More than 70,000 teaching days are lost in primary schools every year because of voice strain. A survey has revealed that teachers in 59 per cent of schools complain of the problem. A third of heads report that it has caused teachers to have taken time off. One in 20 heads say that staff took between 10 and 35 days off. RNID, the charity that lobbies on behalf of deaf people and which conducted the survey, says that the cost to schools is around pound;15 million a year.