Disorders induced by direct occupational exposure to noise: Systematic review.

BACKGROUND: To review the available scientific literature about the effects on health by occupational exposure to noise.
MATERIALS AND METHODS: A systematic review of the retrieved scientific literature from the databases MEDLINE (via PubMed), ISI-Web of Knowledge (Institute for Scientific Information), Cochrane Library Plus, SCOPUS, and SciELO (collection of scientific journals) was conducted. The following terms were used as descriptors and were searched in free text: "Noise, Occupational," "Occupational Exposure," and "Occupational Disease." The following limits were considered: "Humans," "Adult (more than 18 years)," and "Comparative Studies."
RESULTS: A total of 281 references were retrieved, and after applying inclusion/exclusion criteria, 25 articles were selected. Of these selected articles, 19 studies provided information about hearing disturbance, four on cardiovascular disorders, one regarding respiratory alteration, and one on other disorders.
CONCLUSIONS: It can be interpreted that the exposure to noise causes alterations in humans with different relevant outcomes, and therefore appropriate security measures in the work environment must be employed to minimize such an exposure and thereby to reduce the number of associated disorders.

INTRODUCTION

Urbanization, the need for new branches of industry, and automation, mechanization, and computerization of work result in a new health problem for workers and change the character of occupational diseases and its development mechanism. The changing nature of work decreases significantly the number of “traditional” works and raises a new problem − the prevalence of work-related disorders.[1]

In this sense, noise, defined as undesirable sound, in one of the most common hazards at occupational and environmental level,[2] and sound level exceeding 85 dB is harmful to health; this effect is further increased depending on the duration and systematic exposure,[3] as well as the intensity and frequency of the sound,[4] the risk factors among the exposed population, the individual susceptibility, ethnicity,[5] sex,[6] and the exposure to other physical,[7] chemical,[8910] and biological[9] agents.

In this way, rapid expansion and the changes in the Asian economy have led to, among others, an increased number of workers exposed to high noise intensities.[11]

According to the National Board of Occupational Safety and Health, 25–30% of Finish workers are exposed to excessive noise, and this is supposed to be one of the greatest hazards at occupational level.[12] In USA, it is estimated that more than 30 millions of workers are exposed to hazardous noises in their work environment,[6] and of these, there are nine millions of Americans that are at a potential risk of developing Noise-Induced Hearing Loss (NIHL) because of noise levels higher than 85 dB(A),[13] which is the maximum limit of daily exposure according to the recommendations of the National Institute for Occupational Safety and Health (NIOSH), that is equivalent to approximately 3600 Pa2 s, in 8 work hours.[14] In addition, excessive noise is considered the main cause of occupational disease at the European level.[15]

Therefore, since 2009, NIHL is constituted as an occupational disease.[11] Consequently, knowing that the exposure to noise is harmful to health, the aim of this study is to review the scientific literature about the effects of exposure to a physical agent on the worker's health.

MATERIALS AND METHODS

By reviewing the scientific literature, a systematic and critical analysis of the retrieved papers was performed.

Literature search

All data were obtained by a direct consultation via Internet of the scientific literature contained in the following databases: MEDLINE (via PubMed), ISI-Web of Science (Institute for Scientific Information), Cochrane Library, SCOPUS, and SciELO bibliographic database (collection of scientific journals).

The thesaurus developed by the U.S. National Library of Medicine was employed for the recovery of papers. No subject qualifiers (subheading) were used, nor tags application was necessary. The following terms were considered adequate as descriptors as well as when searching the text format in the title and abstract: “Noise,” “Occupational,” “Occupational Exposure,” and “Occupational Disease.” The final search equation was developed for its use in the MEDLINE database, via PubMed, using the Booleans connectors: (“Noise, Occupational” [Mesh] OR “Noise, Occupational” [Title/Abstract]) AND (“Occupational Exposure” [Mesh] OR “Occupational Disease” [Mesh] OR “Occupational Exposure” [Title/Abstract] OR “Occupational Disease” [Title/Abstract]).

The following filters (limits) were used: “Humans,” “Adult (more than 18 years),” “Comparative Studies.” These filters were lately adapted for the databases mentioned above.

The search was performed from the first available date according to the characteristics of each database, until January 2015 (time of the last update).

Additionally, a second search was performed consulting the reference list of the identified articles to reduce the number of unrecovered papers by the review.

