Statements and Advice
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Our position is consistent with those published by ASHRAE, WHO and CDC.
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 can be found here.
And the ASHRAE position can be found here.
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.
Ionizers, electronic air filters, ozone generators, and photocatalytic oxidisers
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.
Heat pumps and fans
Although useful for heating and cooling, most common types of heat pumps used in New Zealand only heat or cool the air that is already within a space. Heat pumps recirculate existing air and will have almost no effect on indoor air quality (outside of temperature and relative humidity) and virus transmission. This is because they circulate air within the room but do not replace it with fresh air.
To reduce viral transmission risk and improve hygiene in dwellings, we recommend that all rooms should have some form of ventilation running at low speed even in winter. Bathrooms should have a constantly running fan-driven extraction system (see Table 2.3 NZ Standard 4303:1990) venting bathroom moisture and odours to outside.
Maintain the 20 ºC +/- 2 ºC temperature setpoint and RH around 50%.
Open windows/flush room with fresh air.
Portable air cleaners (HEPA filtration)
Ventilation with fresh air is preferred to air cleaning. 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.
However, if ventilation is insufficient, or extra protection is required (e.g. masking cannot be maintained), then portable air cleaners should probably be considered ahead of other technologies.
It is important to understand what is meant by “air cleaner” as the terminology is not used consistently. In this note we use the term to mean machines based on HEPA filtration only. For commentary on other technologies (like ionisation and UV irradiation), see our other notes.
The main limitations of portable (HEPA) air cleaners is that they:
- Can be noisy.
- Do not reduce the build-up of CO2 (from exhaled breath) which at high levels may affect comfort and concentration, or any toxic gases that may be present in some specific settings.
- Can require several units to be effective. Air cleaning technologies typically require several devices to achieve more than 3 or 4 ACH.
- 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 the room is already achieving 10 air changes per hour, the additional 2.8 represents only a marginal improvement.
To be most effective, portable air cleaners need to 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.