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Danger! Ototoxic chemicals

By Stefan Dubowski

Some solvents can cause permanent hearing damage

Dan Tadic, head of the Canadian Welders Association, knows a lot about welder safety. He sold welding equipment for 30 years. Without hesitation, he can rhyme off aspects of the CSA W117.2-01 standard, which covers eyewear, ventilation and other safety measures for welders.

One of those measures speaks to hearing protection. The standard calls on welders to wear personal protective equipment (PPE) to reduce the damaging effects of noise from air carbon arc and plasma arc cutting processes.

“Follow that standard and you’ll have all the equipment you need to work safely,” Tadic says.

But many welders may not be familiar with one emerging occupational health concern: ototoxic chemicals  (substances that have a toxic effect on the ear and its nerve supply). In welding, manganese, a material often present in the fumes that welding creates, is ototoxic. That means if it is inhaled, manganese can harm a welder’s ability to hear clearly.

Ototoxicity is not just a concern for welders but for workers across numerous industries, including printing, boat builders, construction and manufacturing.


Harmful to hearing

Ototoxic materials attack the cochlea, the ear’s spiral-shaped cavity and sensing organ that picks up sound. Ototoxic solvents such as styrene, toluene and xylene also affect the auditory vestibular nerve, which transmits sound and balances information to the brain. As well, these substances can harm the auditory cortex, the part of the brain that processes sound.

Damage from ototoxic substances can manifest as different sorts of hearing difficulties. One study conducted by researchers from the University of Queensland in Australia found that members of the Australian Defence Force who were exposed to four or more ototoxic substances were two to four times more likely to experience moderate or severe tinnitus (ringing of the ears) compared to members of the force who were less exposed to the hazardous materials.

In another study, researchers investigating hearing loss among fibreglass and metal manufacturing plant workers exposed to styrene — a material used to make plastic containers and metal products — found that exposure seemed to harm people’s comprehension of spoken phrases.

Researchers at the Université de Montréal conducted a review of studies on ototoxicity and found toluene, a solvent used to make paint and glue, interacts with noise to harm exposed individuals’ hearing even more than either noise or toluene exposure alone. The scientists ranked other substances according to evidence of ototoxicity, too, noting that ethyl benzene, n-hexane and xylene may be ototoxic, and that carbon monoxide may interact with noise to harm hearing. 

Little conclusive evidence

For many of the chemicals, scientists can’t say conclusively that exposure causes hearing loss. Although results from animal testing clearly indicate that certain materials do harm hearing, it’s more difficult to prove that an individual’s hearing loss resulted from ototoxic exposure, especially when exposure to the substance coincided with exposure to noise. Since some chemicals work synergistically with noise to exacerbate hearing loss, it isn’t easy to tell how much of the damage resulted from noise and how much from ototoxicity.

“There are very few things you can say conclusively have an increased risk,” says Bev Borst, technical service specialist at 3M Canada in London, Ont. “Toluene and carbon monoxide are recognized as ototoxic, depending on their concentration. There are others that are of particular interest.”

Adrian Fuente, an assistant professor at Université de Montréal’s School of Speech Pathology and Audiology, sums up the challenge: Scientists can’t expose humans to solvents just to find out how much is too much, say, ethyl benzene. But given the evidence from animal experiments and observed hearing loss among people whose jobs exposed them to ototoxic substances, “it’s very easy to make the link,” he says.

Researchers are working to strengthen evidence of the connection between hearing loss and exposure to ototoxic substances. Thais Morata, member of the hearing loss prevention team at the National Institute for Occupational Safety and Health (NIOSH) in the United States, is deep into this area of research. She says scientists now understand the harmful effects of ototoxicity appear sooner than the effects of noise.

“Investigations which examined the effects of solvents over time indicated that hearing loss is observable two to three years earlier than is usually seen with noise exposure,” Morata says. “This issue of latency is certainly dependent on the ototoxicant and the characteristics of the exposure, and needs further investigation.”

She points to a 1986 landmark study conducted by scientists in Sweden that suggested exposure to ototoxic chemicals is more damaging to hearing than exposure to noise alone. The study looked at the number of insurance claims made for hearing loss from workers in different types of companies. Chemical plant employees made the most claims, even though noise levels at the chemical companies were lower than the noise levels at other businesses.

“Studies conducted with experimental animals have clearly shown that some environmental and occupational toxicants can reach the inner ear through the blood stream,” Morata says. “Some of these chemicals are also damaging to the nerves… What we do not know yet are precise exposure conditions or levels necessary for an effect. The onset, site, mechanism and extent of ototoxic damage from these toxicants vary according to risk factors that include the type of chemical, interactions and exposure level.”

New standard coming

Work on a new CSA standard that identifies ototoxicity as a potential hazard is underway. CSA Z1007 Management of Occupational Hearing Conservation Programs includes information for anyone whose job involves exposure to ototoxic substances and noise. The draft of the standard says managers in such workplaces should submit employees to hearing tests at least twice per year. Borst says Z1007 is expected to be in the public consultation stage before the end of the year and may be published in 2015. It could change between now and then, but for the moment, the draft identifies how workers may potentially be exposed (inhaling, touching or ingesting the substance) and recommends using the right PPE to reduce exposure.

As with any protective gear, compatibility is important. If a worker wears a powered-air respirator with a hood, earplugs would provide excellent fit and protection, Borst says, while earmuffs wouldn’t be compatible.

But protective equipment is the last line of defence against workplace hazards. Morata points out that the hierarchy of controls recommends other solutions first.

“In most cases, the preferred approach is to eliminate the source of hazardous noise or chemical,” she says.

When elimination isn’t possible, substitution of the chemical or the loud equipment for safer alternatives may be the next best alternative. If the hazard can’t be controlled through elimination or substitution, engineering controls may reduce noise at the source. Failing those solutions, companies should reduce exposure through the use of administrative controls, such as changing an employee’s work schedule to avoid the hazards.

While employers face certain responsibilities for employee safety, workers themselves should take steps to protect their hearing health, Morata adds.

“Regardless whether hearing tests are offered at work or not, when one suspects having a hearing disorder, he or she should see a doctor.”

Stefan Dubowski is a freelance writer based in Ottawa. He can be reached at

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