Study selection

The final selection of articles was done according to the following criteria:

The following were the inclusion criteria: observational studies, original articles published in peer-reviewed journals, and pertinent works with available complete text, which must be written in English, Portuguese, or Spanish [Figure 1].

Figure 1

Identification and selection of studies

The following were the exclusion criteria: articles that did not focus the intervention on the effects of noise occupational exposure on human's health (existing cause–effect relation between exposure and symptom/disorder), those that could mask the effect of noise (co-exposure to drugs or other chemical and/or physical products), and those studies that involved individuals under 18 years.

Data extraction

Two authors assessed the adequacy of the studies independently (D-P and S-V). To consider valid the process of selection, it was established that the assessment of the concordance between both the authors (Kappa index) must be higher than 0.60 (good or very good strength of concordance).[16] Whenever this condition was met, any discrepancies would be resolved by consulting the third author (W-B), and subsequently by consensus among all the authors.

Double entry tables were used to have control of the extracted data; this allowed the detection of errors and its correction by consulting again the original documents. Papers were collected according to study variables to systemize and facilitate the comprehension of the results; the following data were considered: first author and year of publication, study design, country where the study was conducted, target population, exposure period, time of exposure, and exposure effect.

Methodological quality assessment

The quality of the selected articles was discussed using as a support the guidelines for reporting observational studies STROBE (Strengthening the Reporting of Observational studies in Epidemiology),[17] containing a list of 22 essential checkpoints that should be described during the publication of these papers. Therefore, each item was scored according to whether it was collected in the articles or not as “1” or “0” information. In case the evaluation of one item was not necessary, then this one item was not recorded for the total of items (not applicable=NA). When several points composed an item, these were evaluated independently, giving the same weight for each one of them, and subsequently obtaining its average (being the final result for that item), so that in no case the score could exceed 1 point per item.

RESULTS

A total of 281 references were recovered when the described search criteria were applied; after purging the duplicated references and after the application of inclusion/exclusion criteria, it was possible to retrieve 23 studies with available full text[7111214151819202122232425262728293031323334353637] [Table 1], proceeding from MEDLINE (n=10; 40.00%), Cochrane (n = 1; 4.00%), ISI Web of Science (n=2; 8.00%), Scopus (n=10; 40.00%), and SciELO (n=2; 8.00%). The concordance between the evaluators about the pertinence of the articles was 100%. After the assessment of the quality of selected articles by STROBE questionnaire, the scores oscillated between 10.65 and 20.4 [Table 2].

