BMC Public Health 2008, 8:151 http://www.biomedcentral.com/1471-2458/8/151 ital hearing deficiency (genetic or acquired); reduced external acoustic ear canal; and presence of excess earwax or otorrhea. Using the criteria outlined above, we excluded 19 subjects from the sound technician group because 11 of them had been exposed to sound-producing activities for less than five years and eight subjects were less than 19 years old. In the control group, we excluded seven subjects: six because they did not fulfill the inclusion criterion regarding age and one because of the presence of excess earwax. The control group was selected by invitation. The partici- pants in this group were health professionals, students and employees at the Federal University of São Paulo, Bra- zil, who were unaware of the possible presence of hearing loss. The evaluations and questionnaires were applied in the Brazilian Cochrane Center and in the studios of the televi- sion channels Record TV and Cultura TV, in São Paulo, Brazil. Portable acoustic booths were used when conduct- ing the audiometric examinations. Rooms were deemed to be silent room when the noise level was between 55 and 60 dB, in accordance with Administrative Technical Standard no. 3214 of the Ministry of Labor and the Work- place Safety Office [10]. The sound pressure levels in the examination locations were measured using devices that conformed to the stand- ard ANSI S1.4-1983. These made it possible to check whether the environment within which the booth had been set up was appropriate for conducting audiometric assessments. The sound pressure levels in the places where the audio- metric assessments were carried out were measured using the TES 1350A sound level meter. In the Record TV studio, the device measured a minimum of 57 dB(A). In the Cul- tura TV studio and in the Brazilian Cochrane Center, the estimated minima were measured as 41.6 dB(A) and 57.8 dB(A), respectively. All three locations where acoustic measurements were made were within the above men- tioned standard set by the Ministry of Labor and the Workplace Safety Office [10]. Individuals who presented a diagnosis of hearing abnor- malities during the examination (at any of the hearing fre- quencies assessed), or a diagnosis of obstruction, were referred to the outpatient clinic of the Speech-Language- Hearing Service of Hospital São Paulo, the Federal Univer- sity of São Paulo, Brazil, for detailed evaluation and fur- ther procedures as required. We investigated the participants' hearing levels at the fol- lowing frequencies: 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 3000 Hz, 4000 Hz, 6000 Hz and 8000 Hz. We also per- formed speech audiometry [11]. The subjects were considered to present high frequency hearing loss consistent with noise exposure if their audio- grams at the frequencies of 3000 and/or 4000 and/or 6000 Hz presented hearing thresholds that were at least 25 dB higher than the thresholds for the other tested fre- quencies (i.e. 250 Hz, 500 Hz, 1000 Hz, 2000 Hz and 8000 Hz), when this compromised hearing was observed in both the air test and the bone test, in one or both ears [4]. The sound technicians were asked to take a 14-hour hear- ing rest prior to the test, in accordance with Administra- tive Technical Standard no. 19 of the Ministry of Labor and the Workplace Safety Office, dated April 22,1998 [4]. We applied a questionnaire about music listening habits (we asked whether participants were going to discos and/ or listening to walkmans or iPods at least once a week, in the past or present) and use of hearing protection devices. Participants in the sound technician group were consid- ered to be using hearing protectors if they reported using them for at least 20% of their total time at work. For exam- ple, if an individual worked as a sound technician for nine years but used hearing protection devices for only two years (9 × 20% = 1.8 years), it would be deemed that this individual had indeed used hearing protection. We also gathered data on correlated complaints, ototoxicity expo- sure and length of exposure to sounds. Visual inspection of the participants' ear canals was performed using an oto- scope, in order to rule out any presence of obstructions that would alter the examination results [10]. A basic tinnitus assessment was performed, including an audiological evaluation and a self-reported assessment. We also defined tinnitus as a ringing or other type of noise generated within the head [12]. The audiological evaluation was performed using the portable AD229 E digital audiometer, calibrated accord- ing to the standard ANSI S3.6 (1989) [4,13,14]. Sample size To determine the sample size to be used for comparing hearing loss prevalences between the sound technician group (PPE) and the control group (PPNE), pairs of ratios were compared by means of the equation proposed by Pocock (1983) [15]: n = (PPNE(1(−PPPPNNEE−)P+PPEP)E2t (1−PPE) Θ2 Page 3 of 8 (page number not for citation purposes)