The first eletrocnic Journal of Otolaryngology in the world
ISSN: 1809-9777

E-ISSN: 1809-4864


Year: 2008  Vol. 12   Num. 2  - Abr/Jun - (7º)
Section: Original Article
Variation of Audiometric Thresholds Among Workers Subjected to Occupational Noise
Jozeane Dias dos Santos1, Maria Inês Dornelles da Costa Ferreira2
Key words:
noise-induced hearing loss; occupational noise; occupational health

Introduction: Workers who are exposed to noise may present variation in their audiometric levels, thus irreversibly damaging their auditory system. Objective: To analyze the variation of thresholds between initial and final audiometric tests for each average frequency; to compare thresholds and the variations of audiometric thresholds according to gender, age, work time, and occupation. Method: A retrospective, individual, comparative cohort study was carried through a database of 4,837 workers who had undergone at least two audiometric tests. Chi-square, Fisher, Mann-Whitney, Spearman, and Wilcoxon tests were used for statistical analysis. Results: The right ear was shown to be more likely to have auditory loss. Among males, there was prevalence of 3 - 6 KHz frequencies, while 0.5 - 2 KHz frequencies were more prevalent among females. Older workers with a longer work time were the most affected. The most affected functions were industry auxiliary, machine operator, washer, welder, general auxiliary, and brick layer. Conclusion: This research confirmed the importance of both development and application of the Auditory Preservation Program.


Since the Industrial Revolution, machine occupied a place of prominence in companies, before kept only by man. The technological development created bigger and noisier machines each time, which contributed substantially to aggravate the problem of the noise.

The noise is potentially capable of causing serious and irreversible injuries in the hearing apparatus. The gradual deterioration of the auditory sensitivity, which occurs in function of the exposure the sonorous intensities bigger than 85 dBNA during eight daily hours for long time are enough to cause damage to cochlear, leading to the noise-induced hearing loss (NIHL) in workers.

The present research aims at discovering whether the occupational noise has variation of the audiometric thresholds in submitted workers. With the present work, the objective is, primordially, to analyze the variation of the thresholds between the audiometries of reference and final, that is, until the moment of the accomplishment of the study for each average of frequencies; to compare the thresholds and the variations of the audiometric thresholds among gender, age, time of service and occupational function.

The relevance of this research is in searching the verification of the audiological health of the workers through a comparative analysis of audiometric data, which is of great interest for the speech therapy. The subject was chosen in order to get subsidies to assist the workers in the prevention of the noise-induced hearing loss, a pathology which is more and more present in workers exposed to occupational noises.

The exposure to noise associated in the work environment is a problem of occupational health which is more and more present in environments of professionals. With the results of this study, it is intended to alert for the necessity and importance of the implementation of programs of auditory conservation by companies, searching solutions in order to prevent and to monitor the possible auditory losses or its evolution, as well as the consequences that the NIHL causes.


The occupational exposure to noise can lead to the auditory loss, which generally occurs in a gradual way. The auditory loss is only perceived by the individual when it reaches the area of speech, that is, when he starts to relate difficulties to understand words in a conversation or to the telephone, for example.

The determination of the auditory threshold of each individual is supplied through the tonal audiometry, carried through by means of an instrument called audiometer, whose functioning will have to be submitted to the procedures of verification and periodic control. The acoustic gauging and the calibration will have to be carried through whenever the acoustic gauging indicates alteration obligatorily and once a year. The audiometric examination is executed by qualified professional, that is, doctor or speech therapist, as resolutions of the respective federal professional advice1.

The tonal audiometry provides the auditory thresholds, allowing the analysis of the degree of the auditory loss. Normal thresholds are considered equal or lesser values of 25 dB in all the frequencies. Values above 25dB indicate abnormality in the examination. When such values are between 26 dB and 40 dB, this means that the individual has a light auditory loss; between 41 dB and 55 dB, means moderate auditory loss; between 56 dB and 70 dB, means moderately severe auditory loss; between 71 dB and 90 dB, indicates severe auditory loss; above of this value, it means deep auditory loss 2.

