Tao Jiang, M.Sc., M.Sc., Aud(C) - mailto:tjiang@is.dal.ca
Nova Scotia Hearing and Speech Clinic
5599 Fenwick Street
Halifax, NS Canada B3H 1R2
Popular version of paper 2pAAa7
Presented Tuesday afternoon, June 17, 1997
133rd ASA/NOISE-CON 97 Meeting, State College, Pennsylvania
Embargoed until June 17, 1997
The detrimental effects of loud sound on the acuity of hearing were known as early as 2000 years ago. According to Bacon, one of the best-known English philosophers, Pliny the Elder first noted this in his book Historia Naturalis. Nevertheless, much of what we know today about noise-induced hearing loss (NIHL) was only accomplished in the last 100 years. Today, noise is recognized as the second most common cause of acquired hearing impairment among adult population. In addition, NIHL is ranked as one of the top ten occupational injuries in the 1990's.
Recent research on NIHL has taken on a new direction, that is, concentrating on areas not directly related to industrial activities. For example, noise at schools has attracted attention from acoustic scientists and audiologists; there is amounting evidence that excessive noise is widespread in educational settings. Poor acoustic conditions reduce speech intelligibility in classrooms and affect attention and academic performance of students. Noise makes teachers uncomfortable and can even be detrimental to their teaching. In one study, 61% of teachers found "uncomfortable" noise levels frequent at schools. Among the teachers involved in the study, physical education teachers (77%) complained of noise in gymnasia most. Furthermore, research has surprisingly shown that noises associated with certain educational activities, such as music teaching and vocational training, can even become hazardous to the hearing of both students and teachers. At the 1997 annual conference of the Canadian Association of Speech Language Pathologists and Audiologists, Tao Jiang presented a well-documented case about a music teacher. The young female music teacher suffered noise-induced hearing loss likely caused by long hours of music teaching. In a survey involved in 27 regional vocational/technical schools in Massachusetts, Allonen-Allie concluded that noise was likely to cause hearing damage in these educational settings and called for immediate intervention. There have also been legal cases against schools in relation to school noise.
Unacceptable noise levels have long been noted in school gymnasia. In 1988, C. Truchon-Ganon conducted a series of noise measurements in eight school gymnasia in Quebec, Canada. They found that the gymnasia were as noisy as in factories. In 1996, Tao Jiang surveyed noises in an elementary school gymnasium. The average sound level reached 94.4 dB, 10 times greater than that of the safety limits established for industrial workers. This level is equivalent to that of jack hammers!
Although the concern with noises in gymnasia is growing, nevertheless, cases are rare that physical education teachers are actually at risk of NIHL. Here, we report a unique case of a 57-year-old physical education teacher who has been recently diagnosed with a permanent hearing impairment linked with teaching. This case gives us a valuable opportunity to find out whether noise-induced hearing loss is related to sound levels in the gymnasium.
At the age of 57, David (not his real name) had been a physical education teacher for 27 years. He came for a hearing evaluation because of his complaint of "missing sounds" and ringing in the ears. He had no history of industrial noise exposure and other ear problems. Besides teaching, he also coached hockey regularly at schools. He contributed his hearing difficulty to high noise levels in the gymnasium.
Before making any judgement as to the cause of his hearing loss, we should answer two fundamental questions: (1) does David have an NIHL? and (2) is his teaching environment noisy enough to cause his hearing loss? To answer the first question, we conducted a complete hearing test on him. The results convincingly showed David had an NIHL. Figure 1 depicts his audiogram, a classical example of an NIHL evidenced by a notch at high frequencies.
Since David was 57 years old, one may argue that his hearing loss was probably related to his aging. To rule this out, the best way is to compare his hearing loss with that of an established reference population of the same age and gender but without any noise exposure. Logically, if he has an NIHL, by comparison, the degree of the loss must be greater than that of the reference population. The dashed line in Figure 2 represents averaged hearing levels of an age and gender matched population adapted from the ISO1999 standard. Undoubtedly, the extra amount of his hearing loss must be caused by something else other than aging.
Are noise levels in the gymnasium intense enough to cause hearing loss? To answer this, we conducted a noise study in one gymnasium where David taught. Table 1 summarizes the results of the noise measurements using a sound level meter. The acoustic conditions were poor with high reverberation in the gymnasium. Screaming and shouting were the main noise sources, further compounded by sound of whistles and balls hitting on the hard floors. The sound levels ranged between 72 and 119 dBA during classes. One teacher and four students wore dosimeters to record actual amounts of noise during classes. The results are presented in Table 2. The levels ranged between 90.8 dB and 106.41 dB. Again, the sound levels were similar to industrial noises. In this case, the projected dose for the teacher was 400%, exceeding the safety limits by 300%! It means that under such a noisy condition, he should only teach two hours a day. Any time more than that is overexposure, thus posing a great danger to his hearing.
Another interesting we did was to assess in situ spectral characteristics of whistle sound emitted in the external ear canal. Because teachers in gymnasia commonly use whistles, therefore, it is important to find out whether their use can add more harmful sound energy. In the laboratory, we tested an ACME whistle approved for use in 41 countries. We used a procedure called the real ear measure. A probe microphone was inserted into the external ear canal of the experimenter at 6 mm in front of the eardrum. While the whistle was being blown, its levels and spectral information were simultaneously recorded by an analyser. The advantage of this method is to capture the actual amount of sound energy reaching the eardrum, giving us a more realistic picture of sound effects.
Figure 3 presents a graph of the whistle sound, a narrow band sound centring at 3150 Hz. Its level reached 130 dB SPL. This value was greater than that measured in the sound field probably because of the external ear resonance effect that amplifies high frequency sound entering the outer ear. Consequently, more harmful sound energy was delivered to the eardrum. We believe that the use of whistles was one of the etiological factors contributing to his hearing loss.
Having ruled out both aging and other medical conditions as causes of his hearing loss, it is confirmed that David suffers permanent NIHL. The field noise study shows that the noise levels in the gymnasium exceed the safety limits, thus constituting a hazardous environment where permanent hearing loss is likely to occur. Furthermore, the blowing of whistles is capable of generating and delivering sound energy as much as 130 dB, equivalent to aircraft noise, at the eardrum. For a physical education teacher who has frequently used whistles for 27 years, the risk of developing the NIHL becomes much greater in this case. Therefore, the evidence has proved that David's hearing loss is caused by noises at his school gymnasia. It is safe to state that we should not only treat noise at gymnasia as a detrimental factor to teaching and learning, but also consider it as a potential occupational hazard to the health of teachers.