The science is clear - you can lower your risk of catching Covid-19, and also other airborne diseases, by not breathing in stale indoor air.

That means it’s time for some ventilation advice. "Crack a window open" is the Government's most important message, and it’s a good one.

But it's also not always that easy, particularly not in the middle of a La Niña winter.

So what are the ways to ventilate the indoor spaces you want to keep living and breathing in? And when and how often should we do it?

At home:

Ventilation at home is a key part of preventing Covid-19 spread in Aotearoa at the moment – your family members and flatmates are close contacts for a reason and the most likely people you’ll catch the virus off.

Breaking those chains of transmission is something we should all be striving to do to lower the peak of this latest wave.

We are dealing with an airborne pathogen and therefore one of the easiest ways to stop it spreading is to regularly recycle the air we are breathing.

Flatting can make preventing Covid spread challenging.

Peter Meecham/Stuff

Flatting can make preventing Covid spread challenging.

Crowded living rooms and bedrooms on a cold winter's day can very quickly have stale air – most easily recognised by an increase in CO2 levels.

For a variant like BA.5 (the most transmissible variant to take off since the pandemic began), a close-contact stale air environment is nirvana.

So it’s important to crack the windows at home if you can, everywhere you can, and regularly.

This is more important in a Covid-19 context than wiping down surfaces or doing a deep clean of your house, though hygiene is obviously important.

Outgoing Director-General of Health Ashley Bloomfield says 15 minutes of regular window opening is enough, but open all windows if possible.

If you can't open a window because you're in a high-rise flat, or because you're living in Bromley, there are other ways to replace air.

Not many homes in New Zealand have full Heating, Ventilation and Air Conditioning (HVAC) systems set up, but most of them probably have either a kitchen fan or a bathroom fan.

Turn one of them on for a bit, and you will shift some stale air out of the room, Bloomfield says.

If you're creative, there's also a ventilation home hack that you can try here, using a desk fan – that one’s courtesy of aerosol chemist Dr Joel Rindelaub, of Auckland University.

At work:

Good ventilation is particularly important in two main areas - where the risk of transmission of infection is high, or where there are vulnerable people, such as disabled or older people.

An office workplace is one area where the risk of transmission can become quite high.

If you're sitting at a desk for eight hours, breathing in and out, with dozens of others around you, the air can get pretty stale, pretty quickly, particularly if windows can't be opened.

The good news is that most commercial buildings do have HVAC systems.

Get to know your HVAC system in your workplace.


Get to know your HVAC system in your workplace.

These systems are mostly known for heating or cooling the air (and there's always one person in the office that wants it warmer) but they can also exchange stale air for fresh air and can be used to alter humidity, and remove pollutants, pollen, or dust from the air.

They can't remove Covid-19 from the air if someone's come to the office with the virus, but the exchange of air being performed by an HVAC helps dilute its spread.

It is important to understand that not all heating systems include ventilation, and some will only heat or cool air.

If you're not familiar with your workplace HVAC, it would pay to get familiar with it.

Does it exchange air? Has it had regular maintenance? Are its ducts and filters clean? Could it be modified (by a qualified engineer) to be even better at preventing virus spread?

At school:

Schools can be a vector of transmission for viruses like Covid-19, but ventilation standards can differ wildly from school to school and even classroom to classroom.

Studies have shown air-exchanging HVAC systems in schools can significantly lower Covid spread between kids, but few of the country's 36,000 classrooms have them installed.

Again, the main advice for teachers and kids remains to crack windows and doors open when and wherever possible.

To help schools assess CO2 levels, the Ministry of Education has distributed portable CO2 monitors to all state and state-integrated schools.


A petition has been launched to change the speed limit in Egmont Village due to safety and noise concerns.

For classrooms where ventilation is particularly challenging, portable air cleaners have been touted by the ministry as a solution.

Their effectiveness is dependent on being correctly sized for the room, running on a high fan speed, and having a quality HEPA filter.

All schools have been offered air cleaners to use at their discretion in spaces that may have a higher risk of airborne transmission such as some staff rooms, music rooms, high-use meeting and break-out rooms.

They can also be used in classrooms and other spaces when adverse weather conditions make it less practical to open the windows.

The ministry said it had distributed 5000 air cleaners from March 2022 and in May allocated an additional 7500 air cleaners for schools.

What else do I need to know?

It's important to remember that ventilation alone won't prevent Covid-19 transmission. It is not a golden ticket to taking off your mask when you're inside.

Even in a room with good ventilation, but especially if you cannot control the ventilation in a room, it's important to continue to wear a well-fitting mask.

Bloomfield says a good rule of thumb is to wear a mask anywhere you are in an indoor environment outside the home.

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