As schools are preparing to return, Education Minister Chris Hipkins has announced 5000 air cleaners have been ordered for New Zealand schools.

"As we know, along with vaccination, testing, good hygiene and physical distancing, good ventilation is important in minimising the risk of airborne transmission of the virus that causes Covid-19," says Hipkins.

"I've heard that schools have done a good job keeping fresh air moving through their classrooms, but we know opening doors and windows to get fresh air flow won't always be practical."

Hipkins expects the first 500 air cleaners to arrive in March and the remaining 4500 to be delivered by June.

These will be used in targeted areas within some schools in the coming months.

"To help schools identify classrooms and other spaces which get good levels of fresh air flow and those that don't, schools will receive a ventilation self-assessment toolkit with a portable CO2 monitor they can use to help identify areas of concern and the right approaches to improve ventilation.

"These 2500 portable CO2 monitors are in addition to more than 8000 Internal Environment Monitors which are already in, or will soon be deployed, in schools early this year.

"I ask any school with concerns about ventilation to reach out to the Ministry of Education for support."

Early observations of a joint study between NIWA and the Ministry of Education support opening windows and doors as the best way to boost the flow of fresh air in classrooms.

It says that good ventilation removes air from inside and replaces it with clean air from outside, preventing the build-up of potentially contaminated air. The level of CO2 in a space is a good indicator of the freshness of the air.

"This aligns with the advice and views of international experts – that is that there is no substitute for fresh air flow," says Hipkins.

"During the study there were days when opening doors and windows was less effective – for example when there was no outdoor breeze, or when it rained and schools were not able to open windows and doors as often. We know there will be cases where schools need to supplement existing natural ventilation.

"It's important we keep our tamariki as safe as possible, so in addition to the investment in portable air cleaners, the Ministry is also exploring simple systems to assist air quality and natural ventilation in schools."