Ann-Christin Johnson

Audiometric findings in workers exposed to low levels of styrene and noise

Learning ObjectivesExplain whether and how exposure of workers to styrene in the course of making fiberglass products, and to high noise levels, interact to produce bilateral high-frequency hearing loss.Recall the personal and environmental factors that were significantly associated with hearing loss in this study.Discuss how these findings relate to past studies in animals and styrene-exposed workers, and take note of relevant public health considerations.Audiometry and exposure measurements were conducted on workers from fiberglass and metal products manufacturing plants and a mail distribution terminal (N = 313). Workers exposed to noise and styrene had significantly worse pure-tone thresholds at 2, 3, 4, and 6 kHz when compared with noise-exposed or nonexposed workers. Age, noise exposure, and urinary mandelic acid (a biologic marker for styrene) were the variables that met the significance level criterion in the multiple logistic regression. The odds ratios for hearing loss were 1.19 for each increment of 1 year of age (95% confidence interval [CI], 1.11–1.28), 1.18 for every decibel >85 dB(A) of noise exposure (95% CI, 1.01–1.34), and 2.44 for each millimole of mandelic acid per gram of creatinine in urine (95% CI, 1.01–5.89). Our findings suggest that exposure to styrene even below recommended values had a toxic effect on the auditory system.

Audiological findings in workers exposed to styrene alone or in concert with noise.

Audiological testing, interviews and exposure measurements were used to collect data on the health effects of styrene exposures in 313 workers from fiberglass and metal-product manufacturing plants and a mail terminal. The audiological test battery included pure-tone audiometry, distortion product otoacoustic emissions (DPOAE), psychoacoustic modulation transfer function, interrupted speech, speech recognition in noise and cortical response audiometry (CRA). Workers exposed to noise and styrene had significantly poorer pure-tone thresholds in the high-frequency range (3 to 8 kHz) than the controls, noise-exposed workers and those listed in a Swedish age-specific database. Even though abnormalities were noted on DPOAE and CRA testing, the interrupted speech and speech recognition in noise tests were the more sensitive tests for styrene effects. Further research is needed on the underlying mechanisms to understand the effects of styrene and on audiological test batteries to detect changes in populations exposed to solvents.

Function of the Auditory and Visual Systems, and of Peripheral Nerve, in Rats after Long-Term Combined Exposure to n-Hexane and Methylated Benzene Derivatives. I. Toluene

Rats were exposed to n-hexane, toluene, or toluene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in combined exposure), 21 hr/day, 7 days/week during 28 days. Neurophysiological recordings were made 2 days, 3 months, and one year after end of exposure. A reduction in auditory sensitivity, recorded by click evoked auditory brainstem response, was observed 2 days after exposure to toluene alone, or to toluene together with n-hexane, but not after exposure to n-hexane alone. The reduction lasted one year after the exposure. Three months after combined exposure, a synergistic enhancement of loss of auditory sensitivity was observed in the mixed exposure group. One amplitude in the flash evoked potentials was lowered in the n-hexane exposed group 2 days after exposure. No such reduction was seen after simultaneous exposure to toluene. Exposure to n-hexane alone caused a marked decrease in peripheral nerve conduction velocity 2 days and 3 months after exposure, while exposure to n-hexane together with toluene had only a small effect on this velocity.

