INTRODUCTION
Noise-induced hearing loss is one of the most frequent irreversible work-related diseases in the world26. Direct consequences of occupational noise exposure are reported by various studies. The term occupational hearing loss is frequently used as a synonym of noise-induced hearing loss, but in addition to noise, many industries present other agents, such as chemicals, which considered isolated or in combination with noise, can cause and/or maximize workers' health damage19.
Upon the specific analysis of lead, we can see that industrial applications are vast, but the production of batteries (accumulator) probably represents the industrial segment responsible for most of the consumption of this substance in developing countries. Owing to lead toxic properties and working conditions present in many industrial facilities, the workers are frequently exposed to high concentrations of lead, and consequently, subject to lead poisoning.
In Brazil, there are thousands of battery manufacturers, plants and repairers from north to south of the country, which use lead as the raw-material without following the appropriate control; moreover, there is occasional exposure, such as for example the existence of accumulators spread on residential backyards without the dully protective measures. This snapshot demonstrates that there is room for significant improvement in the area of prevention.
The international literature presents innumerous studies about the effects of lead poisoning in the body, demonstrating how multiple organs can be affected. There are two poisoning routes: the epithelium of respiratory airways absorbs lead smoke or impels particles into the pharynx where they are swallowed, and the skin, through which compounds such as lead tetraethylate and others are absorbed. In both cases, lead gets into peripheral circulation and accumulates in the liver, spleen, kidney, heart, brain, muscles and skeletal system, and the main harmful effects are manifested in the hematological, nervous, renal, gastrointestinal and reproductive systems.
Conversely, investigations about the effects of lead in the auditory system and the harmful effects of simultaneous exposure to more than one agent, considering noise for example, are scarce. There is still controversy about the effects of lead on hearing and whether it interacts with noise, maximizing its damage. Studies addressing the peripheral hearing system are rare and present contradictory results and the analysis of the central portion of the system is limited to electrophysiological tests, such as brainstem auditory evoked potentials and late evoked potentials (P300).
The purpose of the present study was to critically review the national and international literature concerning the auditory effects of occupational exposure to lead in the auditory system and to try to identify the priorities for future studies.
LEAD EXPOSURE
Lead is a heavy metal, foreign to the human metabolism. The set of signs and symptoms that result from the presence of lead in the human body is named Saturnism.
There are two ways in which lead can be present at different levels in the body: by environmental contamination or occupational exposure9.
Occupational exposure is the main form through which there is excessive absorption of lead in adults. Preventive measures have reduced the number of cases of lead poisoning in developed countries but the long-term consequences of exposure in asymptomatic workers is not fully known7.
Brazilian legislation, provided by the Regulating Standard NR-7, Administrative Rule No. 24, issued by the Occupational Safety and Health Department on December 29, 1994, has defined that plumbemia level is that in which lead concentration in the blood (Pb-B) is equal or greater than 40 µg/dl (Normal Reference Limit) up to the biological tolerance limit (Maximum Allowed Biological Value) of 60 µg/dl17.
Araujo et al.6 reported that there was significant difference between blood lead levels in subjects who worked in battery industries located in developed countries and those who worked in developing countries. Twenty-eight percent of the workers of this kind of industry in Jamaica and 38% of workers in Korea had Pb-B levels greater than 60µg/dl, whereas in the United States, only 6% presented levels above the limit18. This variation clearly shows a significant difference between the work processes and workers protection adopted by those countries.
Even though there are no precise data on the occupational lead poisoning in Brazil, studies conducted in the city of Bauru, SP between 1985 and 1987 revealed that there were 600 cases of lead poisoning or Saturnism among battery manufacturer workers9.
The basic problem of contamination in battery manufacturers is emission and dispersion of dust that contains lead in the workplace, contaminating the air, surfaces (the floor and benches), clothes and workers' hands, facilitating absorption. Therefore, responsibility of contamination is normally attributed to the worker, but it actually depends on the production process, work conditions and maintenance of an unhealthy and inappropriate workplace6.
According to Buschinelli8, lead dusts are of the most unhealthy type. The effect of poisoning is slow and cumulative and people exposed to this metal, if not appropriately protected by collective and personal protective equipment, can suffer severe health damage.
Battery manufacturers in Brazil are the main responsible for lead poisoning, especially because there are accumulators spread throughout the whole country, many times in people's backyards and without any protective measure. These manufacturers are the main consumers of lead, and generally cause mass lead poisoning. It is an activity that does not require high technology and tolerance levels are sometimes exceeded up to 50 times.
