Professor Cath Noakes, one of the two engineer members the Scientific Advisory Group for Emergencies (SAGE), told a webinar hosted by the Building Engineering Services Association (BESA) that it was not enough to simply increase ventilation rates in buildings to mitigate transmission of the Covid-19 virus.
“We don’t yet know how much ventilation we need to get this under control; we might never know that and there will always be some risk, but we can aim to reduce the residual risk as much as possible,” she told the BESA webinar. “It is not enough to just say let’s increase ventilation rates. We know it matters and will be critical for health and wellbeing (including mental health) beyond Covid so we must get this right.
“We can say we have not seen any evidence of high transmission in well-ventilated spaces – so if we are designing and delivering to the standards set in current building standards that will help, but we may need to go beyond that.”
She pointed out that many buildings were not even achieving current standards and many “had no proper ventilation at all”. She also expressed particular concern about naturally ventilated spaces.
“So we might not have all the answers, but we do know we need to ventilate better. We also know that ventilation is forever. Even if we get this health crisis under control; what about the next one? We need to get the engineering solutions right for the long term.”
Noakes, who is Professor of Environmental Engineering for Buildings at the University of Leeds, has been providing advice on airborne transmission to the NHS and the government throughout the pandemic and was recently featured on Radio 4’s ‘The Life Scientific’.
“Who would have thought that ventilation would become so high profile?” she asked BESA chief executive David Frise, who chaired the webinar.
She said bespoke solutions would be needed to deal with the wide range of factors in each indoor space and long-term solutions depended on collaboration between engineers, researchers and policy makers.
“This is a very complex issue and it will take years to build up the amount of data needed to make sure we can do this better. However, as a rule of thumb we should aim for [air change rates of] 10 litres per second (l/s) per person and CO2 concentrations below 800 parts per million.”
She confirmed that studies had shown the risk was higher indoors when ventilation provided less than 3 l/s per person and that household transmission was a particular concern. She also explained that the virus thrived in cool, dry and dark conditions – so controlling relative humidity should also be considered.
Noakes has chaired SAGE’s Environment and Modelling Group (EMG), a cross-disciplinary group that has helped the NHS, Public Health England and government officials understand how the virus survives in the built environment and suggested mitigation strategies using engineering solutions.
She said that recent studies suggested some of the larger water droplets that carry the virus can be suspended in the air for longer by air conditioning and cautioned against using it in places with low levels of ventilation. She also said air cleaning was not a reason to ventilate less, but “should be used where you can’t ventilate effectively”.
Engineering controls should sit above the measures that rely on human behaviour such as distancing and wearing face coverings in any “hierarchy of risk control”, according to Noakes. Building managers should address source control before studying ventilation requirements. This approach would not necessarily lead to increasing ventilation rates.
“This is not just about flow rates as it depends on the size of the space. 10l/s per person is the ideal, but if people are close together and for an extended period we may need more flow rate. You can also have quite a lot of people in a large space with lower ventilation rates.”
She said all these variables showed the need for caution about setting hard and fast rules; suggesting the bespoke solutions should be linked to the relative risk of exposure to the virus. Concentrations of CO2 could be used as a “canary in a cage” to demonstrate whether appropriate ventilation rates were being achieved.
“The spaces that worry me the most are those that are naturally ventilated as these are temperature and wind dependent.”
She said there was also very little ‘real world’ evidence to prove the effectiveness of air cleaning devices – performance data had come mainly from laboratory-based studies – but they may improve indoor air quality (IAQ) by removing some other airborne pollutants.
However, there are other risks with some air cleaning devices including exposure to blue light and secondary pollutants. “Also people might think they are a silver bullet and assume they are protected when they are not,” said Noakes.
She said it was better to have an installed air cleaning system, which was the responsibility of the facilities manager to maintain, rather than a portable device. Deeper understanding of the technologies and more training would be needed if air cleaning was to be more widely deployed, she added.
For the future, the industry will have to find ways of proving it is delivering the ventilation performance required in buildings and demonstrate its compliance with the standards that will emerge. Part F of the Building Regulations, which is currently under review, could be used to beef up the IAQ measures likely to be introduced in the forthcoming Environment Bill, the BESA webinar heard.
“The gap is between what we write down as a standard and how we deliver. There are buildings out there that don’t have any controlled ventilation at all – they don’t meet any building standards, but we can’t just close them down,” Noakes explained.
The government will have to find a balance between its ambition for a net zero carbon built environment and one that achieves good health outcomes for people, she added.
“I hope that people have recognised early enough that ventilation is really important. There is a risk that we try to deal with net zero by sealing [buildings] up more. The problem is that it is always much easier to measure energy performance than ventilation effectiveness – we have to address that too.”
BESA’s head of technical Graeme Fox thanked Noakes for sharing her hugely valuable analysis with the Association’s members and the industry at large. He said her advice was consistent with the messages in BESA guidance on ventilation and IAQ that was attempting to turn the lessons learned during the pandemic into practical measures for building owners and managers as well as engineers.
“Her presentation exposed the full complexity of this issue, which reinforces the urgent need for simplified, practical guidance,” said Fox. “It also confirmed that there are no silver bullets and our industry is in the forefront of efforts to develop properly planned solutions for the long-term health and wellbeing of building occupants.”