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

Health risks from exposure to noise from personal music playersstandard ISO 1999 in 1975 and has been embodied in the National Physical Laboratory(NPL) tables that are still used widely for prediction of NIHL in populations exposed tonoise. The later version of ISO 1999 in 1990 (ISO 1999:1990) synthesised data fromstudies in the US as well as from the studies of Burns and Robinson to derive formula forpredicting NIHL. An advantage of ISO 1999 (ISO 1999:1990) is that it allows the user toinsert different values to account for the effects of age-associated hearing loss. Thisfacility has enabled ISO 1999 to keep up with developing the current understanding ofthe effects of age on hearing and the recognition that there are important socio-economicfactors governing hearing acuity. This is an important achievement because the nonexposedcontrols used in many studies of NIHL have been drawn from different socioeconomicgroups than the exposed participants (e.g. office worker, researchers,university staff).All of the above methods account for the combined effects of age and noise exposure bysimple addition of the hearing losses from the two origins, or by a slight modification ofsimple addition. The modified addition incorporated in ISO 1999 (ISO 1999:1990) slightlyreduces the resultant hearing loss compared to simple addition. However, this effect isnegligible for combined hearing loss lower than 40 dB and for the present purposes canbe ignored.ISO 1999 allows prediction of the distribution of NIHL to be expected from anycumulative amount of noise exposure. This is combined with (in most cases simply addedto) distribution of age-associated hearing loss appropriate to the population in question.This calculated distribution of NIHL allows estimation of the probability that a givenmagnitude of overall hearing loss will be exceeded. In the context of the present study,noise levels in the range from 80-95 dB(A) are of interests. Based on ISO 1999, thefollowing table (Table 5) shows the extent of NIHL to be expected from a working lifetimeof 45 years at daily continuous noise levels of 80, 85, 90 and 95 dB(A). The values arefor NIHL at 4 kHz, which is the frequency predicted to give the greatest hearing loss.Values are given for the median and the 5 th centile (value exceeded by 5% ofpopulation). These data constitute the noise-induced component of hearing loss alone.Note that hearing loss is minimal for exposures at 80 dB(A), even at the 5 th centile, andincreases at higher levels.NIHL at 4 kHz in dBDaily noise level in dB(A)80 85 90 95Median 1.7 6.6 14.9 26.55 th centile 2.2 8.8 19.6 35.1Table 5: NIHL predicted from ISO 1999 as a function of noise exposure level for 45years (adapted from Lutman et al 2008)Table 6 shows similar data for the much shorter exposure durations of 3 years, which ismore relevant to the present opinion. Note that in that case, NIHL is less likely than after45 years of exposure, as expected. However, the proportion is greater than simplydividing the amount of hearing loss pro rata. To a rough approximation, the magnitude ofNIHL after 3 years is 43% of the NIHL after 45 years. The use of this model suggests adeparture from the equal energy principle in the direction that more NIHL occurs in theearly years of exposure and clearly suggests that preventive measures must be aimed atthose who start noise exposure from PMPs when young. Note that at 4 kHz there is moredamage earlier; at 1 kHz the damage is less and is later. Hence overall damage to thecochlear can be related to the equal energy principle. ISO 1999 accommodates this trendby assuming that the noise level has to exceed 90 dB(A) to affect hearing at 1 kHz and93 dB(A) to affect hearing at 500 Hz, but 77/78 dB to affect hearing at 4 kHz.28