A £3.3m scheme to provide schools in Wales with machines that disinfect classrooms after a Covid outbreak has prompted calls for reassurance over their safety.

The Welsh education ministry said on Monday that all schools, universities and colleges in Wales would be supplied with ozone disinfecting machines for cleaning up Covid-hit classrooms.

Plaid Cymru has raised questions about toxic chemicals contained within the technology. And campaigners questioned why the Welsh government was turning to a harmful substance in schools when safer alternatives are available.

The 1,800 machines, currently in production, will cost £3.3m as part of £5.9m initiative to improve air quality in classrooms and lecture halls.

Developed at Swansea University, the machines convert oxygen in a room to ozone, a chemical that kills Covid in the air and on surfaces, along with other viruses and bacteria. Once a room has been disinfected, the machine, each the size of a suitcase, converts the ozone back to oxygen. But ozone is so toxic that no one will be allowed inside the room when the machine is operating, according to Dr Chedly Tizaoui, who was part of the design team.

Speaking to the Guardian, he said: “When we apply the ozone, the room has to be closed and no one is allowed to be in the room, including pets and animals.

“That is the condition of the application. It is like the use of chlorine, you don’t want to be in an environment where chlorine is dispersed at high concentration. Ozone smells but people are not allowed to smell it. That is extremely important from a safety point of view.”

He said the machines would be programmed to give cleaners time to leave a class room while the machine was in operation. “Once the full cycle is finished then it is safe to go back to the room, and the machine will signal to the cleaner that it is safe to return,” Tizaoui said.

He added that they could be password protected to guard against misuse, and that a strict safety guidelines would be required before they were rolled out.

He said: “They are safe if operated properly. Like a car, if you drive it at 100mph in a 30mph hour limit that is not safe.”

Asked whether the benefits outweighed the risks, Tizaoui said: “It not just a benefit for tackling Covid, ozone is also effective against other seasonal viruses.”

Announcing the initiative, Rebecca Evans, the minister for finance and local government, said: “By investing in new technology such as ozone disinfecting machines, we’re ensuring learners can stay in school and colleges as Wales moves beyond the pandemic.”

But Dr Eilir Hughes, a GP on the Llŷn peninsula and a member of the Fresh Air Wales campaign, was not convinced. He said: “If you have to put in place a lot of health and safety guidelines in rolling it out then it really needs to be worth it.

“Adding a toxic substance to the environment raises concerns about how it will react to chemicals particularly in soft furnishings. Is it sensible to be testing this in schools?”

In an article published online on Monday in Nation Cymru he wrote: “Using ozone to disinfect does seems counterintuitive. In an attempt to lower the risk of harm to human health, we are using toxic chemicals when safer alternatives are available.

“And suggesting that disinfecting potentially infected air when no one is present to breath it is an insult to our intelligence.”

Hughes argued that better ventilation in schools was safer and a more effective means of tackling Covid in the classroom.

He said: “Instead of rolling out expensive, untried, unnecessary technology that has the potential of being damaging to the environment and dangerous to human health, we should place our efforts on interventions that provide the greatest benefit in reducing risk … If natural ventilation is provided for indoor settings, transmission reduces by up to 70%.”

Plaid Cymru’s education spokesperson, Siân Gwenllian, said: “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 spokesman said the machines were only for disinfecting empty indoor spaces. He said: “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.”

The Welsh government will also be providing CO2 sensors to schools, colleges and universities to improve ventilation, after a similar move in England.