Introduction
The Department of Health of Brazil (2006) defines Noise-Induced Hearing Loss (NIHL)
as hearing loss caused by prolonged exposure to noise. It is characterized as sensorineural
hearing loss and is usually bilateral, irreversible, and progressive while the exposure
to noise continues.
NIHL is a predictable and preventable disease with an epidemiologically relevant prevalence
in urban communities. It is also called hearing loss from exposure to noise at work,
occupational hearing loss, or professional deafness. It begins and prevails at frequencies
of 3, 4, and 6 kHz, progressing later to frequencies of 8, 2, 1, 0.5, and 0.25 kHz.
However, when we study occupational hearing loss, we should remember that noise exposure
can coexist with other causal agents that may interact with the noise and enhance
its effects on hearing. These include exposure to certain chemicals, vibrations, the
use of some medications, and individual susceptibility.
Morata and Lemasters (1995 apud Department of Health, 2006) proposed the use of the
term “occupational hearing loss” because it is more inclusive, comprising the noise,
which is undoubtedly the most frequently responsible agent, but also other causes,
all of which should be considered in the implementation of diagnostic and preventive
measures, limits, and safety legislation.
The U.S. safety standards commissioned by the Occupational Safety and Health Administration
(OSHA) permit unprotected exposure to noise up to 90 dBA for up to 8 hours with annual
monitoring. In Brazil, the regulation 15 of the consolidated labor laws (NR-15) limits
the exposure to continuous noise at 90 dBA to a period of 4 hours and the decibel
level to 85 dB for a full 8-hour work day.
Hearing loss caused by continuous exposure to noise can cause several functional limitations
of hearing, i.e., changes in frequency selectivity, temporal and spatial resolution,
recruitment, and tinnitus as well as changes in hearing sensitivity (Samelli, 2004).
Discussion and Literature Review
In 1996, the National Institute for Occupational Safety and Health (NIOSH), an agency
of the government of the United States of America, published a practical guide for
Occupational Hearing Loss Prevention using the term “occupational hearing loss,” which
encompasses not only hearing loss caused by exposure to aromatic solvents, metals,
and some asphyxiating agents or to vibration, thus encouraging research into these
and other factors that could potentiate hearing loss, but also noise-induced hearing
loss (Fiorini; Nascimento, 2001).
It is estimated that 25% of the working population is exposed to a level of nose that
will lead to some degree of NIHL (Bergstrom; Nystrom, 1986; Carnicelli, 1988; Morata,
1990; Prospero, 1999). Although NIHL is the most common work-related injury, its prevalence
in Brazil remains poorly understood. This reinforces the importance of reporting,
which enables accurate assessment of the situation and appropriate scaling of the
necessary prevention and assistance measures.
The most striking feature of NIHL is the degeneration of the ciliated cells of the
organ of Corti. Recently, these lesions and the onset of cellular apoptosis have been
demonstrated to stem from oxidation caused by the presence of free radicals formed
by the excessive sound stimulation or by exposure to certain chemicals. These findings
have led to the study of substances and conditions that can protect cochlear ciliated
cells against the insults of noise and chemicals (Oliveira, 2001, 2002; Hyppolito,
2003).
According to the Department of Health in 1998, the National Committee on Noise and
Hearing Conservation defined the characteristics of Pair as follows:
-
Always sensorineural, once the organ of Corti in the inner ear has been damaged.
-
Usually bilateral, with similar patterns in each ear. In some situations, the degree
of hearing loss differs between the ears.
-
Usually does not produce hearing loss greater than 40dB (NA) at low frequencies and
75dB (NA) at high frequencies.
-
The progression ceases when the exposure to intense noise ends.
-
The presence of NIHL does not make the ear more sensitive to noise; as the threshold
increases, the progression of the loss slows.
-
The hearing loss begins and predominates in the frequencies of 3, 4, and 6 kHz and
eventually progresses to 8, 2, 1, 0.5, and 0.25 kHz.
-
With stable exposure conditions, the losses in 3, 4, and 6 kHz range generally plateau
after about 10 to 15 years.
-
The worker with NIHL may develop intolerance to loud sounds and complain of tinnitus
and reduced speech intelligibility, which hampers oral communication.
Changes in frequency selectivity also cause difficulties in auditory discrimination.
This injury increases the minimum time required to solve a sound event (temporal resolution),
which, especially in association with the reverberation of the work environment, limits
the ability of the patient with NIHL to recognize sounds (Bamford; Saundes, 1991).
The worker with NIHL may develop intolerance to loud sounds and complain of tinnitus,
reduced speech intelligibility, and impaired verbal communication. In a study of 3466
workers who applied for compensation because of NIHL, McShane, Hyde, and Alberti (1988)
reported a 49.8% prevalence of tinnitus. Of the affected subjects, 29.2% stated that
tinnitus was their main problem.
The diagnosis of NIHL is based on clinical and occupational research in which the
prior and current exposure to risk is evaluated and the characteristic symptoms described
earlier (Department of Health, 2006) are considered. The diagnostic work-up should
also include audiometry evaluation.
It is important to detail the exposure to allow correlation between the exposure and
the signs and symptoms of NIHL. Occupational anamnesis is thus an essential tool for
risk identification. Such knowledge about the working environment can be obtained
through site visits, review of the company's technical reports, and information from
inspections as well as from the patient's report.
It is essential to confirm the presence of hearing loss by an audiological evaluation.
This consists of the following tests: pure tone audiometry by air conduction (despite
its subjective nature), pure tone audiometry by bone conduction; speech audiometry,
and immittance. To be reliable, this assessment must be performed under the conditions
established by Ordinance No. 19 of the Regulatory Norm no. 7 (NR-7).