The NIHL is an illness whose diagnosis is carried through by the labor doctor from a well defined characterization of the picture. It is a chronic illness and, at certain moment, is irreversible. It results of the aggression to the ciliary cells of the agency of Corti, which elapses of the exposure the noises whose sound pressure levels (NPS) are high3. Moreover, it is insidious and cumulative, getting worse throughout the time. It is generally neurossenssorial, generally bilateral auditory loss, not producing auditory loss bigger than 40 dB IN in low frequencies (0,5; 1 and 2 KHz) and than 75 dB IN t in high ferqeuncies (3 to 4; 6 to 8 KHz), developing in the 6 first frequencies of 3 KHz, particularly in 4 KHz, progressing later for frequencies 8; 2; 1; 0.5 and 0.25 KHz, which takes more time to be harmed. Its evolution is higher in the ten first years of exposure; as time goes by, the progression of the injury becomes slower, becoming stable when the exposure to the noise will be eliminated. The individual with auditory loss will be able to develop conscription and tinnitus. There is a possibility of a reduction of the intelligibility of speech, harming the verbal communication. The tracing of the audiogram is characterized by the auditory drop, which is the notch between the frequencies of 3 and 6 KHz 3.

Despite the existence of noise in the work environment, the NIHL is a preventable disease, which consists of the elimination or the control of risks or inadequate conditions of work.

Occupational Audiology

The occupational auditory evaluation is a task to multidiscipline that it involves the labor doctor, the speech therapist, the professionals of the security and labor medicine, to deal with the auditory health of the worker. The speech therapist participates, together with the team of security of work, on the elaboration of proposals and development of projects of administrative environmental improvements for the reduction of exposure 4. He/she also carries through the tonal audiometry, which is an examination forced by law.

In the clinical audiometry, the Airways (AW) are investigated in the frequencies between 0,25 and 8 KHz and Bone Way (BW) in the frequencies of 0,5 to 4 KHz, whereas, for occupational tests, the AW one in the frequencies between 0,5 and 8 is only searched KHz. The BW is only gotten if the AW is modified 5; 1. For the accomplishment of the occupational examination, it is necessary that the worker remains in auditory rest for a minimum period of 14 hours so that its auditory thresholds are reestablished 1.

The times of exposure to the noise levels do not have to exceed the tolerance limits, once they can cause gradual and irreversible hearing loss, leading to the NIHL. The bigger the tax of exposure to the noise, the greater damage will be able to occur for the hearing. The times of exposure to the noise levels do not have to exceed established limits 6.

Labor Medicine

Another professional involved in the labor health is the labor doctor. It develops the Program of Medical Control of Occupational Health - PMCOH, in which hiring, periodic, dismissal examinations are carried through, of return to work and exchange of position. The doctor is liable to the attention and analysis of the evolution of the audiometric tracings of workers. Such activity is executed through the comparison of the audiometry of reference, which is the first audiometry of the worker, with all the periodic audiometries that will be carried through.

The periodic examination aims at the precocious diagnosis of alterations in the health of the worker, induced for risk agents present in the work environment, still in reversible phase 7. The audiometric examination will have to be carried through, at least, at the moment of the admission, the six months after the admission, annually and in resignation 1. With the guarantee of the legislation, the possible auditory losses or its evolution can be evaluated better, considering themselves the consequences produced for the NIHL.

Consequently, the Program of Hearing Conservation (PHC), is a set of measures to be developed to minimize the risks, with the objective to prevent the installation or evolution of losses of hearing and, consequently, the NIHL, together with the Program of Prevention of Environmental Risks (PPRE), which it prevents the potential risks that the employees are exposed in its labor activity 1;8.

The PHC involves a team to multidiscipline composed for engineers, doctors, speech therapists, nurses, psychologists and the administrative sector. A PHC, to be effective, must contain mapping and control of the noise, auditory protection, indication of the areas of risks, education, training, auditory evaluation and availability of equipment of auditory protection.

The prevention of the damages of the auditory system implies to prevent the start of the auditory loss, while the auditory conservation will initiate since the first moment where the employee is exposed to the occupational agents of risk for the hearing. The prevention starts before this first exposure.