A multicenter study on the audiometric findings of styrene-exposed workers

Abstract Objective: The objective of this study was to evaluate hearing loss among workers exposed to styrene, alone or with noise. Design: This cross-sectional study was conducted as part of NoiseChem, a European Commission 5th Framework Programme research project, by occupational health institutes in Finland, Sweden, and Poland. Study sample: Participants’ ages ranged from 18–72 years (n = 1620 workers). Participants exposed to styrene, alone or with noise, were from reinforced fiberglass products manufacturing plants (n = 862). Comparison groups were comprised of workers noise-exposed (n = 400) or controls (n = 358). Current styrene exposures ranged from 0 to 309 mg/m3, while mean current noise levels ranged from 70–84 dB(A). Hearing thresholds of styrene-exposed participants were compared with Annexes A and B from 2. Results: The audiometric thresholds of styrene exposed workers were significantly poorer than those in published standards. Age, gender, and styrene exposure met the significance level criterion in the multiple logistic regression for the binary outcome ‘hearing loss’ (P = 0.0000). Exposure to noise (<85 dBA p = 0.0001; ≥85 dB(A) p = 0.0192) interacted significantly with styrene exposure. Conclusions: Occupational exposure to styrene is a risk factor for hearing loss, and styrene-exposed workers should be included in hearing loss prevention programs. Sumario Objetivo: El objetivo de este estudio fue evaluar la pérdida auditiva en trabajadores expuestos a estireno, aisladamente o con ruido. Diseño: Se realizó este estudio transversal como parte del NoiseChem, Comisión Europea del 5°. Proyecto Marco de investigación, en institutos de salud ocupacional de Finlandia, Suecia y Polonia. Muestra de estudio: La edad de los participantes fue de 18–72 a os (N = 1,620 trabajadores). Los trabajadores (n = 862) estuvieron expuestos a estireno, solo o con ruido, en plantas de fabricación de productos de fibra de vidrio reforzada. Los grupos de comparación fueron trabajadores expuestos a ruido (n = 400) y los de un grupo control (n = 358). Las exposiciones reales a estireno variaron de 0 a 309 mg/m3, mientras que los niveles reales de ruido, variaron de 80–84 dB(A). Se compararon los niveles de audición de los participantes expuestos a estireno con los de los anexos A y B de 2. Resultados: Los umbrales audiométricos de los trabajadores expuestos a estireno, fueron significativamente más pobres que los que se han publicado como estándar. La edad, el género y la exposición a estireno, cubrieron los criterios de niveles de significatividad de la regresíón logística múltiple para los resultados binarios de la “perdida auditiva” (P = 0.0000). La exposición a ruido (<85 dB(A) p = 0.0001; ≥85 dB(A) p = 0.0192) interactu significativamente con la exposición a estireno. Conclusiones: La exposición ocupacional a estireno es un factor de riesgo de pérdida auditiva por lo que los trabajadores expuestos a estireno deben ser incluidos en programas de prevención de pérdidas auditivas.

Increased vitamin plasma levels in Swedish military personnel treated with nutrients prior to automatic weapon training

Noise-induced hearing loss (NIHL) is a significant clinical, social, and economic issue. The development of novel therapeutic agents to reduce NIHL will potentially benefit multiple very large noise-exposed populations. Oxidative stress has been identified as a significant contributor to noise-induced sensory cell death and NIHL, and several antioxidant strategies have now been suggested for potential translation to human subjects. One such strategy is a combination of beta-carotene, vitamins C and E, and magnesium, which has shown promise for protection against NIHL in rodent models, and is being evaluated in a series of international human clinical trials using temporary (military gunfire, audio player use) and permanent (stamping factory, military airbase) threshold shift models (NCT00808470). The noise exposures used in the recently completed Swedish military gunfire study described in this report did not, on average, result in measurable changes in auditory function using conventional pure-tone thresholds and distortion product otoacoustic emission (DPOAE) amplitudes as metrics. However, analysis of the plasma samples confirmed significant elevations in the bloodstream 2 hours after oral consumption of active clinical supplies, indicating the dose is realistic. The plasma outcomes are encouraging, but clinical acceptance of any novel therapeutic critically depends on demonstration that the agent reduces noise-induced threshold shift in randomized, placebo-controlled, prospective human clinical trials. Although this noise insult did not induce hearing loss, the trial design and study protocol can be applied to other populations exposed to different noise insults.

Auditory degeneration after exposure to toluene in two genotypes of mice.

Two inbred strains of mice, CBA/Ca (with a moderate hearing loss starting late in life) and C57BL/6J (with an early onset of spontaneous auditory degeneration), were exposed to toluene by inhalation (1000 ppm, 12 h/day, 7 days) at either 1 or 6 months of age. Thresholds of auditory brainstem response (ABR) were measured 3–5 days after exposure and assessed repeatedly up to the age of 16 months (C57) or 23 months (CBA). Both strains of mice exposed to toluene at 1 month of age showed a mild loss of sensitivity at a high frequency (31.5 kHz) shortly after exposure. With increasing age, toluene exposure had little effect on the aging process of the auditory system in CBA mice but accelerated age-related hearing loss in C57 mice. The results indicate that toluene exposure can aggravate auditory deterioration only in mice with a strong genetic predisposition to spontaneously precocious age-related hearing loss.

A guinea pig strain with recessive heredity of deafness, producing normal-hearing heterozygotes with resistance to noise trauma.

