Hearing health of workers exposed to noise and insecticides



Cleide Fernandes TeixeiraI; Lia Giraldo da Silva AugustoII; Thais C MorataIII

IFaculdade Integrada do Recife. Recife, PE, Brasil
IICentro de Pesquisa Ageu Magalhães, Fundação Osvaldo Cruz. Recife, PE, Brasil
IIINational Institute for Occupational Safety and Health. Cincinnati, OH, EUA





OBJECTIVE: To examine the peripheral auditory disorders in a group of workers exposed to organophosphate and pyrethroid insecticides, used in vector control campaigns.
METHODS: The prevalence study examined a population of 98 individuals who sprayed insecticides in campaigns for the prevention of Dengue, Chagas disease and Yellow fever. The sampling approach was finalistic, and included the workers in a health district of Pernambuco, during the year 2000. A questionnaire was used to collect data on occupational and non-occupational risks, safety measures utilized, family history of auditory problems and health symptoms. Previous noise exposure history was also investigated, since noise can be a confounding factor for hearing loss. Hearing sensitivity and middle ear function were assessed by pure tone audiometry.
RESULTS: Among those exposed to insecticides, 63.8% demonstrated a hearing loss. For the group of workers exposed to both noise and insecticides, hearing loss was observed in 66.7% of the cases. The median exposure time necessary to detect high-frequency losses was 3.4 years for workers exposed to both agents and 7.3 years for workers exposed to insecticides only. Hearing thresholds were poorest among workers exposed to both agents. Auditory damage for those with combined exposures to the two factors was more severe than the hearing losses observed among those exposed only to insecticides.
CONCLUSIONS: There is evidence that exposure to insecticides was associated with peripheral sensorioneural hearing loss and that noise exposure can potentiate the ototoxic effects of insecticides. It is necessary to evaluate this possible association through epidemiological studies.

Keywords: Hearing loss, noise-induced. Insecticides.Vector control. Auditory system and multiple exposures.




Studies concerning occupational hearing loss and health status have been concerned, almost exclusively with the risks of exposure to noise.3,7,12,14 Other risk factors for hearing impairment have been observed, such as chemical exposure to ototoxic products. However these studies are still insufficient. Sensorial disorders of the hearing system have also been associated to medical drugs (amnioglicosides, quinine, etc.) or to organic solvents used by the manufacturing industry.7,9,10

The ototoxic properties of the majority of different groups of composite chemical substances suspected of being neurotoxic were never tested.1,2,11 Include among the latter are the agro toxins. There are approximately 600 active principals of pesticides available within the market throughout the world and 50,000 commercial formulas based on these active principles are utilized in agriculture.2,10 This information gives us some idea of the dimension of health risks resulting from the use of pesticides.

The first occupational studies suggesting that exposure to chemical products were directly associated to hearing disorders and equilibrium were conducted approximately 30 years ago.7,4 However, it is only in the past 16 years that greater attention has been paid to the potential interaction between noise and other environmental agents. Studies concerning occupational exposure to different solvent mixtures have indicated different alterations in workers’ auditory system. Abnormalities in the recognition of speech and in cortical response audiometry have been observed among workers exposed, for long periods of time, to aromatic and aliphatic solvents. However, these workers did not present alterations in cerebral trunk abnormalities.14 The effects of toluene on hearing among rotogravure printing workers (n=190), exposed simultaneously to noise, indicate the adjusted relative risk of 1.4 (95% CI 1.4-1.75) when compared to an unexposed reference group.12 The authors of this study conclude that the results of acoustic reflex decay tests within the group exposed to both agents – toluene and noise – indicate that retro cochlear or central auditory pathway lesions might have occurred in some of the hearing disorders observed.