The diagnostic work-up should be performed in a way that minimizes the influence of
confounding factors extrinsic to the examination, and it is therefore important to
use a soundproof booth and calibrated equipment, to have the exam performed by a qualified
professional (doctor or audiologist), and to instruct the patient to perform sound
rest for 14 hours before the exam. The examiner must also consider potentially confounding
factors intrinsic to the examination, i.e., those related to the patient and his or
her general condition, motivation, intelligence, attention, familiarity with the task,
and interpretation of the test instructions.
When prior hearing assessments exist, they should be compared with the current assessment
to determine whether there has been progressive hearing loss, which in NIHL begins
and predominates in the frequencies of 3, 4, and 6 kHz and later progresses to 8,
2, 1, 0.5, and 0.25 kHz. Likewise, it should be considered that with stable exposure
to noise, the losses in the 3, 4, and 6 kHz range generally plateau within approximately
10 to 15 years.
It is important to differentiate NIHL from other auditory pathologies that have different
characteristics despite having the same etiology and, possibly, occurring in association
with the work environment. Thus, we should remember that “the effects of noise can
cause: acoustic trauma, temporary change in hearing threshold and hearing loss” (Maia,
2006).
There is as yet no treatment for NIHL. The key is to limit the damage by notifying
the appropriate individuals to initiate health surveillance and then monitor the progression
of hearing loss through periodic audiological evaluations. Even now, careful analysis
of the worker's audiological evaluation can lead to rehabilitation through individual
actions and group therapies.
As noise is an occupational hazard, prevention efforts must be implemented in the
work environment. As described earlier, there is legislation defining exposure limits
as well as guidance on programs to prevent and control risks. Companies must follow
these guidelines in accordance with the Regulatory Standards of the Ministry of Labor:
A Prevention Program Environmental Risks (PPRA-NR-9), which state that the various
risks in the workplace must be identified, quantified, and reported to the Medical
Control of Occupational Health (PCMSO-NR7), which will use the information to determine
and proceed with the appropriate assessments of the workers' health.
The actions taken to control NIHL are related to noise control. Measures should be
taken to control the exposure at the source, trajectory, and individual levels. In
addition, organizational measures, such as reducing travel, establishing breaks, and
rotating shifts through the noisy area, can be effective.
Noise is defined as a “physical agent capable of causing damage to the human body
with the short and medium term effects” (Paraguay, 1999). The word is usually associated
with an unpleasant sound with chaotic and irregular frequencies. Noise produced in
public spaces in such excess that it contributes to an undesirable environment is
called noise pollution. Such pollution, which can be intolerable, is becoming one
of the environmental problems affecting the greatest number of people according to
the WHO (WORLD HEALTH ORGANIZATION, 2009).
As the sensitivity of the human ear is highest in the frequency range of 2,000 Hz
to 4,000 Hz, Regulatory Norm number 15 (NR-15), through the Decree n° 3.214/1978 (BRAZIL,
1978), establishes limits for exposure to continuous noise in this range, as shown
in [Table 1] below.
Table 1.
Noise NR-15.
|
Noise (DB) (A)
|
Maximum allowable daily exposure
|
|
85
|
8 hours
|
|
86
|
7 hours
|
|
87
|
6 hours
|
|
88
|
5 hours
|
|
89
|
4 hours and 30 minutes
|
|
90
|
4 hours
|
|
91
|
3 hours and 30 minutes
|
|
92
|
3 hours
|
|
93
|
2 hours and 40 minutes
|
|
94
|
2 hours and 15 minutes
|
|
95
|
2 hours
|
|
96
|
1 hour and 45 minutes
|
|
98
|
1 hour and 15 minutes
|
|
100
|
1 hour
|
|
102
|
45 minutes
|
|
104
|
35 minutes
|
|
106
|
30 minutes
|
|
108
|
20 minutes
|
|
110
|
15 minutes
|
|
112
|
10 minutes
|
|
114
|
8 minutes
|
|
115
|
7 minutes
|
Source: http://www.areaseg.com/acustica/
According to Miranda et al., the prevalence of hearing loss among workers in the metropolitan
region of Salvador was 45.9% in the population studied. The prevalence of hearing
loss induced by noise (PAIR), including bilateral and unilateral losses, was 35.7%.
When such hearing loss was broken down by occupation, the individual prevalence rates
were: 58.7% in publishing/graphic, 51.7% in mechanical, 45.9% in beverage-related,
42.3% in chemical/petrochemical, 35.8% in metallurgic, 33.5% in steel, 29.3% in transport,
28.0% in food, and 23.4% in textile industry. The prevalence of unilateral NIHL was
notably high in this study at 18% of workers evaluated. The magnitude of the prevalence
of hearing loss induced by noise reported in this study represents an extremely alarming
situation and points to the importance of the implementation of Hearing Conservation
Programs by employers in order to prevent this disease.
Conclusion
Work-related hearing loss, particularly NIHL, is a highly prevalent occupational disease
and a prominent occupational health hazard.
We can also conclude that exposure to high sound pressure levels can cause irreversible
hearing loss, which has serious consequences for individuals' health and quality of
life. Therefore, NIHL warrants increased concern and efforts to take such steps as
are within the individual's control to make the work environment suitable for human
occupation.
As NIHL is a preventable and predictable disease, preventive actions by professionals
may be able to change the incidence of hearing loss in noisy environments. Therefore,
occupational health professionals must emphasize the importance of corporate Hearing
Conservation Programs and audiometric management of workers.