Blood lead levels reflect the dynamic balance between absorption, retention and clearance. In prolonged exposure, blood lead levels provide a reliable indication of the current exposure14. Conversely, a little awhile after reduction of exposure levels, the dosing method becomes a poor indicator. Therefore, research studies directed to the investigation of the abnormalities caused by lead exposure should prefer behavioral and electrophysiological tests, right after blood draw, and when the subject is exposed as a routine in his/her work environment.
ATSDR1 (Agency for Toxic Substances and Disease Registry) classified in 1988 three levels of lead exposure based on Pb-B rates. Low lead exposure was defined as blood lead level between 10 and 20 µg/dl, moderate exposure as levels between 21 and 60 µg/dl, and high exposure for levels greater than 61 µg/dl.
According to Zanini28 long-term exposure has deserved special attention in past years, especially the functional abnormalities caused by low concentrations of lead. According to the author, exposure time and manifestations attributed to lead poisoning resulted in difficulty to correlate lead and functional abnormalities, especially considering the effects of the substance in the central nervous system.
NEUROBEHAVIORAL EFFECTS OF OCCUPATIONAL EXPOSURE TO LEAD
In the international literature, there are many studies demonstrating the association of occupational lead exposure and performance in neuropsychological or neurobehavioral tests. Even though these are tests not applied in audiology, we believe it is important to report such studies because they referred to abnormalities in central nervous system skills caused by lead exposure15.
According to the specialized literature, the performance in neurobehavioral tests demonstrated integrity of the nervous system affected by neurotoxic exposure, through assessment of intelligence, memory, learning, verbal thinking, attention/concentration, visual-motor coordination, visual-spatial organization, motor speed and manual skills. The tests are frequently used to assess functions, such as cognitive, psychological and neuropsychological, which include WAIS (Wechsler Adult Intelligence Scale), WMS (Wechsler Memory Scale), RAVLT (Rey Auditory Verbal Learning Test) and Sant¢Ana Test. All the tests listed here resulted in some central nervous system deficit in the subjects, that is, the results showed strong evidence of mild to moderate neurobehavioral dysfunction caused by lead exposure.
AUDITORY EFFECTS OF OCCUPATIONAL EXPOSURE TO LEAD
The literature directed to lead effects in hearing is not very vast. Many articles were found about hearing effects in children exposed to environmental lead15. However, specialized references related to workers exposure to this metal associated to noise exposure are nearly nonexistent. The present study restricted the review of papers about the effects of occupational lead exposure on the hearing system.
THRESHOLDS FOR PURE TONE DETECTION
The results of investigations about auditory thresholds in workers are still contradictory. Repko and Corum25 reported the occurrence of hearing loss in workers exposed to lead, especially those whose blood lead levels were above 70 µg/dl. In that study, they found hearing thresholds in frequencies 0,5, 1, 3, 4kHz associated to blood lead level. The authors made a critical analysis of previous studies considering size of sample and insufficient information about the noise level to which these subjects were exposed in the workplace.
Baloh et al.7 conducted audiologic assessment in 69 subjects occupationally exposed to lead and revealed that the auditory thresholds found did not demonstrate statistically significant differences between the group of workers exposed to lead and the control group. Thus, they concluded in the study that it was not possible to identify the influence of lead exposure on hearing.
Alternatively, when auditory thresholds of 183 workers submitted to lead exposure were investigated by Forst et al.12, they noticed a statistically significant correlation between blood lead level and reduction of hearing thresholds in 4kHz. However, in that study the authors did not consider some significant variables, such as level of noise exposure.
Occupational exposure to lead in 45 workers from a graphic plant was analyzed by Farahat et al.11, and they observed a significant correlation between blood lead levels and hearing thresholds, especially in the frequency of 8kHz. The frequencies of 1, 2 and 4kHz were also affected, and the mean noise level in the workplace was 42dB. Thus, they concluded that there was an association between high hearing thresholds and blood lead level.
More recently, in a study with 339 workers from a battery manufacturer the following were measured: blood lead level, concentration of lead in the air, level of noise in the workplace, and workers' hearing thresholds26. They were interviewed to collect demographic data and occupational history. The multivariate statistical analysis, which assessed various risk factors to hearing, revealed a significant correlation between high chronic levels of exposure and hearing thresholds. There was no correlation between noise exposure and occurrence of hearing loss (probably because the noise level was low, and the duration was not enough to cause detectable effects). Similarly, no interaction between the two agents was detected (possibly by the same reason presented above, or because lead and noise affect different regions of the hearing system).
