Despite the use of air cleaning units becoming popular, simply opening a window for ventilation remains the most effective way to decrease the transmission of Omicron indoors, explains Dr Julie Bennett.

Comment: There has been much talk in the media about various air cleaning units and their potential ability to filter airborne particles, thereby reducing the risk of Covid-19 transmission.

However, these units are not the “magic bullet” solution and deciding whether to invest needs careful consideration, as does implementation.

The New Zealand Indoor Air Quality Research Centre (IAQRC) – which brings together experts in building science, public health, and air quality – recommends taking actions to optimise ventilation before considering the use of an air cleaner.

Indoor environments increase the risk of transmission of the airborne virus (SARS-CoV-2) that causes Covid-19 by containment and concentration of the virus.

Ventilation is one of the most powerful tools to limit transmission. Ventilating an indoor space with fresh outdoor air is the most effective way to remove and dilute airborne particles that contain the virus.

Where there is good natural or mechanical ventilation, portable air cleaners don’t give much additional protection, as the flow of fresh air from the open windows can be much greater than the airflow through the air cleaner.

Where ventilation issues can’t easily be addressed, air cleaning units with high-efficiency particulate-absorbing (HEPA) filters can help reduce exposure to airborne viruses and other pollutants.

It is possible to use carbon dioxide (CO2) monitors as a guide to whether rooms are sufficiently ventilated. These monitors measure the amount of air previously exhaled by others in a room and provide a proxy estimate of ventilation.

Suppose a monitor is usually reading around 800 parts per million (ppm) or less when the room is occupied. In this case, the ventilation is good, and an air cleaning unit is unlikely to add significant benefits unless the air is very polluted. A reading regularly over 1500 ppm indicates that ventilation is poor and if it can’t be improved by the below measures, an air purifier may be useful.

In warm weather, many indoor environments can be naturally ventilated by opening doors and windows. For natural ventilation to be most effective, we recommend opening windows and doors as soon as you start using a room. If possible, open a window or a door on different walls of a room to create a crossflow of fresh air.

Opening windows or vents at different heights is also effective for boosting the airflow. If the weather is windy, good ventilation can be achieved with the windows open only a small amount. However when there is little wind it is important to open more windows or doors or open them wider. We recommend keeping windows open for a while after people have finished using the space to flush out stale air.

Some buildings are fitted with mechanical ventilation systems that use fans and air conditioning systems to move air. There are many variations between how mechanical ventilation is installed and operated, which makes it difficult to compare systems. We recommend that mechanical ventilation is operated at full fresh air intake (re-circulating as little air as possible), from shortly before you need to use the room, until an hour after people have left the space.

Mechanical ventilation systems should be checked by a qualified engineer to confirm that all components are operating correctly. Heat pumps in homes and small offices, which have a fan unit outside and another inside recirculate air (heating or cooling), but don’t bring in fresh air.

Air cleaners should be seen as a short-term measure while ventilation is improved. Air cleaning units only remove particles from the air, they don’t lower CO2 levels, so they are not a substitute for ventilation.

To filter viruses, air cleaners require very fine filters, which make it hard for the air to pass through the filter and bigger fans are needed. A downside of this is that bigger fans can be noisy.

We do not recommend the use of ionizers, ozone generators or some photocatalytic oxidisers. These work by producing substances which damage the virus; some of these substances are also harmful to human health if present in the air we breathe.

During warmer weather, allowing as much outdoor air inside as possible through open doors and windows, or using ventilation from a well-designed and operated mechanical ventilation system is the best way of ventilating our homes, schools, workplaces and social indoor spaces to reduce the risk of Covid-19 transmission.

We have been closely monitoring the scientific literature and the responses from overseas governments to the Covid-19 pandemic. We have also conducted modelling and assessed ventilation rates in a variety of New Zealand buildings, which has led to advice for the New Zealand Government on ventilation and air cleaning practises to reduce the transmission risk of Covid-19. We are currently exploring effective ventilation strategies to use when the weather is colder.

*The IAQRC has no commercial interests in ventilation or air cleaning equipment.

Dr Julie Bennett is a member of the New Zealand Indoor Air Quality Research Centre (IAQRC) and a Senior Research Fellow in the Department of Public Health, at the University of Otago, Wellington.

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