Table 1

Description of the articles selected for review with the effects of exposure

Author, year	Design	Population: sex and age	Profession	Exposure	Period	Country	Exposure effects
Alonso-Díaz, 2014[36]	Cross-sectional	207 41 ± 8 years and 42 ± 6 years Men	Body shops and assembling	≥85 dB(A)	<10 years 10–20 years> 20 years	Spain	In one third of workers exposed audiometry were obtained compatible with hearing injuries noise
Flamme and Williams, 2013[14]	Cross-sectional	n=321 Men and women Age: ≥18 years. Predominantly between 30 and 60 years	Referees	Whistle (104–116 dB (A) from 90 to 95 s respectively)	Questionnaire: 1 day	Michigan (EEUU)	Whistle use contributes to approximately 36% of hearing lose in referees and to tinnitus appearance
Chung et al., 2012[11]	Repeated measures	N: 81 agriculture equipment factory N: 371 firemen Men	Agricultural equipment factory and firemen	Agricultural equipment factory and firemen: 82 dB (A) (66–97)Firemen: 76–79 dB(A)	Study duration: 4 years (2006–2009) Exposed: 8 h/5 days of week Not exposed: 9 h, 2 consecutive days, 15 evening hours of 2 consecutive days and 2 free days	South Korea	Agricultural equipment factory workers have higher level of hearing loss (P < 0.0001) in both ears, adjusted to age, work duration and yes/no smokers. These results are significant after 10 years of work history in stratified analysis
Kitcher et al., 2012[18]	Comparative cross-sectional	N: 140 stone crushers (42.58 ± 7.85 years) N: 150 healthy workers (42.19 ± 12 years) Men and women	Stone crusher	61.2–99.6 dB(A)	Exposure period: 0–30 years	Ghana	Suggestive noise-induced hearing lose in 21.5% of crushers
Jensen et al., 2009[19]	Case and control	42 team leaders (mean age: 47.8 years); 42 plane mechanics (45.8 years); 17 ex-team leaders Men	Mechanics	144 dB (peak) and 124 dB (L(eq)), during limited time. Highest sound pressure of 2–4 kHz levels	Mean of worked years: 19.6 years for team leaders and 24.1 years for mechanics	Denmark “intuit”	The ex-team leaders show a lower number of diseases related with hearing and a higher number of respiratory diseases (significant)
El Dib et al., 2008[20]	Cross-sectional	n: 82 sound technicians n: 95 controls Age: >19 years Men and women	Sound technicians	Noise (music sound)	Minimum 5 years of exposure Majority between 5 and 14.9 years)	Brazil	A higher number of diseases between sound technicians (26.8%) that controls (11.5%) statistically significant
Kaerlev et al., 2008[15]	Cohort	N: 5994 sailors N: 2740 fishermen 20–59 years Men and women	Sailors and fishermen	Not available	Duration results not contemplated Recruitment period: (1989–1998) Follow-up period: 1994–2003 Exposure period of <6 years to >12 years	Denmark	Workers in engine rooms of vessels had more frequent hearing problems No available data for the accumulative effects of a long period RR = 2.39 (IC 95% 1.74–3.26)
Korres et al., 2008[24]	Case and control	N exposed = 139 (86 men and 53 women) (mean age: 41.9 ± 9.0 years range: 24–54 years) N controls = 32 (18 men and 14 women) (mean age: 38.4 ± 6.9 years range: 25–53 years)	Food processing factory workers (Bakery)	92 dB(A). Steady continuous noise	Follow-up period: 2 years (2005–6) Mean exposure time: 11.8 ± 6.9 years (1–33 years)	Not available	Significant statistical correlation between pure tone threshold and exposure time to all frequencies between 250 and 20,000 Hz except in 10,000 Hz.are not sex-dependent
Vangelova and Deyanov, 2007[7]	Case and control	N: 545 n: 271 intense noise (age 44.7 ± 10.2 years) n: 159 heat (age 40.3 ± 10.6 years) n control: 115 (41.5 ± 9.2 years)Age: <32 years; 33–45 years; >46 years Men	Industry workers	Intense noise (86–92 dB(A)) and environmental heat (35.4°C (28.4–41.7)	<9 years 10–19 years> 20 years	Not available	AP: 140/90 mmHg is significantly higher in individuals of mean age exposed to noise (of mean age) in both groups (P < 0.05).Dyslipidemia levels were significantly higher in mean age individuals exposed to heat and in both exposed age groups (P < 0.05)
Shupak et al., 2007[23]	Controlled prospective cohort	N exposed = 42 N not exposed = 4 Men 18–20 years	Engine rooms workers	Exposed: 87–117 dB(A) (use ear protectors) Not exposed: <80 dB(A)	Follow-up period: 2 years	Israel	Significant elevation was shown in groups exposed to thresholdPTaud to 4000 Hz in both ears and to 6000 Hz in the left ear
Dias et al., 2006[21]	Cross-sectional	N: 284 20–72 years (media: 42.51 years) Men and women	Several professions	Not available	Recruitment: April–October 2003	Brazil	Increase of tinnitus with a progression of hearing damage, controlled by age and wave noise exposure
Dement et al., 2005[22]	Case and control	n = 3510 Audiometric test participants: Age: 56.6 years and mean work time: 12.2 years Men and women	Construction and shop workers (trade workers)	Co: <80 dB(A)	Recruitment: 1996–2003	EEUU	The risk of disability in construction workers was higher. Smokers and exposed to high noises OR 95% = 2.7 (2.0–3.6)
Rios and da Silva, 2005[37]	Case and control	4033–50 years Men	Unknown	≥85 dB(A)	40 h/week more than 8 years	Brazil	Mild and moderate noise-induced hearing loss (P < 0.01)
