COVID-19

In addition to other public health measures (such as mask use, staying home when unwell, vaccination, and reducing numbers of people and time spent in waiting rooms or other places where people mix), good ventilation (bringing in as much outdoor air as possible) and air cleaners are additional layers that can help reduce the risk of COVID-19 transmission.

When assessing ventilation and air cleaning systems, thought should be given to the room usage. Improvements in ventilation are particularly effective in reducing airborne cross-infection risk in high occupancy areas where there is little close contact between people, e.g. waiting rooms.

Ventilation types

Ventilation is the continuous introduction of fresh air and the dilution and removal of stale air. As well as physically removing pathogens from room air, ventilation works to rapidly dilute breath, reducing the risk of both short-range and long-range transmission. Common ventilation often used in GP clinics include:

Natural ventilation

Opening windows and doors supplies fresh outdoor air and removes stale indoor air. Natural ventilation can achieve 10 to 50 air changes per hour (ACH). 1 ACH means that every hour, a volume of air equal to the volume of the room is cleaned or replaced with fresh air.

To maximise natural ventilation, open windows and doors. If possible open windows on different walls of the same room to create a cross-flow, and/or open windows and doors at different heights (this boosts the ‘stack effect’ in which cold, dense air displaces hot air).

Wind and temperature impact natural ventilation. As the outdoor temperature drops, you may be able to reduce the window openings (narrower gaps), as the increased indoor/outdoor temperature difference drives more airflow. You may wish to adjust the openings throughout the day as the wind direction, wind speed and temperature change.

Mechanical ventilation

Air is moved into and out of the building primarily by fans, either sited directly at openings, or as part of a system of ducts and/or plenums (for example, some systems use the space between the ceiling and roof as a plenum to distribute air to multiple rooms). Mechanical ventilation systems are usually designed to achieve 2 to 6 ACH.

Mechanical ventilation systems often recirculate some portion of the indoor air to reduce heating or cooling costs. If you can, set your system for minimum recirculation (maximum fresh air intake).

Have any filters in the systems inspected, cleaned or changed as necessary.

Mixed or hybrid ventilation

Uses a combination of natural and mechanical ventilation. If possible, adjust your window and door openings, and/or fan settings to obtain flow from areas of clean air (from outside, or from low occupancy spaces) to areas of potentially contaminated air (where there are many people). Try not to have strong flows directly from one person to another. Flow direction can be determined by using a strip of tissue paper or thin plastic cut from a bag as a wind vane.

Measuring ventilation effectiveness

Carbon dioxide (CO2) monitors can be used to measure the effectiveness of a building’s ventilation to determine what window openings are satisfactory. Outdoor air normally has 410-450 ppm CO2. CO2 levels in a room will rise and fall with the number of people in a room, fall if more fresh air is brought in, and usually stabilise after 10-30 minutes if nothing changes. A CO2 level of less than 800 ppm indicates very good ventilation.

CO2 monitors based on total volatile organic compounds (TVOC) sensors are inexpensive but too inaccurate for assessing ventilation. CO2 monitors based on non-dispersive infrared (NDIR) sensors are usually more accurate, with prices starting from $250.

Place the CO2 monitor somewhere about head level (1.2 m if people are sitting), where people will not breathe onto the monitor directly (at least a metre from where people stand or sit), and away from windows, doors or vents.

Air cleaning and purification

Various systems can be used to remove airborne particles or inactivate airborne pathogens. However, if ventilation can keep CO2 levels low whilst maintaining a comfortable temperature, and effective mask wearing can be ensured, then the risk of pathogens lingering in the air is low and additional air cleaning will only deliver marginal improvements. Ventilation with fresh air is preferred to air cleaning. This is because air cleaning systems:

• Can be noisy.
• Do not reduce the build-up of CO2, which at high levels may affect comfort and concentration.
• Can require several units to be effective. Air cleaning technologies typically require several devices to achieve more than 3 or 4 ACH.

Two air treatment technologies supported by the literature as safe and effective are portable air cleaners, which use a fan to force air through a filter, and ultraviolet germicidal irradiation (UVGI).

