Teachers should open classroom windows between lessons to help reduce the risk of Covid spreading in schools, an expert has said.
Professor Tim Sharpe, a government advisor, said good ventilation was one way of getting fresh air into often overcrowded school buildings to help drive down infections.
The scientific expert from the University of Strathclyde said: "It's now well established that there is some risk of airborne transmission.
"It is difficult to quantify, but there is some risk so using ventilation is a really good way of mitigating that."
Ministers are coming under pressure to provide ventilation units for all schools to help curb transmission in overcrowded buildings.
They have already announced that all state schools will get CO2 monitors this term, at a cost of £25m, to identify where ventilation needs to be improved.
They have also launched a trial of air purifiers in 30 schools in Bradford after evidence suggested proper ventilation reduces the spread of the virus 14-fold.
Heads have been advised to hold classes and assemblies outdoors where possible.
But Prof Sharpe said there was a balance to be had between good ventilation and making sure people are not "freezing cold".
He added: "It doesn't mean opening all the windows all the time, it's a managed process and obviously we're balancing things.
"We want to get reasonable levels of ventilation, but we don't want people to be freezing cold, so trying to sort of get that middle ground is something to aim for."
But while he said that air cleaners could work, they should be "an option of last resort" in rooms which cannot be ventilated in any other way.
