‘Effective, but not the silver bullet’ – air cleaning devices in schools


Leading researchers involved in studying how COVID-19 spreads in enclosed spaces have compiled ten tips to help school leaders decide whether to use air cleaning technologies in classrooms.

It is known that good ventilation can reduce the chances of people becoming infected with the viruses responsible for COVID-19 and other respiratory infections – but some schools struggle to ensure adequate ventilation.  

They may have windows that do not open or only open partially. Other schools might not be able to have windows open because of noise and fumes from passing traffic or because of other adverse environmental conditions.   

Air cleaning technologies ... can add to ventilation but they can’t replace it

Professor Cath Noakes, School of Civil Engineering

Some headteachers are now looking at using air cleaning technologies to compensate for poor air quality. The devices take in air and filter it, to remove the virus particles – or expose the air to a disinfecting ultraviolet light.  

The University is involved in a major scientific investigation into the effectiveness of two types of air cleaning technologies in schools, being run through the Centre for Applied Education Research in Bradford. The results will not be known until later in the year.  

Academics leading the study say they expect the devices to have a positive impact on improving air quality and, given the pressure on school leaders to look at ways of improving airflow in schools, have compiled a guide to help schools decide whether the technologies would work for them.  

The full guide is published on the TES magazine website

Professor Cath Noakes, from Leeds’ School of Civil Engineering, is a leading expert on the use of ventilation to control the spread of pathogens. She said: “Air cleaning technologies are not the silver bullet to controlling infection: they can add to ventilation, but they can’t replace it. So, one issue for schools to think about is – is it better to try and invest in improving ventilation or to add air cleaning devices?  

“Of course, some schools will struggle to improve ventilation without substantial investment, and then air cleaning technology is likely to be a good choice. But careful thought needs to go into how the system is configured so it is most effective.”  

The advice to school leaders is:  

1: Try to improve ventilation first  

Carbon dioxide monitors can identify spaces where ventilation is poor. A reading regularly over 1,500 parts per million (ppm) when the classroom is occupied calls for additional measures such as air cleaning if the ventilation cannot be improved.   

2: Evaluate whether air cleaning technology is a good investment   

Air-cleaning technology can be an important measure against COVID-19 transmission and other infectious viruses. But there may be other benefits too. Air filtration also removes other particles such as soot, pollen and dust and can help reduce exposure to hay fever allergens and pollutants from traffic that may exacerbate asthma. So, it is worth considering whether investment in air cleaning is needed as a short-term solution (6-12 months) until ventilation improvements can be made, or whether it is likely to be needed for the longer term (one to five years) to create a healthy learning environment.  

3: Costs and benefits   

Devices cost between £100 and £1,500 and vary in quality and effectiveness.  Air cleaning might not remove all the viral particles and would not stop the virus spreading between pupils in close proximity to one another. Investing in improving ventilation might generate bigger returns.  

4: Chose the right equipment  

To effectively remove viruses, the system must have high quality filters that catch particles around 0.5 micron in diameter, where a micron is one-millionth of a metre.  The amount of clean air needed to be produced depends on the size of the space and existing ventilation, and if a ventilation assessment is carried out this can be calculated with a reasonable degree of accuracy. 

As a rule of thumb, in a typical school classroom with 32 people, a total clean air delivery rate (CADR) of around 720 cubic metres an hour (m3/hr) is usually a good estimate. This is often better provided by two or three smaller units with a CADR of 240 - 360 m3/hr each rather than a single large unit. 

5: Where are the devices going to be located  

Portable devices should be positioned away from walls and should not be in a confined position where furniture or curtains could affect the airflow. Wall-mounted units, such as UV-based units in the Bradford schools' trial, may be more suitable when floor space is tight, however these will usually require more substantial electrical work to provide power.  

6: Maintenance and running costs  

Factor in the cost of replacement filters and determine where these can be purchased from, as well as the frequency with which they need to be replaced. If air cleaning devices are used in more polluted environments, the filters may need to be replaced more frequently.  

7: Schedule filter replacement into the school year  

Filter changes can often be done in-house but if you have a larger school with multiple units, it may be more effective to have a maintenance contract with a supplier. Filter changes pose a low risk of the virus being released but are better carried out when children are not in the room.   

8: Provide staff training  

It is important to remind staff that they still need to open windows and doors, although probably not as wide or as often.  An air filter only removes particles from the air, it does not flush out the carbon dioxide. It is important to ventilate the room even when an air cleaning device is in use. There are lots of studies that show that ventilation is important for health and wellbeing beyond COVID-19.  

9: Complete a health and safety risk assessment  

Carry out a risk assessment for using and maintaining the air cleaning devices.    

10: Someone to take responsibility for the units   

One person should be designated as a responsible officer for managing the devices. This could be someone from the school estates team, a business/site manager, a technician, or a member of teaching staff.  

For further details

For media enquiries, email University of Leeds Press Officer David Lewis at d.lewis@leeds.ac.uk.

The full guidance is available on the TES magazine website.