The survey team randomly picked one school from each of the 18 districts, interviewing about 360 teachers, whose larynxes were examined and voices assessed.

It discovered those who sang in classes had a lower chance of having the disorder than those who did not. Women teachers were twice as likely to develop problems.

The study also established that the more students there were in a class, the more likely the teacher would develop a voice disorder. For instance, having 10 extra students made it almost twice as likely.

Teachers who taught continuously for more than 35 minutes were more prone to problems than those who taught shorter classes.

Even the teachers who did not develop a full disorder had unpleasant symptoms, for instance dry throats, fatigue and the feeling of having a lump in their throats.

Monica Chan Lui-fung, who teaches English and music at a Kwai Tsing primary school, said the ailment was common among her colleagues and some had quit their jobs because they could not stand it.

'Most of us already use microphones, but sometimes pupils get too agitated and we have to raise the roof to keep things under control,' she said. 'Many of us are used to living with a hoarse voice.'

Professor Michael Tong Chi-fai, who heads the department's academic division, suggested the education industry should offer therapies and workshops to the affected teachers and introduce smaller classes.

The issue seems more serious among primary school teachers than secondary ones.

Cyrus Hung Kai-lung, who teaches economics in a secondary school in Sha Tin, said: 'Primary students are often so naughty teachers have to shout, but I'm glad we don't.'

Voice problems also incurred costs for treatment, sick leave and the need for substitute teachers.