Portable air cleaners

The effectiveness of any one unit can be assessed by determining the number of times the volume of air in the room is filtered each hour.
The manufacturer will specify the Clean Air Delivery Rate (CADR), which is the rate at which clean air is expelled from the device.
The number of air changes per hour that are cleaned can be calculated by dividing the CADR by the volume of air in the room.
Volume of air=room length x room width x average ceiling height (if the floorplan is not rectangular, divide it up into rectangles and add the volumes of each section).
Number of air changes cleaned per hour =CADR/volume
For example, a portable air cleaner with a CADR of 350 cubic metres per hour (at maximum fan speed) operating in a room 10 metres long x 5 metres wide x 2.5 m ceiling height. The room volume is 10 x 5 x 2.5=125 cubic metres. The cleaner will provide 350/125=2.8 clean air changes per hour. If the room is poorly ventilated and achieving only (say) 1 air change per hour then these additional 2.8 air changes represent a substantial improvement in air cleaning. However, if room is already achieving 10 air changes per hour, the additional 2.8 represents only a marginal improvement.

To be effective, portable air cleaners must be placed so that they process most of the room air, which is harder to achieve if the room is already well-ventilated. They should run continuously if there is more than one person in the room. The best placement of an air cleaner is:

• Roughly in the centre of the room.
• At table top height, or on the floor, or anywhere in between.
• Where it doesn’t cause a draft from one person to another.
• At least a metre from walls or furniture if possible.
• Away from curtains or anything which might be sucked into the inlet.
• Away from open windows (so they are not filtering the already clean air from outside, or blowing the cleaned air out of the room).
• Not blocking an emergency escape route and not creating a trip hazard.

It is best to choose a filter that is High-Efficiency Particulate Arresting (HEPA) to H13 level or better. True HEPA filters should have been tested to reach the HEPA standard: for example, H13 filters remove 99.97% of particles down to 0.3 microns in size. Some filters are advertised as “HEPA like”, which does not guarantee their ability to filter small particles, although they may perform well.

As portable air purifiers are unable to clean all of the air in a room at the same time, a lower standard of filter (MERV 13, or better still MERV 16 or E10) is acceptable. Such a filter will remove at least 75% of the virus-bearing particles with each pass of air through the unit.

Filters should be replaced at the intervals recommended by the manufacturer, usually every 6-12 months. As the SARS-CoV-2 virus inactivates rapidly on dry surfaces, changing filters is not particularly hazardous, and filters can be disposed of to landfill using the precautions you would use for disposing of used facemasks etc.

Ultraviolet germicidal irradiation (UVGI)

Properly installed, UVGI is a safe and effective way to inactivate pathogens in the air. Most UVGI air treatment devices are either:

• ‘Upper-room UV’ with UV light fixtures shining across the room, above head height, sterilizing the air as it circulates through the room.
• UV systems inside mechanical ventilation ducts, sterilizing the air passing through the duct.
• Standalone units with a fan, which draws in air from the room, passes it through a UV sterilization chamber, and expels it back into the room.

The first two types must be installed correctly by a reputable supplier, to ensure that room occupants are properly shielded from the UV light. Any UV unit should be certified against a suitable standard to ensure any ozone generated is within safe limits.

The CDC published advice on upper-room UV at https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation/uvgi.html

There are also UV systems for disinfecting surfaces, including disinfecting the heating or cooling elements in heat pumps and air conditioning systems. These do not sterilize the air so may have little impact on airborne cross-infection.

Final notes

We do not recommend the use of ionizers, electronic air filters, ozone generators, and photocatalytic oxidisers. Some devices using these technologies can release additional chemical species into the air, which may be harmful to health. Most common types of heat pumps used in New Zealand do not bring in fresh air. Instead, they circulate air within the room. Bathrooms should have a constantly running fan-driven extraction system. Consulting and treatment rooms may need special consideration when they are not equipped with adequate mechanical ventilation, or windows that cannot be opened e.g. due to patient confidentiality. Apart from an air cleaner, these areas may need extra precautions. Solutions which bring in fresh air (e.g. supply or extract fans) will help keep CO2 levels low with associated benefits for comfort and well-being.

This advice was compiled by members of the New Zealand Indoor Air Quality Research Centre (IAQRC) with additional input from Phoebe Taptiklis (Motu Research/Massey University) and Dr Jason Chen (Dept. Mechanical Engineering, University of Canterbury). The authors have conducted modelling and assessed ventilation rates in a variety of NZ buildings, which has led to advice for the NZ Government on ventilation and air cleaning practises to reduce the transmission risk of COVID-19.