Combined effects of noise and styrene exposure on hearing function in the rat.

Workers are commonly exposed to multiple factors and substances that are hazardous to health. For instance, in 2005, approximately 30% of the European workers reported being exposed to noise during at least a quarter of the time spent in their work environment, 11% reported inhaling vapors such as solvents and thinners; 19% reported exposure to smoke, fumes, powder, or dust; and 14% reported handling chemical substances.1 Although occupational noise exposure has long been recognized in the United States and in Europe as the most deleterious factor to hearing2,3 the impact of chemical-induced hearing loss on workers should not be underestimated.4 A cursory survey of the literature for the last three decades reveals that concern about the effects of chemicals on hearing has grown steadily. The proliferation of work-related substances and medicinal drugs (mostly antitumor drugs and aminoglycosides) has been accompanied by an equivalent increase in the number of scientific publications on the hearing risks encountered by chemical-exposed persons. Cause for concern is even greater when the synergistic risks of co-exposures are considered. For example, physiological factors may increase the severity of a chemical’s effect on hearing. Today, robust evidence also confirms that the effects of ototoxic substances on ear function can be aggravated by noise, which remains a well-recognized cause of hearing impairment. In an expert forecast supervised by the European Agency for Safety and Health at Work5 (EU-OSHA, 2009), a “combined exposure to noise and ototoxic substances” was rated as an emerging risk.

Industrial ototoxic chemicals have predominantly been assessed through animal studies, which are supported by data from epidemiologic studies on human workers from various industries. Bergstrom and Nystrom (1986) published the seminal results of a 20-year longitudinal study performed with 319 Swedish employees from different industrial sectors.2 This study began in 1958 and involved testing workers’ hearing regularly. Its findings showed that a large proportion (23%) of the employees working in a chemical division suffered from hearing impairment, despite their exposure to lower noise levels than other divisions. Based on this type of evidence, researchers have long hypothesized that industrial solvents pose an additive risk to hearing. Scientific findings in animals are generally considered qualitatively relevant to human health, provided no substantial difference in biological response (e.g. metabolism) exists between test animals and humans. Despite these findings, current research is limited by the following: 


a lack of detailed exposure histories;


the presence of confounding factors (ototoxic drugs, tobacco, alcohol consumption, aging, and exposures outside the workplace);


and the fact that chemical exposure scenarios used in experimental investigations are qualitatively different from real-world occupational settings.





In this extensive, but not exhaustive, review of the literature, the authors hope to share the evidence indicating that noise is not the only source of work-related hearing damage and to draw greater attention to the matter of chemically induced hearing loss. The present publication intends to provide occupational physicians and other health professionals with a clear picture of what is known about how chemical substances may affect hearing ability in the general population, and more specifically, in workers. The article describes the following: 


chemicals that can be noxious to the inner ear;


work areas where exposure to ototoxic substances are likely;


and the basic features of the physiological mechanisms leading to hearing impairment.





This article also addresses the limitations of pure-tone air-conduction audiometry when assessing chemical-induced hearing loss and proposes a more complete approach to evaluate the auditory neurosensory hearing receptor and the neural pathways involved in the stapedial reflex. With the proposed battery of tests, physicians will be able to evaluate both ototoxicity and neurotoxicity. Throughout this manuscript, emphasis is placed on the need for stronger links between primary care and occupational physicians. This could help preserve patients’ hearing status as a part of their overall health.