Gitanjali and Dhamodharan, 2004[35]	Retrospective cohort	8 participants in each group 24 controls (20–45 years) Men and women	Conductors of “autorickshaws”	Occupational noise >75 dB during 1 or 2 years. Occupational noise >75 dB during 5–10 years Occupational noise a >75 dB during >15 years	Study period: 2000–2001	India	Could be concluded that workers exposed to high levels of occupational noise increase the risk of low sleep quality But the adaptation to this effect probably takes some years
Leme, 2001[26]	Case and control	Ca: 61 employees of San Francisco Oliveira Public Health Service State Hospital Mean: 44.67 ± 9.45 years (29–62 years)Co: 30 Sex not available	Ca: San Francisco Oliveira Public Health Service State Hospital	Carpentry: mean 94.65 dBSawmill: mean 94.65 dBMechanics: mean 97.6 dBLaboratory: mean 85.5 dBFrequencies used: 250, 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz	Study from 1996 to 1997 Test was performed after 14 h of sound response	Not available	Significant differences in all the frequencies even for right (P < 0.005) or left ear (P < 0.02)
Tomei et al., 2000[25]	Case and control	N = 52 bedframe N = 65 light metal N = 64 office workers Men Mean age: 48.2 ± 7.2 years	Bedframe factory workers	>90 dB(A) (exposed)	Mean exposure period: 21.2 ± 6.7 years	Not available	Mean systolic and diastolic of bedframe workers is significantly higher that control groups Bedframe workers exposed to >90 dB present higher means of systolic and diastolic that those exposed to <90 dB
Lee, 1999[27]	Cross-sectional	43 exposed employees (mean age: 23.9 years) 37 controls Sex not available	Nightclub workers	>85 dB	Mean exposure time: 5.1 h (3.6–6.9 h)	Singapore	The exposed group had a higher significant prevalence of (41.9%) sensorineural hearing lose respecting to their control group (13.5%), also suffer more tinnitus (21% versus to 2.7%)
Casson et al., 1998[28]	Cross-sectional	Fishermen: 139 (45.39 ± 9.57 years) Control: 136 (40.75 ± 8.37 years) Men	Deep-sea fishermen	Not available	Not available	Italy	Deep-sea fishing RR = 3.64 (1.07–12.4), years working as fisher (RR = 1.60 (1.16–2.20) and the age (RR = 2.04 (1.47–2.83) have influence on the noise-induced hearing lose in a significant way. Also, the exposure to noise produce an increase in systolic pressure (RR = 190 (1.03–3.51)
Szczepanński and Otto, 1995[29]	Case and control	Not available	Vessels crew and engine room workers	93–102 dB(A)3000–4000 Hz	Study workers: 1975–1988	Poland	The higher increase of hearing threshold in the range of 5.0–5.8 dB was shown in engine rooms workers in different types of vessels
Hirai et al., 1991[30]	Cross-sectional	N: 2124 industry workers (age: 20–59 years) Exposed to 85–115 dB: 615 Exposed to <85 dB: 1141 Silent office: 368 men	Industry	Exposed to 85–115 dBExposed to <85 dB Silent office	10 years	Japan	The prevalence of hearing lose in exposed group to levels of 85–115 dB is of 16.5% higher that moderate group (7.5%) and than the silent one (2.8%) Significant
Tarter and Robins, 1990[31]	Cross-sectional	N: 150 White workers N: 119 Black workers of 35–65 years Men	Car factory	>85 dB(A), 8 h/day	≥5 exposure years	Not available	Hearing lose to 4000 Hz and worked years in jobs with high level of noise are significantly associated with the mean of arterial pressure and for hypertension between black workers but not for white ones
Thiery and Meyer-Bisch, 1988[32]	Cross-sectional	N: 234 Not exposed: exposed to 80 dB(A)Exposed: >95 dB(A) (3 groups divided by age) Sex not available	Sheet metal workshop in a car factory	95 dB(A)	<20 years	Not available	Significant hearing lose after 9 years of exposure in comparison with a quasi-steady exposure to the same extent Hearing lose higher at 6 kHz than a 4 kHz
Mäntysalo and Vuori, 1984[12]	Case and control	N:99 Group 1 exposed 3 and 4 years to impulse noise (age: 24.6 ± 2.83 years): 10 Group 2 exposed 5 and 6 years to impulse noise (28.3 ± 2.83 years): 10 Group 3 exposed 7–10 years (30.1 ± 2.23 years): 10 Exposed to continuous steady-state noise (28.3 ± 4.87 years): 12 Group control (23.8 ± 3.36 years): 10 Men Born in 1940 or later	Exposed to boost: Noise: electroplating workers and welder Exposed to continuous steady-state noise: Workers of cable factory	Environmental noise: 80–85 dBPeaks from 130 to 140 dB and inclusive 150 dB from 300 to 800 ms	Exposed to impulse noise: 3–10 years Exposed to continuous steady-state noise: 5–42 years	Finland	Most of the exposed to impulse noise used hearing protectors, practically none used it in steadyThose exposed to impulse noise (using or not ear protectors) had higher hearing lose than those exposed to steady Higher sensibility was shown with frequencies between 4000 and 6000 Hz
Sataloff et al., 1983[33]	Cross-sectional	N: 295 Sex and age not available	Different type of factories: wood, paper, “pulp” water company, steel, gas company, “smelting company,” foundry, electrical	99–118 dB(A)	1 to >20 years	Not available	An intermittent exposure to intense noise frequencies results in a very sever lose but is relatively lower or not the hearing lose at lower frequencies even after many years of exposure
Sulkowski et al., 1981[34]	Retrospective cross-sectional	14,81129–60 years Men and women	Majority miners and textile industry employees	≥90 dB(A)	Follow-up period: 1971–9	Poland	It was shown a noise-induced hearing lose (INHL) in all the professions Incidence: 16/100,000 employees The highest rates were for industry transporters, miners and textile industry