LEAD EFFECTS ON AUDITORY CENTRAL NERVOUS SYSTEM
The occurrence of abnormalities in different brain evoked potentials, including cognitive potentials (P300), was reported in some research studies. Holdstein et al.14 investigated brainstem auditory evoked potentials (BAEP) in 16 subjects aged between 18 and 56 years (mean age 40 years) with blood lead levels between 30 and 80µg/dl (mean of 48 µg/dl). BAEP waves were recorded in 10 and 55 milliseconds (ms) and there was increased interpeak latency in I-III (10 and 55ms) and III-V in 10ms in subjects exposed to lead.
Brainstem auditory evoked potentials and somatosensory potentials were studied by Lille et al.16 in 13 subjects with mean blood lead level of 100 µg/100ml ands mean age of 37 years, being that four of them had history of concomitant exposure to alcohol. The authors reported one single case of abnormal result with increase in interpeak latency I-V in one subject exposed to lead and alcohol. In the study, the authors concluded that there was significant correlation between level of Pb-B and latencies obtained in the test.
The results of brainstem auditory evoked potentials were also analyzed by Discalzi et al.10 who assessed 49 workers exposed to lead, mean exposure of 7.4 years. The concentration of lead in the blood was analyzed based on the samples collected on the first day of the study (mean of 54.6 µg/dl) and the mean from the past three years of lead exposure (mean of 53.5 µg/dl). They also considered the interpeak latencies I-V, I-III and III-V. The authors found a significant increase in interpeak values and latency increase was more significant in interpeak I-V in workers whose blood lead levels were greater than 50 µg/dl in the past three years of exposure. The authors concluded that BAEP is a sensitive test to detect subclinical effects of lead in the auditory pathways of the brainstem. The conclusion was then confirmed by other researchers. Hirata and Kosaka13 used BAEP and demonstrated an increase in interpeak latency III-V. In a similar study, Murata et al.20 performed the test in 36 women occupationally exposed to lead, with mean level of Pb-B of 55.6 µg/dl, and they did not find significant differences between latencies when comparing the results to those of the control group.
Araki et al.3 studied the cognitive function using late evoked potential (P300) in 22 workers, whose blood lead concentration was between 12 and 59 µg/dl. P300 latency was significantly prolonged when compared to the control group and demonstrated correlation with the lead blood level. The authors also assessed the peripheral nervous conduction speed. The results demonstrated that exposed workers had slower speed, and it was correlated to blood lead level. No significant correlation was found between nervous conduction speed and P300, and the authors concluded that the mechanism of lead effects in the CNS seems to be different from the mechanism that affects the peripheral nervous system. In addition, the researchers stated that the results of the study suggested that lead affects the cognitive function, as well as the functions of the auditory central nervous system.
Similar results were described by Murata et al.21. Latency of P300 proved to be significantly increased and dependent on Pb-B level when compared to the control group, and there was increased interpeak latency I-V in the BAEP assessment. The sample of the study comprised 22 workers with lead levels below 65 µg/dl and absence of clinical signs characteristic of lead poisoning. Despite the results, the authors suggested that auditory brainstem pathways are affected by lead exposure, as well as the cognitive function, even in asymptomatic workers.
Similarly, the occurrence of abnormalities in brainstem auditory evoked potentials and cognitive potentials were described in workers exposed to lead and studied by Araki et al.4,5. The authors concluded that the effect of lead on latencies (peak and interpeak) obtained in the test are present with Pb-B levels greater than 40-50 µg/dl and in P300 in blood lead levels greater than 30-40 µg/dl.
In a transversal study that tried to investigate the effects of simultaneous exposure to lead and noise on the auditory central nervous system in 43 workers of a battery manufacturer in Brazil, the following procedures were performed: measurement of blood lead level (Pb-B), pure tone audiometry, dichotic digit test, Synthetic Sentence Identification (SSI), Spondaic Simultaneous Words (SSW), and Filtered Speech (Jacob15, Jacob et al., in press). Out of 43 subjects, 17 had been exposed to noise levels of 96 dB HL and 26 were exposed to noise (84 dB HL) and lead. Behavioral tests that assessed the auditory skills of closure, figure-ground and sequential memory showed differences between the groups, because the abnormalities were more prevalent among workers exposed to both agents.
In 1993, Otto and Fox24 suggested directions for future studies about the effects of lead in the hearing system:
- histology and morphology studies of all levels of the hearing system to identify the topodiagnosis of auditory pathway lesions;
- electrophysiological tests of cortical and subcortical structures to identify topodiagnosis and auditory deficit induction mechanisms;
- brainstem auditory evoked potentials varying the stimulus presentation patterns, especially in humans;
- studies with complex auditory stimuli (that is, dichotic hearing, masking) to identify the effects of lead on central auditory processing;
- electrophysiological and psychological tests to determine reversibility/irreversibility of damage caused by lead.
