Air purifiers eliminate 90% of aerosols in a school classroom

A research pre-print published on 6 October 2020, entitled Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS-CoV-2, provides the details on a recent experiment conducted by professor Joachim Curtius, at the Gœthe University (Institute for Atmospheric and Environmental Sciences) in Frankfurt.

Four commercially-available air purifiers, equipped with medical-grade HEPA filters, like those in NatéoSanté’s EOLIS Air Manager, were tested for a week in one of the school’s classrooms that usually hosts students. A classroom without any devices served as a control.

After 30 minutes, 90% of the aerosols in the occupied room’s indoor air were eliminated, while the levels of PM10 fine particles also dropped significantly.

These are results worth considering with regard to the risk these aerosols pose in potentially transmitting Covid 19, along with droplets and fomites (infected objects). All the more so in enclosed group areas, classrooms and places of business, enclosed shared offices, open spaces, shops and any places open to the public, including hotels, restaurants, gyms, etc. Especially at a time when the Auvergne Rhône-Alpes Region has recently announced the installation of air purifiers in its 565 secondary schools.

• Read more on the topic: ‘Businesses: why buy a professional air purifier?’

Still, social distancing and physical hygiene measures, starting with regularly opening windows for ventilation, are the first precautions to take to protect yourself and others during this high-risk second-wave of the pandemic.

The four air purifiers tested used HEPA 13 filters

The abstract of the work carried out by professor Joachim Curtius and his team begins with the passage: ‘The airborne transmission of SARS-CoV-2 through virus-containing aerosol particles has been established as an important pathway for Covid-19 infection’.

• Read more on the topic: ‘America’s health authorities now consider that the virus can be spread by airborne transmission’

Regarding the methodology:

‘Here we tested the efficiency and practicability of operating air purifiers in a high school classroom while regular classes were taking place. Four air purifiers equipped with HEPA filters were installed in a classroom.

We monitored:

• the total aerosol number concentration for particles > 3 nm at two locations in the room[...]
• the aerosol size distribution in the range from 10 nm to 10 µm [...]
• PM10 [...] as well as CO2 concentrations [...]

For comparison, we performed similar measurements [...] in a neighbouring classroom without air purifiers.’

Aerosol concentration reduced by more than 90% in 30 minutes in the classroom equipped with air purifiers

Regarding the measurements observed and conclusions drawn:

‘Reduction in aerosol particle concentration in a closed classroom without air purifiers (blue line)
and with 3 or 4 air purifiers operating at stage 3 (3 x 257 m³/h per air purifier, green lines;
4 x 257 m³/h per air purifier, black lines) or stage “turbo” (4 x 365 m³/h, red line).
Data are normalized to a starting value of 10,000 particles cm-3. Data are displayed for the time intervals
until door or windows were opened again’

‘In times when classes were conducted with windows and door closed, the aerosol concentration was reduced by more than 90 % within less than 30 minutes when running the purifiers (air exchange rate 5.5 h-1).

The reduction was homogeneous throughout the room and for all particle sizes.

The measurements are supplemented by a calculation estimating the maximum concentration levels of virus-containing aerosol from a highly contagious person speaking in a closed room with and without air purifiers.

Measurements and calculation demonstrate that air purifiers represent a well suited measure to reduce the risks of airborne transmission of SARS-CoV-2 substantially.

Staying for two hours in a closed room with a super infective person, we estimate that the inhaled dose is reduced by a factor of six when using air purifiers with a total air exchange rate of 5.7 h-1.’

• Download the source paper

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