Table 2

Methodological quality of the studies by the 22 point assessment of the STROBE guide

Article [Reference]	Questionnaire items
	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	Total	%
Vangelova and Deyanov[7]	1	1	0.5	1	0.34	1	1	1	1	0	1	0.67	0.5	1	1	1	1	1	1	1	0	NA	17.00	80.95
Chung et al.[11]	1	1	0.5	1	1	0.5	1	1	1	0	1	1	0.92	1	1	1	1	1	0.5	1	0	NA	17.42	82.94
Mäntysalo and Vuori[12]	1	1	1	1	1	1	1	1	1	0	1	1	0.6	1	1	0.75	1	1	1	1	1	NA	20.35	92.50
Flamme and Williams[14]	1	1	0.5	1	1	0.8	1	1	0	0	1	0.67	0.5	1	1	1	1	1	1	1	1	NA	17.47	83.17
Kaerlev et al.[15]	1	1	0.5	1	1	1	1	1	0	0	1	0.67	1	1	1	1	1	1	1	1	0	1	18.17	82.58
Kitcher et al.[18]	1	1	1	1	0.5	0.8	1	1	0	0	1	0.34	4	1	1	0.75	1	1	0.5	1	0	NA	15.28	72.78
Jensen et al.[19]	1	1	1	1	1	1	1	1	0	0	1	0.34	0.4	1	1	0.75	1	1	1	1	1	NA	17.48	87.42
El Dib et al.[20]	1	1	1	1	0.67	1	1	1	1	1	1	0.67	0.8	1	1	0.75	1	1	1	1	1	NA	19.88	94.68
Dias et al.[21]	1	1	0.5	1	1	0.67	1	1	0	0	1	0.75	0.75	1	0	1	1	1	1	1	0	NA	15.67	74.60
Dement et al.[22]	1	1	0.5	1	1	1	1	1	1	0	1	0.67	1	1	0	1	1	1	1	1	1	1	19.17	87.12
Shupak et al.[23]	1	1	1	1	0.67	1	0.67	1	0	0	0.67	0.34	0.8	0.6	1	0.75	1	1	0.5	1	0	NA	14.98	71.35
Korres et al.[24]	1	1	0.5	1	1	1	1	1	0	0	1	0.67	0.8	1	NA	0.88	1	1	0.5	1	1	NA	16.34	81.71
Tomei et al.[25]	1	1	1	1	0.34	1	1	1	1	0	1	0.67	0.4	1	1	0.88	1	1	1	1	1	NA	18.28	87.02
Leme[26]	1	1	0.5	1	0.34	1	1	1	1	0	1	0.34	0.4	1	1	0.75	0	1	0	0.5	0	NA	14.32	68.17
Lee[27]	1	0	1	1	0.5	1	1	1	0	0	1	0.5	0.8	1	1	1	1	1	1	1	1	NA	16.80	80.00
Casson et al.[28]	1	1	1	1	0.5	0.67	1	1	0	0	1	0.67	0.4	1	NA	1	1	1	0.5	1	0	1	15.73	74.92
Szczepański and Otto[29]	1	1	1	1	0.5	1	0.34	1	0	0	1	0.17	0.6	0.8	NA	0.25	0	1	0	0	0	NA	10.65	53.25
Hirai et al.[30]	1	1	1	1	0.5	1	1	1	1	0	1	0.67	0.8	1	NA	0.9	1	1	0.5	0.75	1	NA	18.12	86.27
Tarter and Robins[31]	1	1	1	1	0.5	0.75	1	1	1	0	1	1	0.92	0.5	1	1	1	1	1	1	1	NA	18.67	88.89
Thiery and Meyer-Bisch[32]	1	1	1	1	0.5	1	1	1	1	0	1	1	0.6	1	NA	1	1	1	0.5	1	1	NA	17.60	88.00
Sataloff et al.[33]	0.5	1	0.5	1	0.5	1	1	1	0	0	1	0.25	0.8	0.8	NA	0.88	1	1	0	0.75	1	NA	13.98	66.55
Sulkowski et al.[34]	1	1	1	1	1	0.34	1	0.5	0	0	0.67	0	0.4	0.75	NA	0.75	0	1	0	0.75	1	1	13.15	62.62
Gitanjali and Dhamodharan[35]	1	1	1	1	1	1	1	1	1	0	1	0.5	1	1	1	0.75	1	1	1	0.34	1	1	19.58	89.02
Alonso-Díaz[36]	1	1	0.5	1	0.67	0.67	1	1	1	0	0.67	0	0.6	0.75	NA	0.88	1	1	0.5	0.75	1	NA	13.98	69.88
Rios and da Silva[37]	1	1	1	1	0.33	0.8	1	1	1	0	0.67	0.67	0.4	1	1	0.75	1	1	0.5	1	0	NA	15.12	71.98