Data related to hearing loss as a consequence of exposure to a mixture of toluene, ethyl acetate and ethanol were also observed in another study conducted among rotogravure printing workers. The occurrence of hearing losses was directly correlated to the levels of toluene in the urine.13

Jacobsen et al,9 in his longitudinal study involving 3,284 male residents of Copenhagen (aged 53 to 74 years old) exposed to solvent mixtures, the relationship between occupational exposure and self-perception of hearing disorders was investigated. The exposure to chemicals (without exposure to noise for five years or more) indicated an adjusted relative risk for hearing loss of 1.4 (95% CI: 1.1-1.9). The prevalence of hearing loss among men not exposed to organic solvents was 24% and the risk attributed to those exposed to solvents was 9.6%. Among those workers exposed for less than 5 years, no hearing disorders were observed. On the other hand, those exposed to noise for five years or more had a relative risk of hearing loss of 1.9 (95% CI: 1.7-2.1). The mentioned study demonstrates that there is a positive association between exposure to solvents and hearing loss. The chemical compounds of the agro toxic type, some of which are known for their neurotoxic properties, are frequently considered responsible for intoxicating workers that handle them and apply agro toxins in developing countries.4,17,18

Workers exposed to organophosphates and with low pseudo cholinesterase activity were compared with workers with normal pseudo cholinesterase activity.6 A high incidence of peripheral neuropathologies was observed in the group with low values of pseudo cholinesterase and both groups presented low to moderate degrees of hearing loss of the neurosensorial type. In this study, previous exposure was not taken into consideration. One case of hearing loss was found by Harel et al8 after acute intoxication through the emission of a spray containing a mixture of two organophosphates (7.5% Malathion and 15% Methamidophos). Approximately nine hours after exposure, the exposed person presented cloudy vision and gradual episodes of nausea. On the fourth day after exposure, the individual felt completely deaf and dizzy. This individual was instructed to undergo hemodialysis the following morning and presented profound bilateral auditory loss as well as residual neuropathies in the extremities.

Peripheral hearing disorders, associated with exposure to agro toxic organophosphates and pyrethroid were also observed among a group of 98 rural farmers, ranging in age from 15 to 59 years, exposed to these insecticides and which were not exposed to noise. Audiometric screening revealed that 57.14% of the exposed group had high-frequency hearing losses of the sensorioneural type. Recently, an analysis of equitable distribution undertaken by Beckett5 in a study involving farmers (n=59) in New York found peripheral hearing loss associated to pulverization with insecticides (including mixtures of pyrethroids and organophosphates). More recent studies have also observed hearing disorders of the central auditory pathways among workers that pulverize these types of insecticides in campaigns to combat endemic vectors.16

In Brazil, the Fundação Nacional de Saúde (FNS) [National Health Foundation] recommends the use of organophosphates and pyrethroids in vector campaigns against Chagas’ disease and Dengue/ Yellow Fever. In past campaigns, mixtures of aromatic and lymphatic Organochlorines were utilized, just as in the past few years Malathion and Cypermethrin have been utilized in vector control. Malathion is an organophosphate known to be toxic for human beings, and carcinogenic for animals. This situation characterizes a condition of risk for multiple exposures among health agents which were recruited to work on these programs and campaigns, applying these products in the environment and within households (Table 1).

The objective of this study was to estimate the prevalence of hearing losses among the occupational group exposed to organophosphates and pyrethroids. Considering that many neurotoxic products, depending upon the period and mode of exposure, can lead to alterations in the sensorial systems, we attempted to examine the effects of this exposure on the peripheral auditory system. In this sense, this study may be useful for the development of biological indicators of the effect of exposure to neurotoxic chemicals and intends to contribute towards the adoption of preventive measures and policies of control.



This is a cross-sectional study of prevalence.

The study group was composed of ninety eight male workers (N=98), whose average age was 41.6 years (SD=6.9), and who underwent chronic exposure to organophosphate as well as pyrethroid insecticides for a minimum of three years. All subjects were workers of a health district of the FNS in the tate of Pernambuco, which was selected merely due to logistic reasons, once it does not differ from the other three health districts as far as health measures and policies put in practice. Amongst these workers, 25% stated they were exposed to organophosphates of the DDT and BHC types when they began carrying out their functions. The workers were subdivided into groups according to their present and/or previous exposure to noise within the contexts of work and leisure. Group 1 (n=47) did not state they were exposed to noise in present or previous occupational or leisure activities. At work, the chemical substance was only applied manually, by means of hand-held plastic sprayers, with no exposure to the pulverization of insecticides, or by means of hand-held pressure sprayers (neither of which are noisy). Group 2 (n=51) members stated they were exposed to noise at present or in their previous occupational history or leisure time activities. These participants used thermal foggers (FOG) or ULV cold foggers placed on pickups, or portable motorized atomizers, (weighing 10 kilos on the average). When the latter are turned on, they produce noise above 85 dB (SPL).