One of the prevention ways is the use of Individual Protection Equipment (IPE), which are objects used by a person against possible threatening risks of his/her health or security during the exercise of one certain activity 9. IPE can be constituted by some ways or devices in order to to protect its user against one or some simultaneous risks, preventing injuries or minimizing its severity. Therefore, the function of IPE is to neutralize or to attenuate a possible aggressive agent against the body of the worker who uses it. The company is obliged to supply to the employees gratuitously adequate IPE to the risk and in perfect state of conservation and functioning 9. The employer is liable, still, to guide and to train the employee on the adequate use and substitution of IPE when embezzled or damaged.

The employee, in turn, must use, keep and conserve IPE, to fulfill the determination of the employer in relation to its adequate use and to communicate to the employer any alteration that makes IPE improper for use.


The present research characterizes for a cohort retrospective, individual and comparative study. The searched factor is the workers submitted to noise, and the outcome is the possible variations of the audiometric thresholds.

The data of the research were collected from referring audiometric data to 6,515 employees of 213 companies of the city of Porto Alegre and metropolitan region, through already existing data base. Therefore, this research was characterized for being a population study. The data were supplied by a medical clinic that has been running for 33 years in the area of labor medicine in Porto Alegre. The employees who belong to the data base all supplied by the clinic were enclosed , totalizing 6,515 participants exposed to the superior sound pressure levels of 85dB NPS.

As exclusion criterion, the employees who only had the audiometry of reference, those that they had related not to use IPE or reported illnesses as acoustic trauma, skull trauma, diabetes and exposure the raised sound pressure levels of non-occupational origin were discharged from the study. With this, 1,651 employees were excluded from the study. Being thus, the present research was carried through with 4,837 participants. It is important to mention that the NIHL diagnosis was carried through by the labor doctor that considered clinical occupational anamneses, the physical examination and the complementary examinations.

The data are the auditory thresholds, the time of service, the occupational function, the age, the gender and the auditory loss.

The present research was analyzed by the Committee of Ethics, being approved with the protocol number 42/2007, on July 05, 2007.

The doctor in charge of the clinic of labor medicine signed a declaration allowing the use of the collected data, and the researcher signed the Term of Commitment for Use of Data, making commitment to the allegiance and the secrecy of these data.

The audiometries were carried through by the speech therapist of the clinic after occupational anamneses, and the inspection of the external auditory canal. The preliminary tonal audiometry was carried through by AW in the frequencies of 0.5 to 8 KHz. In alteration case, the VO also was carried through in the frequencies of 0,5 to 4 KHz. For the accomplishment of these procedures, the audiometer of the Vibrasom brand was used, model AVS - 500, duly calibrated, respecting the time of auditory rest of the worker.

For statistics analysis, the computational program SAS System for Windows (Statistical Analysis System), version 8.02.

To describe the profile of the population according to variables in study, tables of frequency of the categorical variable were through, as gender and occupational function, with values of absolute frequency (n), percentage (%), and descriptive statistics of the continuous variable (age, time of service and audiometric thresholds) with values of average, standard deviation (SD), values minimum and maximum, median and quartiles.

For the comparison of the categorical variable between groups, the Qui-Square test or, when necessary, the exact test of Fisher was used (presence of expected values lower than 5). To compare the numerical variable between the groups, the tests of Mann-Whitney and the coefficient of correlation of Spearman were used.

To study the evolution or variation of the audiometric thresholds between the initial and final measures, the delta or difference between the values was calculated and the test of Wilcoxon for related samples (Signed Rank test) used . After that, the values of the deltas of the thresholds were compared with the other variables through the tests above. The results of the audiometric variation were also classified in worsening and not-worsening and compared between the groups. The level of significance adopted for the statistical tests was of 5%, that is, P<0.05.


Concerning the characteristics of the studied population, it was observed that the average of age of the participants of this study was of 36.8 years, with standard deviation of 10.9 years; the minimum age found was of 16 years, and the maximum, of 69 years.

Amongst the 4,837 participants, 100 were less than 20 years (2%); 1.382 were between 20 and 29 years old(28.6%); 1.375, between 30 and 39 years old(28.4%); 1.302, between 40 and 49 years old(26.9%); 578, between 50 and 59 years old(12%);and 100 participants were 60 years older(2.1%).

