or cannot be heard, by the human ear. However, it may still affect hearing and produce other health effects. Factors to consider regarding ultrasonics include:

• The upper frequency of audibility of the human ear is approximately 15–20 kilo-Hertz (kHz).
  • This is not a set limit and some individuals may have higher or lower (usually lower) limits.
  • The frequency limit normally declines with age.
• Most of the audible noise associated with ultrasonic sources, such as ultrasonic welders or ultrasonic cleaners, consists of subharmonics of the machine's major ultrasonic frequencies.
  • Example: Many ultrasonic welders have a fundamental operating frequency of 20 kHz, a sound that is at the upper frequency of audibility of the human ear. However, a good deal of noise may be present at 10 kHz, the first subharmonic frequency of the 20 kHz operating frequency, and is therefore audible to most persons.