Meatoscopy and the study of immittance audiometry were performed on all individuals, with the objective of verifying the functional integrity of the middle ear and the existence of possible conductive or mixed disorders (obstruction of the external acoustic meatus, perforated tympanic membrane, previously diagnosed hearing impairment of a non-occupational origin). These would be the possible disorders mentioned which would exclude individuals from this study. The frequent use of ototoxic medicines and degenerative diseases also led to the exclusion of individuals from this study. The pure-tone audiometry test was performed by two phonoaudiologists which were not given any information on the previous type of exposure within each individual’s history.

Data collection

A semi-structured questionnaire, covering information concerning personal identification, occupational and non-occupational security measures, previous history of auditory problems and health symptoms in general was applied. Data on occupational history included a description of activities performed, type of physical exposure and types of chemical substances to which the informant was exposed. The results of the hearing tests were attached to the worker’s questionnaire after each interview. All tests were performed by one phonoaudiologists according to standardized methods. These results were validated by a second test, performed by the other phonoaudiologist.

Testing procedures

The auditory system of all workers was evaluated by means of pure-tone audiometry at the frequencies of 0.5 to 8 kHz. Bone conduction testing was performed when the hearing thresholds, in the 0.5 to 4 kHz frequency range, exceeded 25 dB HL. The BELTONE 114 clinical audiometer was calibrated according to the ISO R389 (1991) norm prior to data collection. Audiograms were classified as normal if no single threshold exceeded 25 dBHL.

The 6.12 version of the SAS Statistical Analysis System software was utilized for proceeding with the statistical analyses of the data. The CHI square test made it possible to compare study groups and to determine whether there were correlations between the audiometric tests and the period of exposure. Results of the audiometric tests performed on each ear among both study groups were compared by means of the Mann-Whitney test. It was thus possible to verify whether there were significant differences between groups with normal and altered audiometry with respect to the duration of exposure to the insecticide. It was also possible to verify in each of the groups, with normal and altered audiometry, if there were significant differences among the groups with respect to the period of exposure.

Ethical implications

The present study was preceded by visits to the working sites, with the support of the administrative spheres, of the division of occupational health of the Fundação Nacional de Saúde - FNS (National Institute of Health) and workers’ union representatives. Informed consent was obtained from all individuals who participated in this study. The project was approved by the Ethics Committee of the Aggeu Magalhães Research Center.



Among the study population, 72% of the participants had either begun or completed their high school education, however it must be stressed that 94.8% had not undergone any kind of professional training course associated with the activities they did at work. Only 35.7% of the population smoked cigarettes and the majority (84.4%) did not drink alcoholic beverages frequently. Although the efficacy of hearing protectors is questionable, only 2.9% of the exposed group mentioned that they used them. Workers’ hygiene at the end of the working shift were considered inadequate; 80% of the participants did not take a shower before they went home and only 19% mentioned they changed their clothes. The majority stated they went home wearing the clothes they used at work (93.4%) and that they used the same garments more than once before they washed them (73.2%). The most frequent complaints reported in reply to the questionnaire were: irritated eyes (54.1%), headaches (44.9%), dizziness (35.7%), nauseas and torpor (16.3%), and 20.4% of the individuals stated they had suffered from intoxication. As to symptoms related to the central nervous system, the most frequent were: difficulties in understanding what people say (46%), in remembering some facts (43%), in paying attention (24%), and hearing noises inside the ear (24%).

The mean period of exposure within the present function was 7.7 years (SD=4.5 years), and the coefficient of variation was 58.12%. It is noteworthy that the majority of the exposed population had been working in the present function, in which they were exposed to insecticides, for 4 to 6 years and 31.6% indicated more than 10 years of exposure. Few workers utilized any sort of individual protective equipment (34.7%). The majority alleged that there was some kind of incompatibility with respect to the equipment (dust/mist masks or respirators, helmets, hearing protectors or ear muffs) which impeded them from using them in an effective manner. The results of the pure-tone audiometry tests were classified according to clinical criteria which consider the auditory thresholds equal to or below 25 dB (NA) to be a normal standard. As was already expected, the previous occurrence of hearing losses among the study groups was not significant statistically (Table 2).