In relation to the gender, 4,266 participants were of men (88.2%), and 571 were women (11.8%).

To make the analysis easier, the population was divided in groups of service time. Of the total of participants, 1,338 had less than 1 year of service (28%); 2.443 had between 1 and 5 years (50.4%); 442, between 6 and 10 years (9%); 161, between 11 and 15 years (3.3%); 312, between 16 and 20 years (6.4%); and 141 participants more than had 20 years of service time (2.9%). The average time of service of the workers was of 4.2 years, and the maximum was of 34 years.

In relation to the occupational function, it was opted to analyze those who had more than 50 employees. Therefore, the group consisted of 1,113 drivers (23%), 262 workers of general services (5.4%), 256 maids (5.3%), 206 employees of the production sector (4.3%), 161 blacksmithes (3.3%), 147 operators of machine (3%), 128 workers of operational functions (2.6%), 128 masons (2.6%), 104 employees of the maintenance sector (2.2%), 94 mechanics (2%), 91 plant assistant (2%), 90 carpenters (1.9%), 84 installers (1.7%), 82 sewers (1.7%), 80 workers of administrative functions (1.6%), 77 assistants (1.6%), 65 washers (1.3%), 59 electricians (1.2%), 57 wireworkers (1.2%), 56 workers of the assembly sector (1.2%) and 53 welders (1.1%), totalizing 3,393 employees.

The time between the audiometry of reference and the audiometry carried through until the moment of the study was also divided in groups. Of the total number of participants, 1,867 had less of one year (38.6%) between the initial and final examinations; 1.337 had between one year and one month and two years (27.6%); 551 had between two years and one month and three years (11.4%); 664 had between three years and one month and four years (13.8%); 204 had between four years and one month and five years (4.2%); e 214 had more than five years (4.4%) between the examinations.

Picture 1 shows the average variation of the audiological thresholds, considering the studied tritonal averages. For the group of 0,5 to 2 KHz in the RE, an average of 10.3 dB was obtained in the audiometry of reference and 10,5 dB in the final audiometry. In the same group, for the LE, average 9.8 dB was gotten in the audiometry of reference and 9.7 dB in the final audiometry. On the other hand, the group of 3 to 6 KHz in the RE, got 16.8 dB of average in the audiometry of reference and 17 dB in the final audiometry; in the same group in the LE, 17.4 dB were gotten in the audiometry of reference and 17.6 dB in the final audiometry. Consequently, bigger increase of the auditory thresholds for the audiometric group between 3 and 6 KHz is observed.

Picture 1 - variation of audiometric thresholds considering the averages of 0.5 to 2 KHz and of 3 to 6 KHz (average) from Right Ear (RE) and Left Ear (LE). n= 4.837. - n=number of employees (n=4837); RE - Right ear; LE - Left Ear.

Picture 2 presents the frequency of the participants whose audiometric thresholds had been analyzed of categorized form (above 25 dB), considering the average of 0,5 to 2 and 3 KHz 6 KHz for both ears. For the group of 0.5 to 2 KHz in RE, 3.8% of the participants had auditory loss in the audiometry of reference and 4.2% in the final audiometry. For the same group in LE, 4% of the participants had auditory loss in the audiometry of reference and 4.1% in the end. For the group of 3 the 6 KHz in the RE, of 17% of the participants had auditory loss in the audiometry of reference and 17.5% in the end. In the same group for LE, 18.7% of the participants had their examinations modified in the audiometry of reference and 19.8% in the final audiometry.

Picture 2 - frequency of patients whose audiometric thresholds were analysed in a classified way (>25dB), considering the averages of 0.5 to 2 KHz and of 3 to 6 KHz (average) from Right Ear (RE) and Left Ear (LE). n= 4.837. - n=number of employees (n=4837); RE - Right ear; LE - Left Ear.

Of the 4,837 workers, one evidenced that 23.5% already had entered the job with auditory loss; in the end of this study, 24.3% presented alteration of the auditory thresholds (1% of the employees).

