The Government is not against Hepa filters in schools, the Taoiseach has told the Dáil.
Micheál Martin told the Dáil during Leaders' Questions that the Government will make funding available "in some circumstances" for Hepa filters.
The Taoiseach said that "where possible, fresh air is the most effective form of ventilation" but that there are issues in keeping windows open during spells of cold weather.
A number of opposition leaders asked the Taoiseach about the air filters, which have yet to be rolled out in schools.
In response, Mr Martin said that it might cost as much as €90m to kit out every classroom in the country with a filter, but he said that they were not silver bullets.
"There has been a very significant wave of Delta in primary schools. Funding will be made available but there could be different solutions for different schools around ventilation and air quality. Some may be more medium term.
"The immediate short-term solution is portable Hepa filters or some improvement of that kind. Funding will be made available for that. It is not the silver bullet by any means," he said.
"We are not against Hepa filters. We are for them and will provide resources for them. I have said this three times today already.
"There are 50,000 classrooms across the country, between primary and post-primary schools. The department and the minister are adopting a targeted approach to address ventilation issues, which may include air cleaners, where required."
People Before Profit's Richard Boyd Barrett said that it was "known that Covid is an airborne disease", while Sinn Féin's leader Mary Lou McDonald urged the Government to "use the Christmas break, use the opportunity of the kids being on holidays, to install these ventilators and to ensure that we have clean air and that we do not have frozen children and staff members".
Earlier, Mr Martin said that tens of thousands of additional vaccine appointments had gone unused in recent weeks. He said that up to 120,000 appointments had not been taken up.
"Vaccination is protecting people, more than anything else, against severe illness, hospitalisation, and admission to ICUs," he said.
"The booster vaccination jabs have been shown to produce very strong antibody responses and are likely to provide protection against severe disease, hospitalisation, and death from most variants, including Delta and Omicron. The reason I am saying this is that in the week commencing 22 November, 208,000 appointments were offered, of which about 80,000 turned up. Last week, of 180,000 appointments, 93,000 turned up.
"The same urgency does not seem to be there in respect of availing of the option to take the booster as was there when we were offered the first and second doses.
"In the over-75 group, where people have been taking up the offer of a third dose of a Covid-19 vaccine in high numbers, we are seeing a significant reduction in the incidence of the disease, so it does work.
"The booster will have a really significant impact on Delta. I cannot stress that enough."