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Chair of expert ventilation group says Nolan's comment on HEPA filters 'defies laws of physics'

Professor John Wenger said the filters could help schools with poor ventilation.

Professor Philip Nolan, Chair of the NPHET Irish Epidemiological Modelling Advisory Group (file photo)
Professor Philip Nolan, Chair of the NPHET Irish Epidemiological Modelling Advisory Group (file photo)
Image: Sam Boal/RollingNews.ie

AN EXPERT HAS called on the Government to reconsider its stance on using HEPA filters to improve ventilation in some schools amid the ongoing risks posed by Covid-19.

Professor John Wenger, the chairperson of the Expert Group on the Role of Ventilation in Reducing Transmission of Covid-19, said not every classroom needs an air filter but many could indeed benefit from one.

The group was set up specifically to advise the Government on the role ventilation can play in combating the spread of Covid-19

Health Minister Stephen Donnelly has repeatedly said that open windows and C02 monitors provide “sufficient” ventilation in schools.

Speaking on RTÉ Prime Time last night, Donnelly confirmed that the Government is basing this stance on advice received from the HSE’s Antimicrobial Resistance and Infection Control Oversight Group (AMRIC).

In a report submitted to the Government in March, Wenger’s group recommended the use of HEPA (high efficiency particulate air) filters in poorly ventilated areas. However, AMRIC’s advice is at odds with this and has been favoured by the Government.

Speaking to The Journal today, Wenger said he was “surprised” that Donnelly was ignoring the advice given to it by the expert group specifically set up to examine ventilation.

“Why did he turn to this group? I know that the AMRIC group are leading the response to Covid, but we were called in specifically because we have expertise, multidisciplinary expertise, in the area of ventilation – understanding respiratory particles or aerosols, how to measure them, how to remove them, how buildings can be improved with ventilation and so on. So it is a bit unusual at this stage now to to the defer to some other experts, so I’m a bit surprised.”

When asked about the use of HEPA filters, Professor Philip Nolan today told a NPHET press briefing that the devices can play a role in “preventing transmission of the virus” in certain circumstances.

However, he said their use would not necessarily improve ventilation in settings such as schools.

Nolan, chair of NPHET’s Irish Epidemiological Modelling Advisory Group, said: “Their application is in small spaces where you have a known source. So I think the important thing to remember is that somebody who is infectious, there are lots of viral particles close to them, and very few viral particles at a distance from them – so ventilation helps disperse the ones that are both close and at a distance.

“If you’re going to use a HEPA filter, you really have to know the source of the infection and you have to put that HEPA filter beside the source. So they’re useful in hospitals, for instance, where there’s a known aerosol generating procedure or a known source of infection.

“They’re not useful in the far corner of a room with 20 or 30 people in them, where you don’t know if there is a source of infection where you have your HEPA filter.”

‘Defying physics’

Wenger disagrees with Nolan’s comments, saying they “defy the laws of physics”.

“The virus particles move around the room, right? So, if you have one infectious person in the room, those viral particles can move around. That’s a bit like saying if there’s a smoker in one corner of the room and if you’re in the other corner, you can’t smell it. Do you see what I’m saying? It goes against the laws of physics.

“The viral particles will move around the room. If you have very, very good ventilation you will dilute things. What we’re talking about is poorly ventilated spaces and in poorly ventilated spaces, the C02 is building up and so viral particles will build up.

“Any space that is demonstrated to be poorly ventilated would benefit from a HEPA filter and there are many classrooms where that’s been the case. It’s as simple as that.”

Wenger said his group did not call for the use of HEPA filters in all schools, instead noting that classrooms with poor ventilation could benefit from them.

hepa-filter-air-pollution-close-up A close-up of a HEPA air filter (file photo) Source: Alamy Stock Photo

“People are talking about the layers of protection, and ventilation is a layer of protection. And so when the ventilation isn’t working well, this is where HEPA filters step in. I’m not saying that every classroom needs a HEPA filter. If the ventilation is working well, then you might get a marginal benefit from the HEPA filter.