 In order to explore the variable duration of exposure, participants were classified into two groups – those exposed for a period of time inferior to 6 years (median time of exposure to the insecticide) and those exposed for a period greater than 6 years. The highest percentage of hearing disorders was found among individuals in the latter group. These results, when submitted to statistical analyses, were not found to be significant, however, they demonstrated that the relative risk of hearing loss was greater amongst members of the group which was exposed for a longer period of time (Table 3).

It can be noted that the average period of exposure was higher within group A1 than within group A2 for individuals with normal audiometry, the opposite occurred among individuals with altered audiometry. However, significant statistical differences were not found between individuals with normal and altered audiometry either within each group or between both groups (P>0.05).

Analyzing the audiometric distribution according to the level of intensity of hearing loss for the frequencies from 2 to 8 kHz (the frequencies from 0.5 to 1 kHz did not present disturbances), it can be verified that the intensity of the hearing loss within the group exposed to the insecticides and noise was greater than within the group exposed only to insecticides (Figure 1). The median values of intensity of hearing loss increased from 2 kHz to 6 kHz, and decreased at 8 kHz when compared to 6 kHz. Except for the 8 kHz frequency of the right ear, for the remaining frequencies, the median values were greater in group 2 than in group 1, however, in terms of significance, no significant difference was verified for any frequency in either of the two ears (Table 5).




Table 4



This study indicates that the use of organophosphate and pyrethroid insecticides may be responsible for peripheral hearing loss whether or not exposure to noise occurs. The results of the symptoms related to the central nervous system found in our study are compatible with those referred to in the literature concerning chronic or acute intoxications due to chemical substances with neurotoxic effects, such as organophosphates and pyrethroids.2,8 Epidemiological studies report different periods of exposure considered necessary for the chemicals to induce peripheral impairments to hearing (from 3 to 5 years for solvents).9,12,13 In this study, the minimum time of exposure necessary to affect high-frequencies was compatible with the findings of Morata et al.13 Hearing loss within the high frequencies was observed after three years of exposure and, for the medium high frequencies (2 and 3 kHz), hearing loss occurred after 3.4 years of exposure to noise and insecticides. From 4 kHz on, an increase in the differences between the period of exposure of the group exposed only to insecticides and the group exposed to both insecticides and noise was observed. This tendency exacerbates itself at 3 kHz, when the difference is twice as large, indicating a probable synergism between both agents. At 6 kHz and 8 KHz the results change, a minimum mean period of exposure of 2.4 years was necessary for hearing impairments to be noted amongst those exposed only to insecticides (Figure 1). An important observation should be taken into consideration with respect to disturbances in the medium high frequencies. It was verified that, for the group exposed to both insecticides and noise, not only the intensity of the hearing loss was greater but the extension of the spectrum of frequencies affected was broader than that observed within the group exposed only to insecticides. In this case, the problems related to speech comprehension (group 2) are more significant, being detrimental to work performance and occupational health.

In conclusion, chronic exposure to pyrethroid and organophosphate insecticides may affect the peripheral auditory system, whether or not there is concomitant exposure to noise. Data from this study and other recent publications indicate that chemical exposures should be monitored and controlled as part of an effort to prevent hearing impairment. Workers exposed to neurotoxic chemical substances should be included in programs for hearing conservation, whether or not they are exposed to noise.



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Correspondence to
Cleide Fernandes Teixeira
Av. Eng. Abdias de Carvalho, 1678
50720-635 Recife, PE, Brasil
E-mail: lcteix@elogica.com.br

Received on 17/5/2002.
Reviewed on 15/2/2002.
Approved on 4/3/2003.



*Based on the thesis presented to “Centro de Pesquisa Ageu Magalhães da Fundação Osvaldo Cruz”, Recife, PE, Brazil, 2000.

Faculdade de Saúde Pública da Universidade de São Paulo São Paulo - SP - Brazil
E-mail: revsp@org.usp.br