A new strain of waltzing guinea pigs arose spontaneously in a guinea pig breeding facility in Germany in 1996. In addition to obvious vestibular dysfunction, the waltzing animals appear deaf already at birth. Histological analysis revealed that the waltzers lack an open scala media due to the collapse of Reissner’s membrane onto the surface of the hearing organ. Subsequent breeding has shown that this strain has a recessive mode of inheritance. The homozygotes are deaf and display a waltzing behaviour throughout their lives while the heterozygotes show no significant signs of inner ear injury despite being carriers of this specific mutated gene of hearing impairment. However, the heterozygous animals offer the opportunity to study how hereditary factors interact with auditory stress. In the present study, the susceptibility of the carriers to noise was investigated. Auditory brainstem responses were obtained prior to and after noise exposure (4 kHz, 110 dB, 6 h). The carriers were significantly less affected by the noise as compared to control animals. This difference was still significant at 4 weeks following noise exposure. It is suggested that the heterozygous animals have an endogenous resistance to auditory stress.

Relationship between styrene exposure and hearing loss: review of human studies.

Styrene is an aromatic solvent belonging to the alkylbenzene family. Occupational exposure to styrene occurs mainly in the manufacturing of fiberglass-reinforced polyester products, e.g. reinforced plastics and composites. Since 1988, nine studies have been published on the relationship between occupational exposure to styrene and hearing loss. All studies were the cross-sectional epidemiological studies or clinical studies from occupational health clinics. A total of more than 1000 workers exposed to styrene, both with and without concurrent noise exposure, were examined using different outcome measures for hearing loss. Exposure assessment was usually based on styrene measurements in the breathing zone during several hours of one working day. Some of the studies employed also the biological monitoring of styrene exposure based on determination of its urinary metabolites. The current exposures to styrene varied between 2 and 35 ppm. In some studies, lifetime exposure was calculated using company records and questionnaire data. The current exposure to noise was estimated by noise dosimetry or standard noise measurements. Lifetime noise exposure was assessed using questionnaire data and occupational noise estimates. In many studies, noise-exposed groups were used as controls together with the unexposed workers. Of the nine studies, seven show some effects on the auditory system that were associated with styrene-alone exposure. These effects are examined using different outcome measures such as pure tone audiometry, high frequency hearing loss, and central hearing tests. In some studies, an increased risk for hearing loss was associated with exposure estimates.

The Role of Genetic Factors for Hearing Deterioration Across 20 Years: A Twin Study

BACKGROUND Hearing deterioration at advanced ages is associated with environmental exposures (eg, to noise and solvents) and genetic influences may also be important. Little is known about the role of genetic influences on hearing when evaluated longitudinally. We sought to investigate longitudinal hearing loss in a cohort of adult male twins to evaluate the importance of genetic and environmental factors for hearing deterioration over time. METHODS Hearing using conventional clinical audiometry was assessed in 583 male twins (128 monozygotic twin pairs and 111 dizygotic twin pairs) aged 34-79 at baseline and again two decades later. The hearing thresholds at two time points were compared at each frequency and in two different frequency regions. Genetic analyses were based on structural equation models. Bivariate Cholesky decomposition was used for longitudinal analysis. RESULTS The prevalence of hearing loss increased over time in better and worse ear. The hearing threshold shift was more pronounced in the high-frequency region, especially at 8000 Hz. Genetic influences were moderate (heritability: 53%-65%) for pure-tone averages at both lower and higher frequencies, and were of equal magnitude at baseline and follow-up. In contrast, environmental influences were of substantial importance (55%-88%) for rate of change of the hearing threshold over the 18-year period. CONCLUSIONS Genetic factors are of considerable importance for level of hearing acuity, but environmental factors are more important for rate of change over an 18-year period.

A demonstrated positive effect of a hearing conservation program in the Swedish armed forces

Abstract Objective: A revised hearing conservation program (HCP) was implemented in the Swedish Armed Forces in 2002. The aim of this study was to evaluate the incidence of significant threshold shifts (STS) in male conscripts heavily exposed to noise after the implementation of the new HCP, comparing the results to those of an earlier study from 1999/2000. Design: The study was prospective and longitudinal, covering the period from reporting to military service to discharge. The outcome measure was the incidence of STS. Statistics from the military insurance system was analysed. Study Sample: A total of 395 conscripts were included in the study (mean age 19 years). The control group (n: 839) consisted of men of the same age. Results: In 2004/2005 the incidence rate of STS was 2.3% compared to 7.9% in 1999/2000 and compared to 3.7% among the controls. The number of cases of auditory complications reported from conscripts to the insurance system has decreased, from 16 to 5/100 000 days of military training, during the last decade. Conclusions: The new HCP apparently reduced the incidence rate of STS to one third compared to before the program was introduced and leveled it to the incidence rate in the control-group not exposed to military noise.