How puzzled would you be if workers in your plant's quietest area performed worse on hearing tests than those in the noisiest machine shop? Any number of explanations could exist: genetics, old age, dangerous hobbies.

Or, there's a chance they'd be suffering the effects of on-the-job exposure to ototoxic substances.

Ototoxins -- "ear poisons" -- are substances that can impair hearing. Like any chemical hazard, they can be ingested, absorbed or inhaled into the body. Once in the blood stream, scientists suspect ototoxins do their damage by getting circulated to the ear and absorbed by the auditory nerve or, like noise, by breaking the cochlear hair cells. The result can be mild hearing loss, tinnitus (ringing in the ears), or total deafness.

In medicine, certain prescription drugs have long been linked to impaired hearing. Commonly used antibiotics like tetracycline and ampicillin are potentially ototoxic. So are some diuretics, aspirin, and drugs used to treat cancer. Even caffeine, alcohol, and nicotine have been shown in some studies to have ototoxic effects.

Known workplace ototoxins include solvents like styrene, xylene, and trichloroethylene; metals like lead and mercury; and asphyxiants like carbon monoxide. Studies have shown that excessive exposure to any of these agents on its own is enough to induce hearing loss. More severe effects can result when ototoxin exposure is combined with noise exposure. In fact, some researchers indicate that even when noise and chemicals are at permissible exposure levels, the impact of a combined exposure can do more damage than a higher exposure to either hazard alone.

A significant minority

The implications are disturbing. "Not taking ototoxic exposures into account when looking at an audiogram could be detrimental," says toxicologist Katherine Hasal. Audiometric tests, which can't discern between noise-induced hearing loss and ototoxic effects, could mislead safety and health professionals into controlling noise exposures when ototoxic exposures are the real problem. Or worse, workers not considered for audiometric testing could be suffering occupational hearing loss completely unaware.

"Noise is so pervasive and closely connected to hearing loss that people assume it is always the cause," says Laurence Fechter, Ph.D., director of the University of Oklahoma toxicology program. "It may in fact be that noise causes 90 percent of all hearing loss. But we have no way to know that because the other causes get overlooked." Noise is, indeed, by far the bigger hazard. According to NIOSH, more than 30 million workers are exposed to hazardous noise on the job, making hearing loss the most common occupational disease in the U.S.

But the numbers at risk of ototoxic exposures are significant in their own right: Some 3.5 million workers are exposed to trichloroethylene. Several million more face carbon monoxide exposures. Back in 1987 NIOSH estimated that nearly ten million American workers are regularly exposed to organic solvents.

But NIOSH is cautious about advising employers on protecting workers from ototoxicity. "We've observed certain groups exposed at certain levels. We know that at those levels there is harm. But it's hard to extrapolate that information to other groups," explains NIOSH visiting scientist and audiology expert Thais Morata. "To be more assertive, NIOSH needs more data." Morata, who is currently conducting several ototoxicity research projects, says it's only a matter of time. Scientific evidence The data that already exists is fairly convincing. Consider the sort of cases turned up by a search of medical and scientific literature: A 1986 study reported that of 319 workers observed in a variety of job environments over a 20 year period, 23 percent of those exposed to lower noise levels (80-90 dBA) but who worked with industrial solvents suffered pronounced hearing loss. Only between 5 and 8 percent of workers exposed to higher noise levels (95-100 dBA) in nonchemical environments had impaired hearing, according to the report published in Scandinavian Audiology. A study presented in 1990 at the Fourth International Conference on Combined Effects of Environmental Factors in Baltimore showed how workers exposed to noise and a mixture of solvents including toluene, benzene, styrene, xylene and butyl acetate had an increased prevalence of hearing disorders. Workers exposed simultaneously to noise and toluene faced a 24-fold greater risk of hearing loss than workers exposed to either noise or toluene separately, a study conducted at the University of Cincinnati found. In a 1976 study of hearing damage in workers exposed to trichloroethylene, 26 out of 40 who underwent audiological testing showed hearing loss. The study, published in Minerva Otorinolaryngolica, also indicated that the longer workers were exposed to trichloroethylene, the more likely they were to suffer auditory impairments.

Scientific evidence

The data that already exists is fairly convincing. Consider the sort of cases turned up by a search of medical and scientific literature: A 1986 study reported that of 319 workers observed in a variety of job environments over a 20 year period, 23 percent of those exposed to lower noise levels (80-90 dBA) but who worked with industrial solvents suffered pronounced hearing loss. Only between 5 and 8 percent of workers exposed to higher noise levels (95-100 dBA) in nonchemical environments had impaired hearing, according to the report published in Scandinavian Audiology.

