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ARTICLES Table of Contents   
Year : 2002  |  Volume : 4  |  Issue : 14  |  Page : 73-87
Suggested guidelines for studying the combined effects of occupational exposure to noise and chemicals on hearing

1 National Institute for Occupational Safety and Health, Cincinnati, OH 45226, USA
2 National Institute for Occupational Safety and Health, Cincinnati, OH 45226; US Army Medical Department Student Detachment, FT Sam Houston, TX 78234, USA

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  Abstract 

The present document, which describes recommended standardized procedures, aims to assist individual investigators plan a study on the effects of industrial chemicals on the auditory system, collect and analyze environmental and hearing sensitivity data that are accurate and comparable to data acquired by others. This draft document is currently being reviewed by the NoiseChem Research Group. In this peer review stage we are currently accepting critiques and suggestions to this proposal.
Investigations on the aforementioned topic are necessary since there is strong evidence that occupational hearing loss may be caused not only by noise but also by exposure to certain chemicals in the work environment. Since some industrial chemicals are known to be ototoxic, it is plausible to expect that if these chemicals occurred in high enough concentrations in the workplace they could affect hearing. Laboratory studies have yielded a finding not expected, namely that when simultaneous exposure to noise and chemicals occur, the hearing loss observed was greater than the expected hearing loss from noise added to the expected hearing loss from the chemical. If this synergism is verified in humans, then changes will be required in the limits that are set for occupational hazards in order to prevent occupational hearing loss.

How to cite this article:
Morata TC, Little MB. Suggested guidelines for studying the combined effects of occupational exposure to noise and chemicals on hearing. Noise Health 2002;4:73-87

How to cite this URL:
Morata TC, Little MB. Suggested guidelines for studying the combined effects of occupational exposure to noise and chemicals on hearing. Noise Health [serial online] 2002 [cited 2020 Oct 28];4:73-87. Available from: https://www.noiseandhealth.org/text.asp?2002/4/14/73/31807

  Study Objectives Top


The main objective of this document is to assist in the design of field studies that aim: (1) To determine whether exposure to selected chemicals at levels commonly found in industry affects workers' hearing.

(2) To determine whether these chemicals interact with noise in a way that the resulting hearing losses are not equivalent to the expected hearing loss from the noise exposure added to the expected hearing loss from the chemical exposure.

(3) To investigate adequate means to assess and prevent hearing disorders that may occur due to the exposures to be studied.

The identification of the mechanisms underlying the lesions shall not be part of this effort since it can be better addressed by laboratory animal research.


  Literature Review/Background Top


The combined effect of noise and chemicals on hearing

The approach to isolate or vary a single parameter to determine an effect is often utilized in the investigation of occupational hazards and results in hazards being studied as if they occurred in isolation in the work place. In fact, most work environments are complex, consisting of a myriad of physical and chemical agents that are potentially hazardous to health. The results of studies of isolated work place hazards often are used to develop occupational safety criteria that may not be adequate for protecting workers in environments where simultaneous or sequential exposures to a variety of agents occur.

Over the past two decades, research on the effects that simultaneous exposure to noise and chemicals might have on the auditory system has increased significantly (Barregard and Axelsson, 1984; Bergstrom and Nystrom, 1986; Johnson et al., 1988; Morata, 1989; Lataye et al, 1997, 2000; Teixeira et al, 2002; Morioka et al., 2000; Sliwiflska-Kowalska et al., 2001, Morata, 2002).

In a 20-year longitudinal study of hearing sensitivity in 319 employees from different sectors of industry, a remarkably large proportion of the workers in the chemical sector showed pronounced hearing loss (23%) as compared to groups from non-chemical environments (5-8%) (Bergstrom and Nystrom, 1986). This effect was found despite the lower noise levels in the chemical sector (80-90 dBA) when compared to other divisions (95-100 dBA). The authors Bergstrom and Nystrom (1986) were among the first to suggest that exposure to industrial solvents (not identified in the article) could be implicated as an additional causative factor for those hearing losses.

Clinical studies have suggested that exposure to certain industrial chemicals can have retrocochlear effects. The hearing and balance of workers exposed to mixtures of aliphatic and aromatic solvents for periods ranging from 9 to 40 years have been examined (Odkvist et al, 1987). Their scores on speech audiometry were lower than would be predicted by the person's pure tone audiogram (38-64% incidence of abnormality) and their cortical responses to frequency glides were abnormal (50-64% incidence of abnormality); however no effects were observed on the auditory brainstem responses (0-9% incidence of abnormality). Therefore, it was concluded that the auditory system might be vulnerable at cortical levels, as indicated by the speech discrimination test and the cortical responses, two of the most sensitive tests available today in detecting central auditory lesions at the cortical levels.