CLOSING REMARKS
Occupational hearing losses are most of the times related to high levels of noise exposure for a prolonged period of time. In addition to noise, the literature has also revealed a myriad of other agents present in the environment or the industry, such as chemical agents, which may cause hearing losses. As we could notice in the present paper, there is evidence that lead is one of these agents and that its effects are not limited to the cochlea. This information will force more researchers or health-related professionals in the areas of occupational medicine, audiology and otorhinolaryngology to take into account the exposure to chemicals when studying work-related hearing loss. This decision will directly imply the definition of the population to be studied, which variables to be considered, which auditory tests to be investigated and how to direct data analysis.
Currently, international legislation does not require hearing follow-up of workers exposed to chemicals, which is the case in high noise level environments, despite the action of research agencies such as NIOSH22,23 (National Institute for Occupational Safety and Health) and ACGIH2 (American Conference of Governmental Industrial Hygienists) in recommending that exposure to chemicals be considered part of the prevention programs of hearing loss. In 1999, the American army followed the recommendations and changed the standards in the Program of Hearing Conservation, testing periodically the hearing of those exposed to a list of chemicals (including lead), regardless of noise exposure. Considering the evidence present today, it is expected that the example be followed by other institutions.
REFERENCES
1. AGENCY FOR TOXIC SUBSTANCE AND DISEASE REGISTRY (ATSDR). - The nature and extent of lead poisoning in children in the United States: a report to Congress. DHHS, ATSDR, 1988.
2. AMERICAN CONFERENCE OF GOVERNMENT INDUSTRIAL HYGIENISTS - ACGIH. - Threshold Limit Values and Biological Exposure Indices for 1998-1999. ACGIH, Cincinnati, 1999.
3. ARAKI, S.; MURATA, K.; YOKOYAMA, K.; UCHIDA E. - Auditory event-related potential (P300) in relation to peripheral nerve conduction in workers exposed to lead, zinc, and copper: effects of lead on cognitive function and central nervous system. Am. J. Ind. Med., 21:539-47, 1992.
4. ARAKI, S.; MURATA, K.; YOKOYAMA, K. - Application of neurophysiological methods in occupational medicine in relation to psychological performance. Ann. Acad. Med. Singapore, 23(5):710-8, 1994.
5. ARAKI, S.; SATO, H.; YOKOYAMA, K.; MURATA, K. - Subclinical neurophysiological effects of lead: a review on peripheral, central and autonomic nervous system effects in lead workers. Am. J. Ind. Med., 37(2):193-204, 2000.
6. ARAUJO, U. C.; PIVETTA, F. R.; MOREIRA, J. C. - Avaliação da exposição ocupacional ao chumbo: proposta de uma estratégia de monitoramento para prevenção dos efeitos clínicos e subclínicos. Cad. Saúde Pública, 15(1):123-31, 1999.
7. BALOH, R. W.; SPIVEY, G. H.; BROWN, P.; MORGAN, D.; CAMPION, D. S.; BROWDY, B. L.; ET AL. - Subclinical effects of chronic increased lead absorption - a prospective study: results of baseline neurologic testing. Journal of Occupational Medicine, 21(7):490-6, 1979.
8. BUSCHINELLI, J. T.; BARBOSA, C. Q.; TRIVELATO,.G. C. - Chumbo x trabalhadores: um jogo lento e fatal. Proteção, 7(2):42-50, 1990.
9. CORDEIRO, R. - O saturnisno em Bauru. In: Pimenta AL, Costa Filho DC (org.). Saúde do trabalhador. São Paulo: Hucitec, 1988. p. 47-83.
10. DISCALZI, G. L.; CAPELLARO, F.; BOTTALO, L.; FABBRO, D.; MOCELLINI, A. - Auditory brainstem evoked potentials (BAEPs) in lead-exposure workers. Neurotoxicology, 13(1):207-9, 1992.
11. FARAHAT, T. M.; ABDEL-RASOUL, G. M.; EL-ASSY, A. R.; KANDIL, S. H.; KABIL, M. K. - Hearing thresholds of workers in a printing facility. Environmental Research, 73:189-92, 1997.
12. FORST, L. S.; FREELS, S.; PERSKY, V. - Occupational lead exposure and hearing loss. J. Occup. Environm. Med., 39(7):658-60, 1997.
