Calls have been made for HEPA filters to be installed in all Northern Ireland schools to help reduce Covid transmissions.
There are just over 1,000 schools here, but only 200 HEPA units are in place.
High Efficiency Particulate Air (HEPA) is a common component in air purifiers and is frequently used in vacuum cleaners and industries that need high air quality.
Education Minister Michelle McIlveen said the most effective mitigating measure in relation to Covid-19 is natural ventilation and that her department and the Education Authority are supporting schools in prioritising the effective implementation of this.
"Air cleaners were reviewed in detail by SAGE and its environment and modelling group concluded that there is currently very little evidence that air cleaners are an effective control to prevent Covid-19. However the principle of air cleaning suggests that they may be useful in some cases and may be suitable for spaces where there is insufficient ventilation and the ventilation can’t be improved," she said.
"Given that advice from SAGE, my department does not plan to install HEPA air filtration units in all schools.
"All schools have received guidance in relation to ventilation, and supplied with CO2 monitors to assist in identifying areas where ventilation needs to be improved. Schools with concerns about ventilation have been advised to contact the EA maintenance helpline for advice/support on the best approach for their individual circumstances.
"There is a sufficient stock of air filtration units to meet the current demands and additional units will be purchased if there is a requirement for them."
SDLP MLA Sinead McLaughlin said: "The Education Minister’s refusal to take on board the research and evidence that HEPA filters help drive down the transmission of Covid-19 in our schools is a source of deep frustration.
"I have been highlighting the benefits of these filters for some time and the minister and her department have failed to act, typifying their poor response to the challenges posed to our schools by the pandemic.
"As we head into winter, it will be harder to keep our schools properly ventilated with the onset of cold weather. We have had politicians, schools and medical experts calling for these filters to be bought and installed for some time and yet there have only been 200 distributed across the north, with seemingly no plans to acquire anymore at this time.
"Throughout the pandemic, our schools have faced significant challenges as a result of Covid-19, many teachers have been forced to take time off and pupils have had their education disrupted as a result of this virus.
"We should be doing everything within our power to keep both teachers and pupils safe and to allow our schools to operate in the most normal way possible and I would urge the minister to reconsider her stance on this important issue."
Figures from the Office of National Statistics released last Friday show that Northern Ireland is the only UK nation to have seen a week-on-week rise in Covid cases, while across the rest of UK, infections had fallen for the first time in two months.