What is HEPA?

Australia doesn’t have a standard for HEPA. Global standards for HEPA filters differ slightly between regions. The main U.S. standard requires HEPA filters to remove at least 99.97% of particles sized 0.3 microns or larger (0.3 microns is about 300 times smaller than the diameter of a human hair). Europe’s standard is based on the size of particle most likely to get through, known as the most penetrating particle size. By this standard, most HEPA filters are rated H13, which means they should be 99.95% efficient for the most penetrating particle size. The good news is they’re even more efficient at filtering larger or smaller particles.

Watch out for fake products or appliances claiming to be ‘HEPA-like’. These filters may be constructed in a similar way, but do not meet HEPA’s high standards. They may not remove particles as small in size or to such high percentages as HEPA filters can. Make sure your product contains a HEPA filter rated to a grade of at least H13 (not lower).

Buy your HEPA unit and its replacement filters from reputable brands and retailers to ensure they are genuine. If buying online, read reviews and check a seller’s reputation to avoid fakes.

HEPA filters can remove airborne particles from smoke and other air pollution, as well as pollen, mould, dust and bacteria. They also capture tiny droplets released as people breathe, called aerosols, that contain bacteria and viruses. This makes HEPAs an important tool for preventing the spread of airborne bacteria and viruses, including COVID-19, RSV and influenza.

HEPA filters aren’t designed to remove gases from the air. This means they can’t remove odours or gases like carbon dioxide or nitrogen dioxide. Activated carbon, sometimes included with a HEPA unit, can help remove some gases such as volatile organic compounds.

It’s correct that individual virus particles are small enough to pass through a HEPA filter. But most respiratory viruses like COVID 19 are spread on aerosols emitted when people exhale, cough or sneeze. These suspended particles are large enough for HEPA filters to trap.

Here are some web resources to help you choose:

• Clean Air Stars Air Filter Recommendation Tool
• Australian product comparisons

When buying, as well as the unit’s cost, key factors to consider include:

Room size: Your room’s volume will help determine how many HEPA units you need to meet the World Health Organisation recommended rate of 6 air changes per hour or 10L/p/s.

Filter efficiency: This affects how many units you’ll need to achieve the filtration rate recommended above. A high clean air delivery rate (CADR) means the unit is more efficient. Powerful purifiers generally range from 500-900 cubic metres per hour (m³/hour). Larger units are generally more powerful. Clean air delivery rates refer to a unit’s highest setting; depending on the model, this can be quite loud.

Noise level: A noisy HEPA unit can distract and interfere with learning. Consider using two slightly smaller units, separated from each other, to reduce the noise level and allow for more uniform filtration across the room.

‘Set and forget’ function: If possible, choose a model that allows users to program start/stop times. If that’s not possible, purchase a programmable digital timer to plug into the room’s power point, and plug the HEPA unit into that. Some models have smartphone apps to set start/stop times.

Filter replacement: If you plan to use your HEPA unit for several years, consider the cost of filter replacement. Choose a reliable company that will make it easy for you to buy filter replacements over the next few years (or buy a supply at the time of buying the HEPA unit).

The number of HEPA units you’ll need depends on room size. As a rough guide, a good choice for a typical Australian classroom size (~65 square meters) would be two medium-sized (roughly adult thigh-height) HEPA units, each with a clean air delivery rate of around 500 m³/hour or greater.

This guide is an ideal scenario. It’s important to remember that in the real world of budget constraints, some HEPA filtration is better than none at reducing risk.

This web tool can help you choose the right number of HEPA units:

Clean Air Stars Air Filter Recommendation Tool

Smaller units tend to be less noisy than larger ones. Running two less powerful units may be quieter than one larger unit. As an added advantage, two or more units placed around a room clean the air more effectively than a single unit, because they provide more uniform filtration of airborne particles.

Yes — if air quality is good outdoors, it’s best to keep windows open when running a HEPA unit. Be aware this may counter advice from the HEPA manufacturer. This is okay. Natural ventilation through windows and doors improves air quality by reducing carbon dioxide levels, and the HEPA unit cleans the air by removing polluting particles and airborne viruses.

But if air quality outside is poor, such as during bushfires, it may be best to close the windows and run the HEPA unit on full power. Then when people leave the room, for example during lunchtime, open the windows to purge carbon dioxide from the air. Then close the windows again, and run the HEPA unit at its highest filtering speed for 20 minutes or more to clear smoke particles from the room before people re-enter.

For one HEPA unit:

• Place the HEPA unit on the floor, ideally in a central location where it can filter and catch airborne particles.
• Make sure the power cord doesn’t create a trip hazard.
• Ensure the unit’s in/out vents are not blocked by furniture or other objects.
• If placing near a wall, make sure the unit is far enough away from the wall to allow airflow.
• Avoid placing HEPA units near open windows or doors, where their clean, filtered air may get sucked straight out.
• Never place them near heat sources or flammable objects.
• If the HEPA units out-vent blows air in a sideways direction, make sure the vent side is at least 2m away from a person.

