Air pollution is today’s greatest environmental health risk. Good air quality supports children’s healthy development and contributes to staff wellbeing.
Explore the topics below for answers to frequently asked questions about HEPA air filters and natural ventilation. Learn why indoor healthy air is so important, and what air quality monitors can tell us.
This FAQ is for schools or other buildings with natural ventilation or hybrid (natural combined with mechanical ventilation) systems. It’s not for schools that use only mechanical ventilation or have HVAC systems.
Find practical information on how how to buy, use and maintain HEPA filters to clean indoor air.
What are HEPA units?
The air filtering appliances commonly called HEPA (High Efficiency Particulate Air) filters or HEPA units look like large rounded or square boxes with lots of small holes on their sides. Inside a typical unit you’ll find a motor and a fan that draws air through a pre-filter, and behind that, the HEPA filter itself.
Each HEPA filter consists of a mat of dense fibres, arranged in pleats to increase its surface area. Air and any pollutants it contains is drawn through the filter, which traps pollution particles across a wide range of sizes. The unit vents clean air back into the room and the pollutants stay trapped inside the filter.
HEPA filters need to be replaced when they fill with dirt and dust.
How do HEPA filters work?
As air flows through a HEPA filter, most airborne particles collide or get stuck to the filter’s fibres. Technically speaking, HEPA filters work through a combination of interception (trapping by fibres), impaction (colliding with fibres) and diffusion (erratically colliding with gas molecules before colliding with fibres).
What do HEPA filter ratings mean?
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.
If a product’s marketing material says HEPA, can I assume it really is?
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.
What can HEPA filters do?
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.
What are things HEPA filters can’t do?
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.
If virus particles are small enough to pass through HEPA filters, how can HEPAs capture them?
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.
How do I choose the right HEPA unit?
Here are some web resources to help you choose:
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).
How do I know how many HEPAs I need for a room?
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:
Is it better to have one larger HEPA unit or two or more smaller HEPA units in a room?
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.
Should a room’s windows be open when running the HEPA unit?
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.
Where in the room should I put a HEPA unit?
For one HEPA unit:
For multiple HEPA units:
Follow the instructions above, but try to space the HEPA units around the room to provide uniform air filtering coverage.
What steps do I take to get my new HEPA unit running?
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.
What maintenance do I need to do for the HEPA unit, and how often?
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.
When should I turn my HEPA unit on or off?
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.
What can I do about a HEPA unit that’s too noisy to teach or hear students’ responses?
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:
Should any of these types of air cleaners be used instead of HEPA: ozone generators, ionic air purifiers, plasma air cleaning technology, air disinfectants or air fresheners?
Avoid the above types of air cleaners due to their potential health and safety issues. Use safe and effective HEPA units instead.
Learn how ventilation through windows and doors can make indoor air healthier, and read tips to make the most of it.
Why should I open a room’s windows and doors?
Better indoor air quality: Natural ventilation refreshes the air we breathe. It exchanges indoor air, containing contaminants like viruses and carbon dioxide, with air from outdoors. By improving indoor air quality, natural ventilation creates a healthier learning environment for students and staff.
Fewer illness-related absences: Natural ventilation helps reduce the spread of airborne respiratory pathogens and improves indoor air quality, leading to fewer absences due to illness.
Improved academic performance: Natural ventilation can help create a comfortable learning environment that can improve academic performance.
What are some tips to get the most out of natural ventilation?
Between classes, refresh the air by opening doors and all windows and vents as much as possible. If conditions permit, keep windows open during class. Even having windows open just a few centimetres can improve air quality.
Cross ventilation is the most effective way to refresh a room with clean outdoor air. Open windows, doors and vents to create more through-flow, especially if these openings are on different sides of the room.
On warm days, open low and high windows to increase airflow between them, cooling the air and improving indoor air quality.
Advice on how to use HEPA and natural ventilation on bushfire smoke days.
When it’s smoky outside, is it OK to close the windows?
During bushfires and hazard reduction burns, outdoor pollution may be so bad you need to close the windows. Here’s what to do:
Read about indoor air quality and why it’s so important to student and staff well-being.
Why is clean, healthy indoor air so important for children?
Children are more vulnerable to air pollution than adults. Clean indoor air helps prevent allergies, asthma and damage to children’s lungs caused by pollution. It can limit the spread of infections and viruses. Good air quality supports children’s healthy development, leading to improved alertness, concentration, mood and overall academic performance. Finally, improving indoor air quality can reduce the number of sick days taken by students.
Why is good indoor air quality so important for staff wellbeing?
Clean, healthy indoor air is as important for staff as for students. Poor indoor air quality can contribute to development of asthma, bronchitis and other lung disease, and can worsen existing respiratory and cardiac conditions. Good indoor air quality reduces the risk of infectious disease transmission. This is important for teachers and other staff in routine contact with large groups of children and young adults. Finally, clean, healthy indoor air enhances cognitive function, productivity and comfort.
Why is healthy air important for children with asthma?
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.
What are other benefits of good indoor air quality?
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.
What are some common air pollutants?
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.
What about bacteria and viruses – are they types of airborne pollution?
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.
How does outdoor air pollution get into schools and other buildings (for example, from outdoor areas with heavy traffic, or on days with bushfire smoke)?
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.
What about carbon dioxide? Why can it become a problem for students and staff?
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.
Learn why it’s helpful to monitor indoor air quality, and what air pollution sensors can tell you.
What can air pollution monitoring tell us about indoor air quality?
Air pollution varies from place to place and even from room to room. Air pollution monitoring can provide valuable insights to help schools create safer, healthier environments for students and staff. By monitoring air quality schools can:
What can carbon dioxide monitoring tell us tell us about indoor air quality?
Monitoring carbon dioxide levels in rooms can help us understand some aspects of indoor air quality, as follows.
Monitoring carbon dioxide levels does not, however, indicate the effectiveness of air filtration or purification systems to remove indoor air pollutants or airborne viruses. These contaminants may still be present in a room with low carbon dioxide levels.
This FAQ is supported with funding from the Australian Government’s National Environmental Science Program.
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