Solvents are a threat to hearing

During work in industrial processes, the inhalation, ingestion, and absorption of organic solvents through the skin slowly brings about deterioration in hearing, and this should be taken heed of. So said Chilean researcher, Adrián Fuente, on highlighting the role of audiologists in detecting this problem. Fuente was speaking during an AAA workshop in Boston.

Awareness that some solvents damage hearing has been around since the 1980s, yet health and safety regulations in some countries do not bear this in mind when it comes to protecting those who work with such substances. Often, not even hearing health professionals take into account the possible effects of these agents, such as xylene, toluene, styrene, ethelbenzyne, n-hexane, and carbon disulfide, among others.

The issue came under scrutiny as part of a Learning Module at the American Academy of Audiology's (AAA) March Congress in Boston (see dossier). This workshop was taken by doctors in audiology, Thais Morata, of the US National Institute for Occupational Safety and Health (NIOSH), and Adrián Fuente, an academic and researcher from the Laboratory of Audiology based at the University of Chile's School of Speech, Language, and Audiology.

Fuente has spent ten years investigating the effects of organic solvents on hearing function. His studies, started in Chile, have taken him to Poland, Hong Kong, the USA, and Australia, where he is currently undertaking post-doctorate studies at the University of Queensland.

Last November, this expert finished research which identifies for the first time the effect that use of a specific solvent has on human hearing health. The substance is a compound called xylene, exposure to which was analysed amongst workers in histopathology laboratories at state hospitals (see boxed details).

"To date, similar studies have analysed exposure to solvent mixes, which makes it difficult to determine which particular agent has the greatest impact on hearing health and what levels are damaging for hearing," explained Fuente.

Long-term effects

Exposure in the workplace to organic solvents is recorded in various industries, among them plastics, printing, aviation fuels, dry cleaning, pigments and thinners, footwear manufacture, synthetic leather manufacture, rubber, perfumes and detergents, and others. The chemicals enter the body through inhalation, ingestion, or skin absorption. "It is widely accepted today that the effects of organic solvents on the hearing system is down to their oto-and neuro-toxicity," stresses Fuente. Both mechanisms can occur in separate, simultaneous, or alternating ways and cause adverse effects in structures such as the cochlea and the central auditory pathways, respectively.

"As a result, those exposed may present light hypoacusis, which today may not be restrictive yet as the body ages the problem will become more significant. It is likely that these patients, as opposed to workers not exposed to organic solvents, will present difficulties in localising sounds or with speech discrimination at earlier ages, and the likelihood of them needing a hearing aid is higher than that for the normal population, bearing in mind that the effect of ageing is added to that of chronic exposure to these agents. This is a tendency that is now being observed," warns the researcher.

For example, he adds, "in people around 40-yearsold, we have seen signs of hearing ageing that we usually see in those over 55-years-old."

The attraction of solvents to lipid structures is, to a great extent, what is behind the damage caused to the hearing system.

In laboratory animals, it has been observed that the probable route of intoxication of the inner ear is through blood flow. Solvents arrive along this route to the stria vascularis, a structure that filters liquids from the cochlea. From there, they spread by penetrating lipid structures such as Hensen and Deiters cells, which support the organ of Corti, finally reaching and damaging the external hair cells (EHC).

Their neurotoxic effect is due to solvents' capacity to penetrate myelin, a layer of lipids covering nerve fibres such as the central auditory nervous system (CANS). In this way, the solvent gets through directly to auditory fibres and degenerates them.

Detecting and taking action

Despite the great number of scientific studies which link solvent exposure to damaging effects on hearing health, and due to which the European Union and countries such as Australia, New Zealand, and Brazil have initiated protective actions for workers exposed to chemicals and noise, Fuente warns that among occupational health professionals there is still ignorance about the issue.

To tackle the problem, this researcher points out that it is fundamental that professionals investigating patient anamneses delve into possible exposure in the workplace to oto-and neuro-toxic chemical agents. This needs to be done, in the interests of taking early action, even if the person presents normal hearing thresholds. "If we know someone is exposed to chemicals and damaging solvents, they need to be placed on an auditory conservation programme which includes education on personal protection and the harmful effect of solvents. A monitoring plan also needs to be set up, as well as an annual or bi-annual hearing check-up to detect minimum damage, which is a way of enabling early action if there are any abnormal changes," he advises.

Actions that can be taken, according to this researcher, include rotating or re-situating the person at work and establishing shorter days, as well as providing a mask, gloves, and the right clothing. On top of this, and considering that the poisonous effects are boosted if workers are also exposed to workplace noise, the specialist stresses that hearing protection should be provided at exposure levels above 80 dBA, and not at the 85 dBA traditionally stated in work and safety regulations.