“But when your ventilation isn’t working well, then that’s where it’s most beneficial. And that’s the bottom line for me and that’s what we recommended. It’s not a blanket yes or no, it is in certain cases they are going to be valuable.”

In a report submitted to the Government in March, the Expert Group on the Role of Ventilation in Reducing Transmission of Covid-19 stated that air filtration is “known to be effective in removing airborne particles, thereby providing a similar effect to outdoor air ventilation”.

“SARs-CoV-2 [Covid-19] is carried in respiratory droplets and aerosols which are typically 0.5-100 microns in diameter. Devices that use high efficiency particulate air (HEPA) filters or other high grades of filter will be most effective in removing respiratory partides in this size range.

“While there is no direct evidence that use of HEPA filters can reduce the transmission of SARS-CoV.2, it is widely accepted that they will be effective at removing a substantial proportion of the airborne virus.”

The report by the Wenger-led group goes on to note that the European Centre for Disease Control (ECDC), the US Center for Disease Control (CDC) and the World Health Organization (WHO) “all recognise that stand-alone air cleaning devices equipped with a HEPA filter may be useful in reducing airborne transmission in spaces with Insufficient ventilation”.

The group added that, in summary, HEPA filter devices “can be useful in reducing airborne transmission in spaces with insufficient ventilation”.

Guidance from the Department of Education

Wenger noted that, despite Donnelly’s comments, the Department of Education seemingly had taken the advice from Wenger’s group on board when devising guidelines for schools earlier this year.

Advice sent from the Department to schools in May notes the following: “Where the practical measures for the deployment of good ventilation practices have been undertaken (such as open windows and C02 monitors), and poor ventilation continues to exist in a particular room/area, air cleaners may be considered as an additional measure in conjunction with other methods of ventilation that are available.

“Room air cleaner selection is dependent on the particular setting and it is not possible to give a “one size fits all” solution, or a simple rule that everyone can follow. The Air Infiltration and Ventilation Centre notes that measures to reduce risk of exposure that causes Covid-19 from spreading indoors generally fall into three categories: source control, ventilation control and removal/control.

“Air cleaners can assist in removal control and provide an additional measure of precaution where poor ventilation exists. They should not be used to fully replace ventilation and should be used in conjunction with and to support other methods of ventilation that are available.”

Donnelly told Prime Time last night: “Advice from the experts at the moment is that C02 monitors, fresh air circulating with the windows [open] and so forth, that that is the way to go.”

The minister said that the Government is receiving its advice on the issue from both a subgroup within NPHET and a group within the HSE called AMRIC – the Antimicrobial Resistance and Infection Control Oversight Group.

Donnelly stated that AMRIC “was set up [to examine] antimicrobial resistance, but is now providing the recommendations on this”.

When presenter Sarah McInerney pointed out that the ECDC, CDC and WHO all recommend the use of HEPA filters in certain environments, Donnelly said: “Some [experts] are concluding one thing, some are concluding the other. Ultimately the advice to Government, which is what we have to go on, is that the ventilation measures in place in school – C02 monitors and the rest – is sufficient.”

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Other developments

Cabinet yesterday decided that children aged nine and older will be asked to wear a face covering in certain public settings such as schools, shops and public transport, subject to a review in mid-February.

The measure came into effect today and applies to children in third class and above in schools.

Meanwhile, a People Before Profit Bill on workplace ventilation was passed by the Dáil today.

The Bill defines clean air as having fewer than 900 ppm (parts per million) of Co2 – and puts the onus on employers to achieve this through ventilation or air filtration.

Writing in The Journal yesterday, PBP TD Paul Murphy said: “The rationale for doing this is that almost every building where people gather aside from private homes is someone’s workplace.

“Many of the worst superspreading events have occurred in workplaces, with previous outbreaks in meat plants, restaurants, airplanes, schools, nursing homes and hospitals. It is through ensuring the right to clean air for workers that we can protect public health more broadly.”

With reporting by Rónán Duffy at the NPHET briefing

About the author:

Órla Ryan

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