A study of 190 workers was carried out with rotogravure printing workers where the hearing and balance functions of groups of printers exposed simultaneously to noise and toluene were compared with a group of printers exposed to noise alone, a group exposed to a solvent mixture and a group neither exposed to noise nor toluene (Morata et al., 1993). The adjusted relative risk estimates for hearing loss were 4 times greater (95% C.I., 1.4 to 12.2) for the noise group; 11 times greater (95% C.I., 4.1 to 28.9) for the noise and toluene group; and 5 times greater (95% C.I., 1.4 to 17.5) for the solvents group. The acoustic reflex measurements suggested that the hearing losses found in the group exposed to both agents might be due to lesions beyond the peripheral auditory system.

Another study on printing workers (n=124) failed to detect an interaction of toluene and noise, other than addictive (Morata et al., 1997). Forty-nine percent of the workers had hearing loss. From the numerous variables that were analyzed for their contribution to the development of hearing loss, age and hippuric acid (the biologic marker for toluene in urine) were the only variables found to be associated with the outcome. The odds ratio estimates for hearing loss were 1.07 times greater for each increment of 1 year of age [95% CI 1.03-1.11) and 1.76 times greater for each gram of hippuric acid per gram of creatinine (95% CI 1.00-2.98). The findings suggest that exposure to toluene has a toxic effect on the auditory system. Since the workers who participated in the study had relatively short noise exposure time, it is likely that they have not been exposed long enough to allow for the noise effects to be detectable.

The relationship between self-assessed hearing disorders and occupational exposure to solvent mixtures was investigated in a cross-sectional design with 3284 men (Jacobsen et al., 1993). Exposure to solvents for 5 years or more resulted in an adjusted relative risk for hearing impairment of 1.4 in men without occupational exposure to noise. A sub-sample of 51 men was examined with pure-tone audiometry and 20 of the 21 men who reported abnormal hearing also were found to have a hearing impairment. Occupational exposure to noise had an effect twice that of solvents; and in the case of combined exposures, the effects from noise dominated.

A more recent study conducted in Poland examined 517 subjects, who were divided into three groups of unexposed workers, workers exposed to organic solvents only, and workers exposed to both organic solvents and noise (Sliwinska-Kowalska et al., 2001). Hearing thresholds were significantly poorer in a wide range of frequencies (1-8 kHz) for both groups exposed to solvents, when compared with the reference group. The mean hearing thresholds at frequencies of 2-4 kHz were poorer for workers exposed to solvents plus noise than for the solvent-only group; this finding suggested an additional effect for noise. The results indicate that occupational organic solvent exposure at moderate concentrations increases the risk of hearing loss, and the ototoxic effects should be considered when the health effects of exposed workers are monitored.

Selection of chemicals to be prioritized

A literature search will indicate that the chemicals that can be considered as ototoxicants as numerous (for a literature review of the effects of chemicals on hearing, see Rybak, 1992). For the purpose of elaborating this protocol, these chemicals were divided into priority lists. The placement of a chemical in the high priority category took into consideration available evidence of ototoxicity, severity of the problem, accessibility and number of occupationally exposed workers. The following is the priority ranking of the elements discussed.

HIGH PRIORITY OTOTOXINS

Toluene

Xylenes

Styrene

n-Hexane

Mixtures containing the above

Trichloroethylene

Lead and derivates

Carbon Monoxide

Cyanide

ADDITIONAL OTOTOXINS

Mercury and derivates

Stoddard Solvent

Arsenic

Carbon Disulfide

Benzene

Manganese

The present protocol focuses on the study of the high priority list chemicals, although some of the recommendations such as hearing assessment procedures, noise assessment and data analysis strategies can be used in studies of other agents.

Magnitude of the problem

Two of the most common hazards that occur simultaneously in many work environments are noise and chemicals. It has been estimated by NIOSH that 30 million people are estimated to work in potentially hazardous noise levels, an estimates that includes workers in manufacturing, construction, agriculture (Franks, Stephenson and Merry, 1996). Approximately 10 million workers are exposed to solvents in the manufacturing sector where frequently noise is also a potential exposure (Morata, Dunn and Sieber, 1994).

At least one million workers in manufacturing have sustained job-related hearing impairment (defined as greater than a 25-dB average threshold hearing level at 1,2 & 3 kHz), and about half a million of these have moderate to severe hearing impairment (defined as greater than or equal to 40-dB average threshold hearing level at 1, 2 & 3 kHz (OSHA, 1981). The cost of workers compensation claims for hearing losses thought to result from occupational noise exposure for the period of 1977-1987 has been estimated at 800 million dollars (Ginnold, 1979).