Table 1 shows the variation of the audiometric thresholds between first and the last audiometry, considering the averages studied of 0,5 to 2 and 3 KHz to 6 KHz for both ears. In the RE, statistically significant difference for the group of 0.5 to 2 KHz is observed (P=0,006) and trend for the increase (P= 0,095) in the group of 3 to 6 KHz.

On the other hand, for the LE, significant difference was not verified statistically. Consequently, a trend of bigger auditory loss for the RE is evidenced.

Table 2 presents the comparison of significant numerical variables between genders. For women, the prevalence was for the group of 0.5 to 2 KHz. In the RE, in the audiometry of reference, one got P=0,032 and, in the end, P< 0,001. In the LE, only for the final audiometry, P=0,011 was found.

For men, the significance was evidenced for the group of 3 the 6 KHz for the examinations of reference and final ones and for both the ears.

Table 3 shows that there was significant correlation of the age with the audiometric thresholds, and the biggest thresholds were found in the participants with bigger age for the analyzed groups (0,5 to 2 KHz and 3 to 6 KHz). Also, it is possible to visualize the significant correlation for the time of service with the audiometric thresholds, mainly for the high frequencies (3 to 6 Khz).

Picture 3 shows the frequency of alteration of the audiometric thresholds, considering the totality of the participants in the occupational functions. Consequently, it is observed that the functions with bigger alteration of the audiometric thresholds were of assisting of plant (60.4%), machine operator (58.5%), washer (55.4%), welder (50.9%), maid (50.4%) and mason (50%).

Picture 3 - Frequency of worsening of audiometric thresholds when considering the total amount of participants in their occupations. n = 3.393. - n=number of employees


The average of age of the studied population is of 36.8 years, as the other studies consisting of young adult workers, with the age between 34 and of 40 years shows 10; 11; 12. Another data found in these studies are the prevalence of masculine sex 10; 11; 13; 14.

The NIHL is the reduction of the hearing due to long and habitual exposure the intense levels of occupational noise. It is characterized for neurossensorial auditory loss, generally bilateral, initiates with bigger loss in the frequency of 4 KHz, progressing for 3 and 6 KHz. Later, it advances for the other groups of frequencies - 8, 2, 0,5 and 0,25 KHz. The tracing of the audiogram is characterized by the acoustic drop, called notch between the frequencies of 3 KHz and 6 KHz 3.

In this study, which involved workers exposed to the noise from 85 dB NPS, a decrease in the auditory thresholds was observed in the average of the final audiometries, in the group of 3 to 6 KHz, of 17 dB for RE and 17.6 dB for the LE, as it shows Picture 1. On the other hand, in the analysis of frequency of the participants with the auditory thresholds above 25 dB, for the same group, we observe that 17.5% of the participants had auditory loss in the final audiometry in RE and 19.8% in the LE, as it is observed in Picture 2. The result found is in agreement with the other studies. Miranda et al 11 had found prevalence of NIHL in 36% (n=7,925) of the workers. Days et al 14 had evaluated 284 workers approximately, finding the related pathology in 63% of the population. Finally, Caldart at al 15, in the analysis of 184 workers of a textile industry, had evidenced prevalence 28.3% of compatible auditory loss with NIHL.

In this research, we find, initially, 23.5% of workers with auditory loss and 24,3% in the final examination. Thus, we evidence exactly that 0.8% of the workers had worsening of the thresholds, being monitored by the clinic of labor medicine, place of the collection of data.

In relation to the laterality of the auditory loss, an unexpected finding was of that the individuals exposed to the noise had a statistical significant difference of the value of the auditory threshold for the group of 0,5 to 2 KHz, and trend for the increase for the 3 to 6 KHz group in the RE, as Table 1 shows. Thus, future researches need to be carried through to clarify this question, since both ears of the individuals are exposed to the noise simultaneously. This study points to respect to the necessity to investigate referring aspects to the auditory processing in workers exposed to the noise, as well as conditions of exposure of the worker to the same noise or of the work environment. In another research, Days et al 14 also found greater auditory damage for the RE, without finding a determinative factor that justified this difference.