A study presented in 1990 at the Fourth International Conference on Combined Effects of Environmental Factors in Baltimore showed how workers exposed to noise and a mixture of solvents including toluene, benzene, styrene, xylene and butyl acetate had an increased prevalence of hearing disorders.

Workers exposed simultaneously to noise and toluene faced a 24-fold greater risk of hearing loss than workers exposed to either noise or toluene separately, a study conducted at the University of Cincinnati found.

In a 1976 study of hearing damage in workers exposed to trichloroethylene, 26 out of 40 who underwent audiological testing showed hearing loss. The study, published in Minerva Otorinolaryngolica, also indicated that the longer workers were exposed to trichloroethylene, the more likely they were to suffer auditory impairments.

Is anyone listening?

It's been more than 30 years since reports first surfaced about the effects of organic solvents on workers' hearing. And 12 since researchers first discovered the dangers of combined exposures to noise and ototoxic substances. But today many health and safety practitioners still aren't aware of the problem.

Ototoxicity is not addressed by the Council for Accreditation on Occupational Hearing Conservation, which, with the support of professional associations like the American Industrial Hygiene Association, the National Safety Congress, the American Association of Occupational Health Nurses, and the American College of Occupational and Environmental Medicine, oversees the nation's only OSHA-recognized training program for occupational hearing conservationists (technicians who conduct audiometric tests). Instructors may discuss ototoxicity during training if they wish, but there's no requirement to do so, according to the council's chairman, Jeffrey Morrill. Morrill guesses that few of the nation's 16,000 accredited OHCs know anything about ototoxic hazards. Nor is the risk of combined noise and chemical exposures widely appreciated among otolaryngologists (ear and throat doctors) according to Leonard Rybak, M.D., Ph.D., professor of surgery in the Division of Otolaryngology at Southern Illinois University. Rybak, who is currently conducting research on heavy metals and hearing loss, says the issue is still emerging. "It's something people haven't looked in to. There are a lot of unanswered questions."

Laurence Fechter explains the ignorance about ototoxins as a communications failure. "This is not a new issue, but it's one that has not been well publicized. Researchers like myself publish in our scientific journals and get a few reports in the press, but they have a short shelf life. There's a basic problem of communication."

NIOSH plans to do something about that. The agency targeted ototoxic- induced hearing loss for priority research as part of its National Occupational Research Agenda earlier this year. According to the agenda, "Noise is the most important occupational cause of hearing loss, but solvents, metals, asphyxiants and heat may also play a role."

NIOSH's Thais Morata says the agency is at work now writing documents to educate the public on combined exposures. The goal she says, "is to suggest that workers [exposed to ototoxins] be tested."

For now, as Morata suggested in a letter published in the British medical journal, Lancet, "The hearing protection and noise reduction strategies currently in use may be insufficient to prevent hearing loss from occupational exposures to ototoxic chemicals."

Until some formal guidance comes along, there are steps she and other experts suggest taking to protect workers at risk of ototoxic exposures: ·

• Consider offering audiometric testing to more employees, especially those exposed to noise at permitted levels who are also exposed to ototoxic substances. ·
• Begin collecting information on employees' exposure to solvents. ·
• Reduce general solvent exposures. ·
• Consider promoting the use of hearing protectors among workers exposed to ototoxins, even when noise levels are below the threshold, to prevent the combined effects of noise and solvent exposure. ·
• And keep your ears open for forthcoming information.
  
  

  For more information ...

  Fechter, L.D., "A Mechanistic Basis for Interactions Between Noise and Chemical Exposure," Archives of Complex Environmental Studies, 1(1):23- 28, 1989. Franks, John and Thais Morata, "Ototoxic Effects of Chemicals Alone or in Concert with Noise: A Review of Human Studies," Scientific Basis of Noise-Induced Hearing Loss, Thieme, New York, 1996. p. 437-444. Haybach, P.J., Ototoxicity, a site on the World Wide Web: http://www.teleport.com/~veda/ototox.html Johnson, Ann-Christin and Per Nylen, "Effects of Industrial Solvents on Hearing," Occupational Medicine: State of the Art Reviews, Vol. 10, No. 3, July-Sept. 1995. Rybak, Leonard P., "Hearing: The effects of chemicals," Otolaryngology -- Head and Neck Surgery, Vol. 106, June, 1992. The Otolaryngologic Clinics of North America, October 1993. Entire issue is devoted to ototoxicity.