Chemical exposure and hearing loss

DISCLAIMER The use of company or product name(s) is for identification only and does not imply endorsement by the Agency for Toxic Substances and Disease Registry. A Toxicological Profile for Styrene, Draft for Public Comment was released in October 2007. This edition supersedes any previously released draft or final profile. Toxicological profiles are revised and republished as necessary. For information regarding the update status of previously released profiles, contact ATSDR at: FOREWORD This toxicological profile is prepared in accordance with guidelines* developed by the Agency for Toxic Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA). The original guidelines were published in the Federal Register on April 17, 1987. Each profile will be revised and republished as necessary. The ATSDR toxicological profile succinctly characterizes the toxicologic and adverse health effects infonnation for these toxic substances described therein. Each peer-reviewed profile identifies and reviews the key literature that describes a substance's toxicologic properties. Other pertinent literature is also presented, but is described in less detail than the key studies. The profile is not intended to be an exhaustive document; however, more comprehensive sources of specialty information are referenced. The focus of the profiles is on health and toxicologic infonnation; therefore, each toxicological profile begins with a public health statement that describes, in nontechnical language, a substance's relevant toxicological properties. Following the public health statement is infonnation concerning levels of significant human exposure and, where known, significant health effects. The adequacy of infonnation to detennine a substance's health effects is described in a health effects summary. Data needs that are of significance to protection of public health are identified by ATSDR. Each profile includes the following: (A) The examination, summary, and interpretation of available toxicologic infonnation and epidemiologic evaluations on a toxic substance to ascertain the levels of significant human exposure for the substance and the associated acute, subacute, and chronic health effects; (B) A determination of whether adequate infonnation on the health effects of each substance is available or in the process of development to detennine levels of exposure that present a significant risk to human health of acute, subacute, and chronic health effects; and (C) Where appropriate, identification of toxicologic testing needed to identifY the types or levels of exposure that may present significant risk of adverse health effects in humans. The principal audiences for the toxicological profiles are health professionals at the Federal, State, and local levels; …

/pdf/toxicological-profile-for-styrene-22w158nz3d.pdf

Toxicological profile for styrene

Comparison of toluene-induced and styrene-induced hearing losses.

The potential chronic health risks of occupational and environmental exposure to styrene were evaluated to update health hazard and exposure information developed since the Harvard Center for Risk Analysis risk assessment for styrene was performed in 2002. The updated hazard assessment of styrene's health effects indicates human cancers and ototoxicity remain potential concerns. However, mechanistic research on mouse lung tumors demonstrates these tumors are mouse-specific and of low relevance to human cancer risk. The updated toxicity database supports toxicity reference levels of 20 ppm (equates to 400 mg urinary metabolites mandelic acid + phenylglyoxylic acid/g creatinine) for worker inhalation exposure and 3.7 ppm and 2.5 mg/kg bw/day, respectively, for general population inhalation and oral exposure. No cancer risk value estimates are proposed given the established lack of relevance of mouse lung tumors and inconsistent epidemiology evidence. The updated exposure assessment supports inhalation and ingestion routes as important. The updated risk assessment found estimated risks within acceptable ranges for all age groups of the general population and workers with occupational exposures in non-fiber-reinforced polymer composites industries and fiber-reinforced polymer composites (FRP) workers using closed-mold operations or open-mold operations with respiratory protection. Only FRP workers using open-mold operations not using respiratory protection have risk exceedances for styrene and should be considered for risk management measures. In addition, given the reported interaction of styrene exposure with noise, noise reduction to sustain levels below 85 dB(A) needs be in place.

https://tandfonline.com/doi/pdf/10.1080/10937404.2019.1633718

Evaluation of potential health effects associated with occupational and environmental exposure to styrene – an update

Twenty workers exposed to styrene and acetone in small fiberglass factories were monitored for 8 h using passive dosimeters Urine samples were collected at the end of the workshift and before the start of work on the next morning The 8-h time-weighted average exposure values for styrene and acetone ranged from 14 to 416 mg/m3 and from 70 to 277 mg/m3, respectively The sum of styrene metabolites, mandelic acid and phenylglyoxylic acid (MA + PGA), in the next-morning urine samples ranged from 81 to 943 mg/g creatinine Different test sensitivity was identified in the otoneurological battery: it was low for audiometric tests and ABR, and relatively high for vestibular tests The vestibular system seems partially sensitive to the toxic effects of styrene in the absence of clinical signs and symptoms The actual exposure levels for styrene cannot be considered devoid of functional subclinical consequences

Otoneurological study in workers exposed to styrene in the fiberglass industry

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Otoneurological study in workers exposed to styrene in the fiberglass industry