Most of the reviewed papers included case and control studies (11; 44.00%)[712192224252627282937] and cross-sectional studies (10; 40.13%),[14171921283031323436] although they also included three of a cohort design (12.00%).[152335] The study by Gitanjali and Dhamodharan[35] had the smallest population size, with 24 participants, contrary to the study by Sulkowski et al.,[34] in which there was a cohort of 14,811 individuals. In 11 of 24 studies,[711121923252830313637] the target population was exclusively formed by men; nine studies considered both sexes,[1415182021222435] and in five studies, the sex of the participants was not specified.[2627293233] Individuals’ age in the selected studies was more heterogeneous and varied from 19 to 65 years. The country of origin for the articles was Brazil[202137] in three studies, and in two studies, it was United States of America,[1422] Denmark,[1519] and Poland.[2934] An article was retrieved from each of the following countries: Japan,[30] India,[35] Ghana,[18] Israel,[23] South Korea,[11] Singapore,[27] Italy,[28] Spain,[36] and Finland.[12] Some studies did not make explicit the country of origin.[7242526313233] Sulkowski et al.[34] published the earliest work in 1981, and the most current was published by Alonso-Díaz[36] in 2014. The current relevance/obsolescence of the selected articles was measured by a median of 10 years.

Most professions that were related to noise exposure were from the industrial field,[7111218192425303132333436] followed by those related to the sea (sailors, fishermen and shipyard workers[1523282934]). The rest of the studies only presented one of the following occupations: referee,[14] sound technicians,[20] firemen,[11] construction,[22] trade,[22] hospital professionals,[26] nightclub employees,[27] and drivers.[35] However, Sulkowski et al.,[34] Dement et al.,[22] and Chung et al.[11] indicated two different occupations and Dias et al.[21] compiled numerous jobs that were related to noise exposure. Rios and da Silva[37] did not explain exactly the occupation of the study population.

Kitcher et al.[18] collected data for the higher time of exposure as 30 years.

Noise exposure was from 61.2 dB(A)[17] with peaks reaching 150 dB(A).[12]

Noise-induced hearing disorders by occupational exposure

Most of the works collected significant information related to the noise-induced hearing disorders. Therefore, two large blocks were distinguished: on one side, those that showed NHIL,[1112141518192021232426272829303233343637] and on the other side, tinnitus was observed.[14182127] Most of the studies that were related with NIHL collected individuals exposed to industry noise (employees in factory,[253033] car factory,[3132] agricultural equipment factory,[11] electroplating and welding fields,[12] stone crushers,[17] textile industry, mines,[34] mechanical industry,[19] food processing factory,[24] body shops, and assembling units[36]), followed by sea workers (sailors, fishermen, and shipyard workers[15232829]), although referees,[14] sound technicians,[20] firemen,[11] shop employees,[22] nightclub employees,[27] hospital professionals,[26] and those from the construction industry[22] were also observed. Tinnitus-related professions are the following: referees,[14] factory workers (stone grinders[18]), and nightclub employees.[27]

During all occasions when tinnitus was presented, concomitant NIHL was observed, although the level of noise exposure was different in these studies as seen in Table 1.