13. HIRATA, M. & KOSAKA, H. - Effects of lead exposure on neurophysiological parameters. Environ. Res., 63:60-9, 1993.
14. HOLDESTEIN, Y.; PRATT, H.; GOLDSHER, M.; ROSEN, G.; SHENHA,V. R.; LINN, S.; MOR, A.; BARKAI. A. - Auditory brainstem evoked potentials in asymptomatic lead-exposed subjects. J. Laryngol. Tol., 100(9):1031-6, 1986.
15. JACOB, L. C. B. - Efeitos da exposição simultânea ao chumbo e ao ruído sobre o sistema nervoso central em trabalhadores de uma fábrica de baterias [Tese de doutorado]. Universidade de São Paulo, Bauru, 2000.
16. LILLE, F.; HAZEMANN, P.; GARNIER, R.; DALLY, S. - Effects of lead and mercury intoxications on evoked potentials. Clinical Toxicology, 26(1-2):103-16, 1988.
17. MANUAIS DE LEGISLAÇÃO ATLAS. - Segurança e Medicina do Trabalho. 36ª ed., v. 16. São Paulo: Atlas; 1997.
18. MATTE, T. D.; FIGUEROA, J. P.; BURR, G.; FLECH; JEROME, P.; KEENLYSIDE R. A.; ET AL. - Lead exposure among lead-acid battery workers in Jamaica. Am. J. Ind. Med., 16:167-77, 1989.
19. MORATA, T.C.; LEMASTERS, G. - Considerações epidemiológicas para o estudo de perdas auditivas ocupacionais. In: Nudelmann AA, Costa EA, Seligman J, Ibañez RN. PAIR - Perda auditiva induzida pelo ruído. Rio de Janeiro: Revinter, 2001, Vol.II, p.1-16.
20. MURATA, K.; ARAKI, S.; YOKOYAMA, K.; NOMIYANA, K.; NOMIYANA, H.; TAO, Y.X.; LIU, S. J. - Autonomic and central nervous system effects of lead in female glass workers in China. Am. J. Ind. Med., 28(2):233-44, 1995.
21. MURATA, K.; ARAKI, S.; YOKOYAMA, K.; UCHIDA, E.; FUJIMURA, Y. - Assessment of central, peripheral, and autonomic nervous system functions in lead workers: neuroelectrophysiological studies. Environ. Res., 61:323-36, 1993.
22. NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH - NIOSH. - National Occupational Research Agenda. Cincinnati: USDHHS, PHS, CDC, NIOSH, publication no.96-115, 1996.
23. NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH - NIOSH. - Criteria for a Recommended Standard. Occupational Exposure to Noise. Revised Criteria. Cincinnati: USDHHS, PHS, CDC, NIOSH, publication no.98-126, 1998.
24. OTTO, D. A. & FOX, D. A. - Auditory and visual dysfunction following lead exposure. Neurotoxicol., 14(2-3):191-207, 1993.
25. REPKO, J. D. & CORUM, C. R. - Critical review and evaluation of the neurological and behavioral sequelae of inorganic lead absorption. CRC Crit. Ver. Toxicol., 6:135-87, 1979.
26. WHO - PDH. - Report of the First Informal Consultation on Future Programme Developments for the Prevention of Deafness and Hearing Impairment. World Health Organization, Geneva, p. 23-24, 1997.
27. WU, T. N.; SHEN, C. Y.; LAI, J. S.; GOO, C. F.; KO, K. N.; CHI, H. Y.; CHANG, P. Y.; LIOU, S. H. - Effects of lead and noise exposures on hearing ability - Arch Environ Health, Mar-Apr; 55(2):109-14, 2000.
28. ZANINI, O. - Fundamentos de toxicologia. São Paulo: Atheneu, 1996.
[1] Professor, Post-Graduation Program, Master in Communication Disorders, Universidade Tuiuti do Parana - Curitiba - PR, Ph.D. in Communication Disorders, University of São Paulo - Bauru - SP.
[2] Ph.D., Professor, Department of Speech and Language Pathology and Audiology, School of Dental Medicine, University of São Paulo, Bauru - FOB/USP-Bauru.
[3] Professor, Ph.D., Post-Graduation Program, Master in Communication Disorders, Universidade Tuiuti do Parana - Curitiba - PR, Researcher with the National Institute for Occupational Safety and Health (NIOSH).
Study conducted at the Center for Audiological Research, Hospital de Reabilitação de Anomalias Craniofaciais, University of São Paulo - Bauru.
Address correspondence to: Lilian Cássia Bornia Jacob - Rua Prof. Pedro Viriato Parigot de Souza, 1100, bl 04, apto 703 - Curitiba - PR - 81200-100
Tel (55 41)331-7847 Fax (55 41)331-7870 - e-mail: lilian.jacob@utp.br
|
