I was very pleased to be consulted for the article on disease risks to schoolchildren (4 November).

I am afraid, however, that you misunderstood what I said. My observation was that mechanical ventilation systems need to be properly cleaned, tested and maintained to ensure that they are kept free from anything that may contaminate the air in accordance with the regulations ACOP 33. This doesn’t actually have anything to do with Legionella, which occurs in stagnant water at temperatures between 20°C and 50°C, not in air ducts.

The wider issue, of course, is the link between poor indoor air quality and poor educational outcomes and the role that mechanical systems can play by being able to guarantee the ventilation rates and therefore the air quality in densely occupied classrooms. It is an area that Whitbybird is researching and our initial results demonstrate beyond any doubt that the carbon dioxide levels (one measure of air quality) in naturally ventilated rooms are unacceptably

high for long periods of time. Mechanical ventilation is often seen as the more expensive option, but our whole-life cost study work shows that it can be better than natural ventilation systems financially and environmentally when the savings in fresh air heating are taken into account.