Regulatory Background

In 1983, the Occupational Safety and Health Administration (OSHA, 1983) promulgated the Hearing Conservation Amendment to the Occupational Noise Standard of 1971, which specified the components of a hearing conservation program. It requires that such a program be started if workers have an exposure of 85 dBA TWA or greater. A hearing conservation program must include an assessment of noise exposure, audiometric tests of exposed workers, noise abatement and/or administrative controls, maintenance of records on noise and hearing data, availability of hearing protectors and employment training and education.

In the United States as in many other countries, the only agent that is considered to be ototraumatic is high intensity noise. Therefore, there may be a large number of workers with unmet needs concerning hearing conservation. Currently, there are no regulations requiring monitoring of a worker's hearing due to occupational exposure to potentially ototoxic chemicals, but agencies as NIOSH and ACGIH have recommended that chemical exposure be taken into consideration when planning for hearing loss prevention measures (Franks, Stephenson and Merry, 1996; ACGIH, 2000).

Hitherto, despite the large number of workers exposed to these chemicals in the presence of background noise, few will be required to have regular hearing tests because the noise exposure may not exceed the regulatory guidelines. In addition, controlling chemical exposures is not seen as a necessary preventive measure, and it is not regarded for hearing conservation. Since there is strong evidence that exposure to these chemicals alone or in combination with noise can produce a hearing loss, it is very possible that current hearing conservation practices are not adequate for this population of workers. Some of the other issues these findings raise include the adequacy of pure tone audiometry testing in screening solvent-exposed workers, the appropriateness of the current threshold limits when certain hazards occur simultaneously in the work place, and finally, the role of hearing assessment as applied to the early identification of those most susceptible to neurotoxic disorders.


  Research Methods Top


Study Design

Occupational hearing losses have a gradual onset and require neither reporting nor hospitalization, which makes it difficult to obtain incidence rates. However, prevalence rates are easier to obtain. Since there is neither recovery nor death from hearing loss, its prevalence rate is less biased than prevalence measures for other chronic conditions such as diabetes or heart disease. So, because of the nature of occupational hearing loss, a cross-sectional design is suggested, following the recommendations of several researchers (Elliot, 1978; Erdreich and Erdreich, 1982; Morata and Lemasters, 1995).

Populations to be studied

Groups of workers with various degrees of chemical exposure should be studied, in comparison with workers not exposed to chemicals. Noise exposure should be accounted for.

Sample size calculations were performed using data from Morata et al., 1993. The results of power calculations based on Hsieh (1989) are shown in [Table - 1]. Morata et al. used logistic regression analysis and found significant exposure effects with 190 subjects. However, in order to get adequate power (.80) in all the exposure groups it is recommended that at least 280 subjects be used in a study of the mentioned topic.

Methods for Estimating Noise Exposure

Area surveys should be performed to estimate noise exposure. In an area survey, one measures environmental noise levels, using a sound level meter to identify work areas where workers' exposures are above or below hazardous levels, and where more through exposure monitoring may be needed. The result may be plotted in the form of a "noise map," showing noise level measurements for the different areas of the workplace. Dosimetry involves the use of body­worn instruments (dosimeters) to monitor an employee's noise exposure over the work-shift. Monitoring results for one employee can also represent the exposures of other workers in the area whose noise exposures are similar. It may also be possible to use task-based exposure methods to represent the exposures of other workers in different areas whose exposures result from having performed the same task (Franks, Stephenson and Merry, 1996).

When available, retrospective noise measurements should also be used to characterize noise exposure. For detailed procedures on noise measurements see Berger, et al., 2000.

Methods for Estimating Chemical Exposure

Until the chemicals to be studied are identified, the specific methods cannot be determined. However, for the case of the chemicals classified as High Priority, air sampling of the chemical should be conducted and, when possible, some form of biological monitoring.

Whenever reliable retrospective exposure records exist, they should be used for estimating past exposure. However, any attempt to quantify the worker's exposure will give only an estimate or approximation of the actual total work exposure.

Air-Sampling Methods for the Selected Chemicals

Personal air sampling is the preferred method of evaluating individuals' exposure to air contaminants (for detailed description see Plog, 1988). As with noise, exposure to chemicals should be determined with task-based measurements. It involves the collection of an air-sample by a small device worn by the subject. The sampling device is positioned as close as possible to the subject's breathing zone so data collected closely approximate the concentration inhaled. Active monitoring, with the use of air sampling pumps, should also will be considered.