For multiple HEPA units:

Follow the instructions above, but try to space the HEPA units around the room to provide uniform air filtering coverage.

Remove any plastic wrap on the outside of the unit. Open the unit and remove any plastic wrap from the filter inside.

Make sure the HEPA filter (folded white filter) and pre-filter (thinner plastic sheet with tiny holes) are installed properly. The unit may also contain a black or dark-coloured filter layer of activated charcoal.

Test the unit’s different filtering speeds for the highest speed that’s acceptable in terms of noise.

Don’t use automatic settings for filtering speed — they’ll cause the unit to run too slowly to properly clean the air.

Automate the HEPA unit’s run times, so no one needs to manually turn it on and off each day if possible. If available, use the unit’s ‘set and forget’ feature to program start/stop times. Alternatively, set up a programmable digital timer to do this. Some units can also be programmed using smartphone apps.

Let others know about the unit, so they understand its health benefits.

Wipe down the air intake on the outside of the unit (the area with holes where air enters). To do this, use a soft, soapy cloth once a month — or whenever you see dust build-up.

If the unit has a pre-filter, clean it every six months — or whenever you see dust build-up. To do this, open the unit, take out the pre-filter, wash it with soapy water, then let it fully dry before you put it back.

Replace the HEPA filter when the unit indicates you need to do so or, depending on use, every year. To do this, you’ll need to open the unit, remove the used filter, and discard it inside a sealed plastic bag. Remove any plastic wrap from the new replacement filter, and insert the filter into the unit.

Run the HEPA unit at the highest setting possible 30 mins before students arrive, all during the day, and 30 mins after classes end, to clear the room’s air of pollutants.

When students or staff are present, run the HEPA unit at its highest setting possible given noise tolerance.

During lunch breaks, purge the room with as much outside air as possible. To do this, ideally open the windows fully, while the HEPA units keep running.

At night, and during weekends or holidays, the HEPA unit can be off if the room is empty.

It’s normal for a HEPA unit to make some noise. Quieter models run at about 40-50 dBA. For reference, 40dBA is the level of noise you’d expect to find in a quiet library.

Here are some things you can do if your HEPA unit is too noisy:

• Temporarily lower the filtering speed. Because HEPA units are quietest at lowest fan speeds, this will reduce noise but still provide the benefit of some filtration. But since lower speeds reduce the amount of air filtered, it’s important to remember to turn it back up as soon as possible.
• Consider moving the HEPA unit to a position further from the conversation.

Avoid the above types of air cleaners due to their potential health and safety issues. Use safe and effective HEPA units instead.

In Australia, about one in 10 children suffer with asthma. Healthy air is crucial for these children, whose airways are more sensitive to pollution. Breathing in polluted air can trigger asthma attacks, causing airways to become inflamed and narrow, and making it difficult to breathe. Exposure to air pollution, including bushfire smoke, can worsen a child’s asthma symptoms over time, and increase a child’s risk of developing asthma in the first place.

Healthy indoor air can save schools money by reducing absenteeism for both staff and students. Clean, healthy indoor air also promotes a better sense of comfort, health and well-being.

A California study found that increasing ventilation rates of 4L/p/s (litres per second per person) to meet that state’s standards for schools would decrease absence due to illness by 3.4%. The economic benefits of this measure would outweigh its costs by a factor of eight to one.

Another study from the U.S. found that every 1L /p/s increase in ventilation rate was associated with a 5.6 decrease in days with absences per year at the classroom level.

Air pollution takes many forms. Particulate matter (PM) includes tiny particles that we can breathe in. It includes pollutants from traffic exhaust and bushfire smoke, and allergens like pollen and dust. Nitrogen dioxide (NO₂), from burning fuels, is a gas that irrigates the lungs. Volatile organic compounds (VOCs) are indoor airborne pollutants that off-gas from paint, cleaning products, furniture and adhesives.

Yes, airborne bacteria and viruses, including COVID-19, are biological pollutants. Viruses and bacteria are extremely small. They may move through the air as individual particles, but can also travel on larger particles, such as airborne droplets that form when people exhale, cough or sneeze.

When present, outdoor air pollution can get into rooms through windows, doors and mechanical or natural ventilation systems. It can pass through small gaps or cracks around door or window frames, even if windows and doors are closed. Schools near busy roads or industrial plants face bigger challenges from polluted outdoor air.

Carbon dioxide (CO₂), the waste gas we exhale when we breathe out, can have negative effects if it builds up in rooms. The recommended level for carbon dioxide is less than 850 parts per million (ppm). This compares to the background level of carbon dioxide which is around 415ppm. Levels higher than 1000ppm can negatively affect learning. It’s harder for students to stay alert, concentrate and make decisions when carbon dioxide levels are high. Elevated carbon dioxide levels can cause headaches, dizziness, tiredness, trouble breathing, and make asthma worse. These physical problems may correlate with emotional, mood and behavioural issues.