The Welsh government said it would pay for 30,000 CO2 sensors and 1,800 ozone disinfecting machines developed by Swansea University.

The National Association of Head Teachers said "clear guidance" was needed on what action to take when poor air quality was identified.

Plaid Cymru called for safety guidance regarding ozone disinfecting machines.

Dr Eilir Hughes, a GP from Nefyn, Gwynedd, and a leading member of the Fresh Air campaign, said he was "really pleased" the Welsh government was taking the importance of ventilation seriously.

"We know that this virus spreads in the air and any measures you can introduce to reduce that risk of airborne transmission is welcome.

"I've been campaigning for these measures for some time and involved in rolling them out on Anglesey."

'Canaries in a coalmine'

Dr Rhys Thomas, a consultant in anaesthetics at Glangwili Hospital, said: "It's the air that infects you, not surfaces. It's breathing in somebody else's air that'll do it.

"Delta is far more infectious and dangerous, and transfers more readily in the air.

"We have to bring in multiple mitigations to defeat this virus. We need fresh air and clear air in indoor environments. CO2 monitors are our canaries in the coalmine. They'll start warning us that there's not enough fresh air into that room."

The Welsh government said it hoped to roll out the new technology during the autumn term, but schools on Anglesey have already been using CO2 monitors for over a year.

Council leader Llinos Medi said the equipment was there to "reassure" staff and pupils about the flow of air within school buildings.

"We need to throw everything at this. This is part of the answer," she said.

"It's disappointing that its nearly a year on since Ynys Môn made the decision, and the Welsh government is now making its decision.

"The practicality of getting them bought and set up will be extremely challenging."

Plaid Cymru's education spokeswoman, Sian Gwenllian MS, said her party had been calling for more guidance and resources for schools, colleges and universities about ventilation since last year.

"The provision of CO2 monitors to educational settings, while long overdue, is welcome, and is in line with other nations.

"It's important that measures we use are in line with the latest scientific guidance and I urge Welsh government to provide reassurance to educational settings on the use of devices such as ozone disinfectant systems.

"The use of ozone disinfecting machines is controversial to say the least and we all need to be satisfied that Welsh government is absolutely certain that they are a safe option before introducing them."

A Welsh government spokesperson said: "The ozone disinfecting machines, developed through Swansea University, are specifically for the disinfection of empty indoor space.

"These machines have been developed to speed-up the decontamination of classrooms following a confirmed outbreak of Covid-19 only, and not as a form of air purification for occupied indoor spaces."