Cardiovascular disorders induced by occupational exposure to noise

Cardiovascular disorders were observed in four[7252831] out of the 23 selected studies. As in the previous case, most of these studies (3; 75%)[72531] focused on factory workers. The fourth study focused on sea workers,[28] that is deep-sea fishermen.

Respiratory disorders induced by occupational exposure to noise

Only one study, about factory workers, and mechanics, explained the association between occupational noise and respiratory disorders.[18]

Other disorders induced by noise

Gitanjali and Dhamodharan[35] demonstrated the association between noise and autorickshaw drivers who were exposed to high level of noise (who did not wear ear protectors). This exposure led to sleep quality disorders.

DISCUSSION

In this review, it has been confirmed that hearing disorders are very common occupational diseases and, as it was declared previously, it is one of the main causes of occupational diseases.[15]

As it was demonstrated, noise-induced hearing disorder by occupational exposure was the most prevalent disorder. In fact, it is estimated that 25% of the active population that is exposed to noise has some degree of Noise-Induced Hearing Lose (NIHL). In United Kingdom, NIHL is the second most frequent occupational disease (according to data from 1990 and 1991[20]). In Canada there are a large number of workers who are exposed to the limit levels of 85 dB during the workday of 8 h.[38] Moreover, according to the World Health Organization (WHO), there are approximately 16% of individuals that suffer of hearing loss caused by noise labor exposure.[39] On the other hand, tinnitus affects about 17% of the world population, including 33% of the elderly.[40]

Anyway, it must be considered that individual susceptibility to noise along with the hearing loss grade varies greatly among individuals, which means that with the same noise exposure, some individuals develop substantial hearing losses, while others do not develop it or develop it minimally.[9]

In addition, other factors, such as age, also seem to influence. A Croatian study from 1996 demonstrated correlation between age, exposure, and the mean of hearing loss at 4000 Hz.[15] In the study of Flamme and Williams[14] age had a significant effect (P< 0.04). In this study, an ad hoc test was performed revealing that participation between 60 and 70 years tended to have worse outcomes than participants under 29 years, after controlling by hearing sensibility. Animal studies had shown that short intervals of high noise exposure produce less temporal and permanent hearing lose and less damage at cochlear level than a continued exposure to the same energy and duration.[41] It was observed that peaks around 120–135 dB induced mechanical damage in the hearing mechanisms, while a chronic exposure to lower noise levels produced adverse effects by metabolic disorders.[42] At the same time, it was demonstrated that when the exposure to harmful levels of noise was interrupted, the significant progression of NIHL was detained.[2] In this sense, the study of the workday distribution is an especially important issue for some authors, particularly about the duration and frequency of the nonexposure periods between work turns.[11]

Individual characteristics, such as race, have been documented in the literature, and it is suggested that the hearing threshold levels appear to be worse in White than in Black.[13] Nevertheless, this item was not observed in the reviewed articles.

In relation to the cardiovascular disorders induced by occupational exposure to noise, Jensen et al.[19] demonstrated the association between noise occupational exposure and hypertension, the increased relative risk of myocardial stroke, and ischemic heart disease. A meta-analysis[7] of 43 papers published between 1980 and 1990 concluded that for each 5 dB(A) that increased in the workplace, the systolic blood pressure increased to 0.51 mmHg and hypertension development increased as 14%. These results are in line with those reached in this review. In fact, another reviewed study showed an increase of arterial pressure, total cholesterol, and/or triglycerides in factory workers who were exposed to noise in comparison to those that were not. In addition, Cortés-Barragán et al.[43] agreed with our results regarding the cardiovascular disease induced by noise. In their review, they found that noise caused increase in blood pressure, enhanced cardiovascular disease, myocardial stroke, and heart rate. Besides, they also showed biochemical and lipid alterations.

