BMC Public Health 2008, 8:151 http://www.biomedcentral.com/1471-2458/8/151 Where Θ = Zα + Zβ, such that Zα and Zβ were respectively the confidence scores linked to the confidence intervals (CI) (1-α) × 100% and (1-β) × 100%. The prevalences of hearing loss among a group of percus- sionists and in the general population were estimated by Cunningham (2006) [16], respectively, as PPE = 0.39 and PPNE = 0.9. In the present study, it was intended to carry out the comparison at a significance level of α = 5% with an elasticity trait of 1-β of 95% and a two-tailed hypothe- sis test. Therefore, the equation obtained was Θ = Zα + Zβ = 1.65 + 2.94, thereby recommending the use of at least 50 participants in each group. Statistical analysis In the inferential portion of the study, we used Student's t test for homogenous independent samples, whenever the comparison variable (outcome variable) was continuous, presented normal or near-normal distribution (evaluated by the Kolmogorov-Smirnov test) and presented equal variances (evaluated by Levene's test). To compare ratios, we used the chi-square test or Fisher's exact test, in accord- ance with the conditions established by Cochran (1954) [17]. The tables and statistical calculations were built up in the SPSS software for Windows, version 13.0. P-values less than 0.05 were taken to be statistically significant. Results The minimum sound pressure levels for each of the sound technicians' workplace environments were measured in the studios of the television channel Cultura TV, in order to exemplify how much noise the workers were exposed to. In the audio operators' studio, 101 dB(A) was detected. In the sound mixing work area, videotape operators' work area and microphone operators' work area, the measure- ments were respectively 91.1 dB(A), 90.7 dB(A), and 94.2 dB(A). In the videotape editing and sound monitors' areas, the measurements were 89.1 dB(A) and 86.6 dB(A), respectively. Table 1 shows the distribution of participants regarding age, gender, hearing complaint, music listening habits and ototoxicity exposure in the groups studied. It shows that the predominant age group of the sound technicians was 25 to 35 years old (40.2%) and among the control group (54.7%), but without any significant statistical dif- ference between age range frequencies (all age ranges pre- sented p > 0.05) or between mean ages (p = 0.122). There was no statistically significant difference regarding the ratio of female professionals (p = 0.718) in the study groups. However, a significant statistical difference (p = 0.009 and p = 0.001) was observed between the ratio of hearing complaints and tinnitus, respectively, in the sound technician group and the ratio in the control group. The data suggest that there were a greater proportion of complaints in the sound technician group (26.8%) than in the control group (11.6%). There was no statistically significant difference between the two study groups in relation to music listening habits and ototoxicity expo- sure. Table 2 shows the distribution of the participants in the sound technician group, according to the hours worked per day, length of time working as a professional and the use of hearing protection devices. It can be seen that the sound professionals predominantly worked for 6 to 12 hours per day (57.3%). Most of them had been working for between 5 and 14.9 years. However, most of them were not using hearing protectors (85.4%). Table 3 shows the distribution of participants according to the presence of hearing loss in both study groups. A statis- tically significant difference between the sound technician group (50%) and control group (10.5%) can be seen in relation to high frequency hearing loss consistent with noise exposure, as demonstrated by the chi-square test (p < 0.001). Discussion Workers exposed to noise levels of 85 dB have a permissi- ble maximum daily average exposure of only eight hours and workers exposed to noise levels of 103 dB(A) have a permissible maximum daily average exposure of only 7.5 minutes [18]. However, most of the participants in the sound technician group at Cultura TV and at Record TV exceeded the sound level permitted for eight hours, given that 57.3% of the workers examined in this study were working for between 6 and 12 hours per day and 13.4% were working for more than 12 hours a day in a place proven to be inappropriate for hearing (Table 2), because the sound pressure level was above the permissible daily level. Most of the participants in the sound technician group said that they had been doing that type of work for between 5 and 14.9 years (47.6%) (Table 2). Studies on different populations exposed to sound have also demon- strated inappropriate mean sound levels of between 108 and 110 dB(A) [19-30]. We considered that individuals whose audiograms pre- sented thresholds of less than or equal to 25 dB at all the frequencies examined presented normal hearing, in accordance with the standards of the Workplace SafetyOf- fice[4]. According to Santos & Morata (1999) [31], factors such as gender and age have been described as worsening factors for high frequency hearing loss consistent with noise exposure. However, in our study, these possible con- founding factors were controlled for, because there was no statistically significant difference between the sound tech- Page 4 of 8 (page number not for citation purposes)