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Literature data show that teachers are exposed to increased risk for diseases of the phonatory apparatus. This circumstance is explained by the high phonatory load of teaching and by classroom noise. As passive acoustic requirements of the school buildings are often not respected, the intelligibility inside the classrooms is poor, increasing the teacher’s voice loudness. This work is aimed at studying relations between long reverberation times, high noise levels and vocal effort.Phonatory effort was evaluated in three school complexes, on a sample of nursery (6 subjects), primary (9), junior high (2) and high (8) school teachers. The classrooms were classified as acoustically treated or not. Speech intelligibility was quantified by the speech transmission index (STI). Phonatory doses were correlated to noise exposure levels and to classroom acoustics. The phonatory effort was evaluated with the Ambulatory Phonation Monitor (Kaypentax).Vocal effort was associated to school degree, showing an increasing at the primary school where the pupils are noisier. A significant association was found between the sound level evaluated 15 cm from the speaker’s mouth and the noise level (coefficient=1.45, p value=0.016). Multivariate linear regression models explained the sound pressure level emitted by the speaker (SPL) by phonation percentage and reverberation time. Average reverberation times at 1 kHz were 0.8 and 2.3 s, while average STI, evaluated by a theoretical model, were approximately 0.6 (sufficient) and 0.4 (poor), in treated and untreated classrooms, respectively. In untreated classrooms the SPL was on average 30% higher (p=0.041).The high phonatory load of teaching is significantly worsened by the classroom inadequate acoustic parameters. Long reverberation times and, consequently, poor speech intelligibility, high noise levels and vocal effort are significantly related. Improved acoustic design of the classrooms would reduce the risk for vocal apparatus pathologies.