High efficiency particulate air (HEPA) filters reduce risk alongside other measures. These include: vaccinations, masks, open windows and cross ventilation, staggered playtimes and outside activities.
YES to true HEPA
Currently available in Australia (as more become available we will add to this list)
NO to ionisers or electrostatic precipitators because they can create ozone which can trigger asthma attacks and aggravate other respiratory diseases.
UV is not effective in standalone units due to the air flow speed.
Look for a rate of over 4 air changes per hour.
The unit should be rated for a room size in square metres, assuming an average ceiling height. If the unit is undersized for your room, it will still work, but not as efficiently.
Similarly, they are designed to work in rooms with closed windows, but they will work with open windows and doors.
HEPA filters will be able to remove bushfire smoke particles but they are not able to remove carbon dioxide (CO2). Bringing in fresh outdoor air is the only way to reduce the build-up of carbon dioxide.
Anything 40 dB or below should be quiet enough to talk and teach over. Anything over 45 dB, or more, not so much. It might be quieter to have two smaller units in one room instead of one larger one.
There may be a range of settings which will change the noise level.
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