Another even more significant finding can be observed in Table 2. For the individuals of the masculine sex, there is a decrease of the thresholds for the group of frequency of 3 to 6 KHz, which was expected for workers exposed to the noise. However, for the feminine sex, significant predominance was evidenced for the group of 0,5 to 2 KHz, differing from the study by Cured et al 16.

In this study, the age and the time of service had been factors associated to the alteration in the auditory thresholds. The average time of service of the workers was of 4.2 years. Miranda at al 11 observed high rotation of workers, evidenced by the low averages of work time, indicating that the time of work in the company does not correspond satisfactorily to the time of exposure to the noise. In some cases, the age seemed to be one better pointer of the exposure time, in case the employee presented history of exposure to the occupational noise.

We verify that the time of exposure to the noise contributes for occurrence of auditory losses, in the same way that the increase of the age is a factor of forecast of the decline of the hearing. According to the performed bibliographical survey, such finding is corroborated by literature 10; 14; 12; 15; 17, with the prevalence of NIHL increasing to the measure that increase the age groups.

Another point to be considered is the existing narrow relation between age and time of service or exposure, once the worker who dedicates a long period of his/her life to the work in noisy environments will have, of course, his auditory acuity diminished. This can be related both with the exposure to the occupational noise, and with the incidence of the presbyacusis. Kwito 5 tells that presbyacusis it is the auditory reduction occurred with the age, affecting initially the high frequencies, also 8KHz. The audiogram does not present acoustic notch. In accordance with Almeida 18, the presbyacusis is one of the most frequent causes of auditory loss in adults, predominating in the acute frequencies, whose evolution acquires aspects of high gravity when preceded of a labor life of exposure to the noise. The present study did not verify the possible correlation between the time of service and the degree of the auditory loss, an aspect which is obviously dependent on the variations of sonorous pressure and the time of exposure to the noise.

The comparison of the prevalence of auditory loss between occupational functions can be observed in Picture 3. The functions that presented greater auditory loss had been of assisting of plant, operator of machine, washer, welder, maid and mason.

Miranda et al 2, in his research, found most of the auditory losses in workers of the production sectors, maintenance, services of support to the production, general quality control and services. On the other hand, on the study by Caldart et al 15, the suggestive auditory loss of NIHL was evidenced for the workers of the sectors of industrial engineering, wiring and weaving.


This study allowed the analysis of the variation of the auditory thresholds of 4837 workers when considering the audiometry of reference and the audiometry before the collection of data, defined in this study as final audiometry. It was evidenced that 24.3% presented occurrences of alteration of the auditory thresholds in the group of frequency of 3 to 6 KHz.

For the results, significant difference of the auditory thresholds between the evaluations was verified, predominating bigger harm for the RE. However, it was not found, in the bibliographical survey, any determinative factor that justified this difference.

In relation to gender, women had statistical significant difference in the auditory thresholds for the group of 0,5 to 2 KHz, whereas men for the indicative group of NIHL (3 the 6 KHz).

In this study, the relation between age and time of service or exposure was verified. The bigger the age and the time of exposure to the noise, the greater the auditory loss. This can be related with the occupational or same exposure with the presbyacusis.

The comparative degree between the functions with bigger auditory loss carried through in this research differs from the other consulted studies, not being possible to conclude whether the function has relation with the auditory loss.

The obtained results point with respect to the efficiency of the PHC used for the clinic of labor medicine, therefore only 0.8% had alteration in the auditory thresholds.

A program aimed at the health of the worker allows the auditory conservation, preventing future auditory damages and promoting the monitoring of possible losses or its evolution. Consequently, the companies will diminish the costs with indemnification actions on the part of the employee. It is important to stand out that the auditory conservation also depends on the worker. He must be aware of the risks to which he/she is exposed and use the IPEs.


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1. Speech therapist (undergraduate).
2. Doctor (Professor of Speech Therapy Major at Centro Universitário Metodista - IPA).

Centro Universitário IPA - Porto Alegre
Jozeane Dias dos Santos
R. Perpétua Telles, 36 - apartamento 201
Bairro Petrópolis - Porto Alegre/ RS - Cep: 90460-120
Fax: (51) 33788050 - Falar com Cristine - Email:

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