Potential health risks of exposure to noise from personal music ...

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Health risks from exposure to noise from personal music players3.4. Hearing impairment3.4.1. Definitions and evaluationHearing impairment may be defined to include as a reduction in hearing acuity orsensitivity, or presence of tinnitus. It relates primarily to the inability of the affectedindividual to hear sounds at certain levels. This is tested by presenting of pure tones atfrequencies of 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 6 kHz and 8 kHz and is shown insteps of 5 dBHL on a chart known as an audiogram. The threshold of hearing is definedas 0dBHL on the basis of testing a number of young people. It is generally accepted thathearing thresholds lying between 0 dBHL and 20 dBHL across the frequency range testedmay be deemed within “normal” limits 15 . Thus any threshold levels at any of theaudiometric frequencies listed above may constitute a hearing loss at that frequency of agiven amount raised above normal.There are two types of hearing impairment, defined according to where the problemoccurs:• Conductive hearing impairment, which is a problem in the outer or middle ear.This type of hearing problem is often medically or surgically treatable, if there isaccess to the necessary services; childhood middle ear infection is the mostcommon example;• Sensorineural hearing impairment, which is usually due to a problem with theinner ear, and occasionally with the auditory nerve going from there to the brain.This type of hearing problem is usually permanent and requires rehabilitation,such as with a hearing aid. Common causes are ageing, excessive noise andototoxic drugs etc.World Health Organisation defines hearing impairment (www.who.int/pbd/deafness/en/)as below:Hearing impairment is a broad term used to describe the loss of hearing in one orboth ears. There are different levels of hearing impairment:ooHearing impairment refers to complete or partial loss of the ability to hearfrom one or both ears. The level of impairment can often be usefullygraded as mild, moderate, severe or profound;Deafness refers to the complete loss of ability to hear from one or bothears.15However, WHO proposed to set a limit of hearing impairment to 25 dB(A) (see Table 4)22
Health risks from exposure to noise from personal music playersGrade ofimpairment 16CorrespondingaudiometricPerformance RecommendationsISO value 170 - Noimpairment1 - Slightimpairment2 - Moderateimpairment3 - Severeimpairment4 - Profoundimpairmentincludingdeafness25 dB or better(better ear)26-40 dB(better ear)41-60 dB(better ear)61-80 dB(better ear)81 dB or greater(better ear)No or very slight hearingproblems. Able to hear whispers.Able to hear and repeat wordsspoken in normal voice at 1metre.Able to hear and repeat wordsspoken in raised voice at 1metre.Able to hear some words whenshouted into better ear.Unable to hear and understandeven a shouted voice.Counselling. Hearing aidsmay be needed.Hearing aids usuallyrecommended.Hearing aids needed. Ifno hearing aidsavailable, lip-reading andsigning should be taught.Hearing aids may helpunderstanding words.Additional rehabilitationneeded. Lip-reading andsometimes signingessential.Table 4: World Health Organisation Grades of hearing impairment (WHO, 2008)The WHO table (table 4) relates to the remediation after the acquisition of hearing lossand not to the purposes of protection to prevent noise damage. For the purposes ofprevention it is important to consider any significant audiometric threshold shift as a signof impairment. In order to prevent hearing impairment from occurring it is important toassess the sensitivity of hearing change as soon as possible. Consequently, changes inhearing sensitivity between 0 and 20 dB maybe important, especially in children andyoung people. Another area of concern is the lack of differentiation between hearingacuity or sensitivity which may be deemed “normal” for children, and young people andolder adults as hearing is considered to be “normal” below 20 dB HL for all groups.The above definitions of hearing impairment reflect one aspect of hearing that of aninability to hear sounds of certain level of intensity. Another factor of importance inhearing and understanding speech is the spectral and timing information. This is crucialfor clarity of speech hearing. Timing information is also necessary for another hearingfunction which allows localisation of sounds in space. These aspects are not consideredby the above definition of hearing impairment which considers only intensity information.Hearing impairment may therefore arise despite a “normal audiometric “threshold whichmay be due to loss of timing information. In these instances a speech test may show arelatively poor score relative to what may be expected from the audiometric evaluation.This is normally seen in neural or central nervous system lesions.Hearing impairment may also result from loss of frequency selectivity resulting in poortuning or selective listening for sounds of interest relative to background sounds whichmay distract from the information of interest. This may be assessed using frequencytuning curves or the relative strength of the efferent auditory system in modulatingincoming signals.16Grades 2, 3 and 4 are classified as disabling hearing impairment (for children, it starts at 31 dB).17 The audiometric ISO values are averages of values at 500, 1000, 2000, 4000 Hz.23
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