When the noise-induced respiratory disorders were studied, as was commented previously, it was only possible to select one paper,[19] and other studies had to be excluded because of a possible masking effect of some chemical agents as monoxide of carbon, xylene, hydrogen, or arsenic, tuberculosis treatments,[8944] and other physical agents, such as pressure,[45] which could influence to ototoxicity. A study that was not included in this review demonstrated the relation between vibroacoustic diseases, among others, and changes in respiratory and gastric epithelium, endocrine disorders, epilepsy of late onset, and autoimmune disease.[4]

It could be commented the Gitanjali and Dhamodharan study[35] in relation to other noise-induced disorders, being this study in line with the association between high night and low sleep quality. This disorder could be caused by factors related to occupational stress, and this situation, which is related to another study,[15] perceives noise as a potential stressor. This was previously shown by Castells-Murillo,[46] and his study also described discomfort, interference with communication, loss of attention, concentration, and performance, and stress. In addition, the relation between noise and irritability[6] and psychiatric disorders was demonstrated.[38] Some studies, as the review by Maqueda Blasco et al.,[47] showed neuropsychological effects. Furthermore, they demonstrated an association between noise and other kinds of alterations like the effect of noise in some hormones and electrolytes and the increase in occupational injuries. In this way, Dias and Cordeiro[48] showed that the calculated attributable fraction between noise and work accidents was 30%, and Barreto et al.[49] demonstrated that the risk of fatal injury related to work increased with the intensity of exposure to noise (P=0.004). Cordeiro et al.[50] associated the relative risk for work-related injuries induced by noise to be about 5.0 (P < 0.001).

The following are the possible limitations of this review: the designs of the reviewed study cohorts and case–controls provide an evidence level and a recommendation grade of IIb and III according to the US Agency for Health Research and Quality, and in consequence to the reached conclusions and more, for the applicability of the interventions that are supported in the observational studies, are insubstantial. However, the theme of study must be considered as noise exposure, and in consequence assume that probably it is not possible to aspire to high-level designs and recommendation grade.[51] Although the systematic reviews must be based on monitoring studies and designs that guarantee the higher scientific rigor, in this analysis, all studies that focused on the studied theme were included. Although the real limitations of this study are due to the own ones of each study per se. In the case of Chung et al.,[11] the groups of comparison (farm hands and firemen) were significantly different (P < 0.0001), and furthermore, farm hands had longer duration of work (P < 0.001). The noise levels were not collected for firemen, but data that suggested exposure levels were collected. In addition, Kaerlev et al.[15] did not consider data for individual level. According to some authors, it is difficult to quantify the noise occupational exposure,[30] as in the study by Jensen et al.,[19] wherein they did not perform measures during the time of study and neither during the recovery time, and only data from previous years were used. Although this was not so in the study of Lee[27] that measured the noise exposure by microphones that were worn by the workers of nightclubs. In three studies,[222831] the differences were shown between comparison groups, and it is difficult to have adequate controls for noisy industry workers because there are other involving factors as smell, high temperatures, or high humidity.[30] In the study of Flamme and Williams,[14] it is important to highlight that referees have a second occupation, and so it is difficult to control their exposure in their main occupation and the ludic activities complicate research in this population.

Working conditions in vessels produce high level of stress in the adaptive body mechanisms of compensation and contribute strongly to the sailor's type and number of morbidity.[29] Even though the vessels are large and machine rooms are isolated, in some jobs as in engine rooms, at present it is impossible to reduce the level of noise below 85 dB, and they should be provided hearing protectors.[15]

It is important to recognize that several studies were found that had intrinsic characteristics that could not be included in this review, which demonstrated an association between noise and psychomotor performance, intellectual attention, and memory; propensity to suffer occupational accidents, as well as gender differences in perceiving noise as a potential stressor, is higher in women.[6]

From all previously mentioned observations and in line with other authors,[4452] it could be concluded that the training for hearing conservations should be held at the beginning in a new job or position. Hearing lose in construction workers could be prevented through a combination of silencing equipments and a hearing exhaustive conservation, including regular audiometric tests, effective hearing protective training, and encouraging workers to use of hearing protectors.[53] Additionally, the use of hearing protectors in noisy areas is a significant predictor to decrease the systolic and diastolic pressures.[7] Furthermore, if it is considered that a worker remains a mean of third of his life in his workplace, this could be interpreted that noise exposure produces disorders of different levels of relevance and therefore appropriate security measures should be considered to minimize such exposure and thus reduce the number of associated disorders.

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Conflicts of interest

There are no conflicts of interest.