Hearing Threshold of Korean Adolescents term or chronic exposure to loud noises.11 Noise exposure is an important risk factor for hearing loss. Although exposure may be involuntary, many indivi- duals are exposed voluntarily to noise, including people who listen to music using personal music players. The use of ear protection caps or the lowering of noise has been recommended for industrial workers who are exposed to an equivalent sound level (Leq) of > 85 dB of noises for 8 hours a day. But, typical listener has the volume at a range between 75-100 dB12 and individuals who listen to 15 minutes of music at 100 dB using personal music players may be exposed to the same level of loudness as industrial workers exposed to 85 dB in an 8 hour day.13 Thus, NIHL in juveniles may be caused by the use of personal music players. The impact of exposure to noise on hearing level can be affected by various factors. For example, susceptibility to inner ear injury due to noise was found to depend on perso- nal susceptibility factors, including differences between left and right ears and time of day, suggesting that inner ear injury may result from genetically variable responses to noise exposure.14 Indeed, we found that male students had significantly higher hearing thresholds than female students, and right ear hearing thresholds were significantly higher than left ear thresholds. Although age was found to be an important risk factor associated with NIHL,14 the study was performed in subjects aged 65 years and older, and thus, is probably not related significantly to our study, which was performed in teena- gers. We found that hearing threshold levels were higher in middle school students than in high school students. This difference may be associated with a relatively higher expo- sure of middle school students to noise at a younger age, due to recent introduction of new types of personal music players, including the MP3 player. Although there was no significant association between hearing threshold and daily use of personal music players, we found that students who had used these players for more than 5 years and students showing cumulative usage times of 15 hr × years had significantly elevated hearing thresholds at 4 kHz. These findings indicate that cumula- tive long-term use of personal music players can have a debilitative effect on hearing levels. Students who used speakers to listen to music had signi- ficantly lower hearing thresholds than those who used earphones and headphones. Furthermore, although not statistically significant, students using headphones had lower hearing thresholds than students using earphones. Earphones are smaller in size, more conveniently carried, and more comfortable to wear than headphones. Moreover, unlike speakers, earphones can be used in public places. Earphones are, therefore, preferred by most individuals; indeed, we found that 81.4% of the students surveyed used earphones. By blocking the external auditory canals, however, earphones can elevate output power by 7-9 dB compared with headphones that cover the entire external ears.15 In addition, more users of earphones than of spea- kers tend to elevate the sound volume to cope with envi- ronmental noises. Headphones may have effects similar to, but not greater than, those of earphones. These findings indicate that the use of earphones or headphone has a greater impact than the use of speakers in the development of hearing disorders. Hearing thresholds of juveniles who used MP3, cassette tape, and CD players were similar, whereas hearing thresholds of students who used com- puters were significantly lower. The superior hearing in the latter is likely due to their listening to music using speakers. Chronic exposure to loud noise can cause noise-induced hearing loss by damaging hair cells in the organ of Corti. At birth, humans have about 16,000 hair cells in each ear, but damage to 30-50% of these cells can cause hearing level debilitation.16 Noise-induced hearing loss is irrever- sible in the absence of effective treatments, and hearing aids improve hearing levels only slightly, making the prevention of NIHL imperative.17 Hearing loss in juveniles can lead to many difficulties in future life, making the prevention of noise-induced hearing loss especially impor- tant in young people. Studies have found that intake of vitamin and mineral rich foods, appropriate exercises, maintenance of clean oral hygiene and non-smoking are helpful in preventing the development of hearing loss.18-21 In juveniles, however, the prevention of NIHL has to include avoiding of loud noises produced during recreation and leisure activities, maintaining lower volumes when using personal music players in quiet places, avoiding long hours of personal music player use and not using earphones. Although there was a limitation for the ratio of female and male in the subjects that were selected, we found in this study that most juveniles in Korea use personal music players, and that most listen to music using earphones. Most adolescents included in the present study had used these players for about 1-3 hours per day for 1-3 years. In this study, the outstanding point was that cumulative usage and the use of earphones or headphones had deleterious effects on hearing levels. Therefore, to conserve hearing level, juveniles should avoid continued long term use of personal music players and use speakers when listening to music. In our study, types of earphones were unconsidered as the factor affecting NIHL. However, there are many type of earphones in the market, though almost all types of earphones are semi-insert type in Korea. Thus, further examination may be needed for the types of earphone. Yonsei Med J http://www.eymj.org Volume 50 Number 6 December 2009 775