Analytical procedures specify the collection media, sample volume, and chemical analysis. They can be found, for an extensive number of compounds, in the NIOSH Manual of Analytical Methods (1994). The first stage should be a direct-reading qualitative exposure assessment to ascertain all potential exposures (chemical phases) and ranges. The second stage involves quantitative exposure assessment of specific entities identified in the first stage. Since it is possible for dermal exposure to aromatics to yield higher doses than air samples, patch samplers should also be considered.

Biological Monitoring

Biological Monitoring involves the measurement of changes in the composition of blood fluid, tissues, or expired air to determine absorption of a potentially hazardous material. Biological monitoring should not be performed as a replacement of personal monitoring but may be used to complement it (Plog, 1988). [Table - 2] summarizes the biological monitoring tests for selected substances (for detailed procedures, see ACGIH, 2000-2001, Lauwerys and Hoet, 1993).

Interviews

An important element of most epidemiologic studies that investigate occupational illnesses is the development of a quality survey instrument. Questionnaire development information can be found in Morata and Lemasters, 1995.

Subjects should be interviewed with regard to health history, work history, chemical and noise exposure, including previous and non­occupational. A complete questionnaire should include demographic data, health history (focusing on hearing) and a work history that includes a job and work environment description and exposure to hazards history. A suggested protocol can be found in Appendix A. Major portions were extracted from a National Institute for Occupational Safety and Health, 1996 Questionnaire (Franks et al, 1996), combined with items from various clinical questionnaires.

Since the exposure to the selected chemicals has been associated with neurobehavioral and balance disorders (Odkvist et al., 1980; Baker, Smith and Landrigan, 1985), the inclusion of portions in the questionnaire that investigate these functions is recommended (for sample questionnaires see Hogsted et al, 1984; Johnson, 1987).

Audiological Testing

A test battery should include pure tone audiometry (air conduction) and tests to complement it. Performing only pure tone audiometry will probably not be enough to meet the objectives of the investigation of chemical effects on the auditory system. The rationale for recommending complementary audiologic tests is that not only prevalence data but also descriptions of the pathologies should be sought. In addition, these minimum complementary tests may enable the research team to differentiate the effects of noise from the effects of chemicals. This information will probably lead to the generation of new hypothesis related to the lesions underlying mechanisms. Some alternative tests and procedures are described in Appendix B.

Data Analysis

The American Conference of Governmental Industrial Hygienists (ACGIH, 2000) alluded to the complexity of investigating industrial chemicals effects, by recommending careful review of audiometric data. The hearing loss from industrial chemicals can be very similar to the hearing loss from ototoxic drugs such as aminoglycosides and cisplatin, as well as to the hearing loss from noise. General descriptors of these disorders are very similar: bilaterally symmetrical, irreversible, high frequency (3 to 6 kHz) sensorineural hearing loss with damage mainly to cochlear hair cells. Comparison of these descriptors reveals how difficult it may be to make a differential diagnosis and to determine causation of hearing loss among workers. For a discussion on some of the alternatives available to researchers in addressing these challenges, see Morata and Lemasters (1995).

Mean audiometric thresholds should be obtained, but hearing outcome could also be treated as a binary outcome variable (normal hearing vs. hearing loss) when examining the relationship between the combined exposure to noise and chemicals and hearing. The prevalence of hearing loss between groups with different exposure conditions should be examined even if audiometric thresholds, by themselves, do not allow for easy identification of the effect of chemicals on hearing, and especially when pure­tone audiometry is the only available test.

The use of Statistical Analysis System (SAS) or other appropriate computer software is suggested for data analysis. Analysis of Variance (ANOVA) or regression analysis can be used for threshold data. Logistic regression can be used for binary outcomes. Confounding variables such as age, length of exposure, gender, previous and non-occupational exposures to the studied agents, use of ototoxic medications, smoking and drinking habits, should be controlled for.

Concluding Remarks

This draft document was submitted to the NoiseChem Research Group for review. This group consists of research institutions from seven different countries. One of NoiseChem's objectives is to examine study designs, hearing assessment alternatives, and strategies for the analysis of combined effects of noise and chemical exposures. Moreover, on the basis of agreed protocols, NoiseChem's goal is to conduct epidemiological studies on factory workers in Sweden, Finland, Poland, and the United Kingdom. In this peer review stage we are currently accepting critiques and suggestions to this proposal, which can be sent directly to the authors.[58]

 
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Thais C Morata
National Institute for Occupational Safety and Health, 4676 Columbia Parkway/ C27, Cincinnati, OH 45226
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1 Self-reported hearing performance in workers exposed to solvents
Fuente, A. and McPherson, B. and Hormazabal, X.
Revista de Saude Publica. 2013; 47(1): 86-93
[Pubmed]
2 Hearing care and quality of life among workers exposed to pesticides [Saúde auditiva e qualidade de vida em trabalhadores expostos a agrotóxicos]
de Sena, T.R.R. and Vargas, M.M. and Oliveira, C.C.C.
Ciencia e Saude Coletiva. 2013; 18(6): 1753-1761
[Pubmed]
3 Auditory dysfunction associated with solvent exposure
Fuente, A. and McPherson, B. and Hickson, L.
BMC Public Health. 2013; 13(1)
[Pubmed]
4 Evoked otoacoustic emissions in workers exposed to noise: A review
De Souza Alcarás, P.A. and Lüders, D. and Franča, D.M.V.R. and Klas, R.M. and De Lacerda, A.B.M. and De Oliveira Gončalves, C.G.
International Archives of Otorhinolaryngology. 2012; 16(4): 515-522
[Pubmed]
5 Prevalence of tinnitus in workers exposed to noise and organophosphates [A prevalência do zumbido em trabalhadores expostos à ruído e organofosforados]
Delecrode, C.R. and De Freitas, T.D. and Frizzo, A.C.F. and Cardoso, A.C.V.
International Archives of Otorhinolaryngology. 2012; 16(3): 328-334
[Pubmed]
6 High frequency audiometric notch: An outpatient clinic survey
Osei-Lah, V., Yeoh, L.H.
International Journal of Audiology. 2010; 49(2): 95-98
[Pubmed]
7 Peripheral and central auditory dysfunction induced by occupational exposure to organic solvents
Fuente, A., Slade, M.D., Taylor, T., Morata, T.C., Keith, R.W., Sparer, J., Rabinowitz, P.M.
Journal of Occupational and Environmental Medicine. 2009; 51(10): 1202-1211
[Pubmed]
8 Occupational styrene exposure and hearing loss: A cohort study with repeated measurements
Triebig, G., Bruckner, T., Seeber, A.
International Archives of Occupational and Environmental Health. 2009; 82(4): 463-480
[Pubmed]
9 Comparative study of audiometrics tests on metallurgical workers exposed to noise only as well as noise associated to the handling of chemical products | [Estudo comparativo de exames audiométricos de metalúrgicos expostos a ruído e ruído associado a produtos químicos]
Botelho, C.T., Paz, A.P.M.L., Gonçalves, A.M., Frota, S.
Brazilian Journal of Otorhinolaryngology. 2009; 75(1): 51-57
[Pubmed]
10 Ototoxicity of toluene and styrene: State of current knowledge
Hoet, P., Lison, D.
Critical Reviews in Toxicology. 2008; 38(2): 127-170
[Pubmed]
11 Audiological findings in individuals exposed to organic solvents: Case studies
Gopal, K.
Noise and Health. 2008; 10(40): 74-82
[Pubmed]
12 Central auditory processing effects induced by solvent exposure
Fuente, A., McPherson, B.
International Journal of Occupational Medicine and Environmental Health. 2007; 20(3): 271-279
[Pubmed]
13 Promoting hearing health and the combined risk of noise-induced hearing loss and ototoxicity
Morata, T.C.
Audiological Medicine. 2007; 5(1): 33-40
[Pubmed]
14 Assessment of central auditory processing in a group of workers exposed to solvents
Fuente, A., McPherson, B., Muñoz, V., Espina, J.P.
Acta Oto-Laryngologica. 2006; 126(11): 1188-1194
[Pubmed]
15 Effect of styrene on postural stability among reinforced plastic boat plant workers in Finland
Toppila, E., Forsman, P., Pyykkö, I., Starck, J., Tossavainen, T., Uitti, J., Oksa, P.
Journal of Occupational and Environmental Medicine. 2006; 48(2): 175-180
[Pubmed]
16 Safety evaluation of chemical mixtures and combinations of chemical and non-chemical stressors
Jonker, D., Freidig, A.P., Groten, J.P., De Hollander, A.E.M., Stierum, R.H., Woutersen, R.A., Feron, V.J.
Reviews on Environmental Health. 2004; 19(2): 83-139
[Pubmed]
17 Chemical exposure as a risk factor for hearing loss
Morata, T.C.
Journal of Occupational and Environmental Medicine. 2003; 45(7): 676-682
[Pubmed]



 

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