Winds of change: Why building ventilation has never been so important
New research suggests mechanical ventilation halves the risk of spreading airborne diseases in schools and offices compared to natural ventilation.
Like electric cars and virtual events, future historians are likely to regard 2021 as the year when indoor air quality (IAQ) finally went mainstream.
For decades, we have been wary of outdoor air pollution. Toxic air is responsible for one in nine early deaths globally. It kills seven million people a year: more than HIV, tuberculosis and malaria combined. Vehicle fumes and fossil fuel burning have been the major culprits, but the Covid pandemic has brought into much sharper focus indoor air quality. We are now very conscious of the air that we breathe indoors, where we spend 90% of our time.
The average adult breathes 18,000 and 20,000 times per day: Sucking in and filtering around 8,000 litres of air every day. At the same time, it’s been proven that poor indoor air quality and inadequate ventilation encourages the transmission of infectious diseases. The safest office will always be your own house. The cleanest restaurant will always be your kitchen. We know these environments are unlikely to harbour invisible clouds of diseases.
But with ‘normal life’ tantalising close as the pandemic hopefully reaches its final stages, in the UK at least, we’re hearing from so many genuinely concerned businesses about what to do next – about how to reopen safely and with confidence. Is opening a few windows this summer good enough? What about the winter, energy efficiency, and the weather, noise and pollution? There’s plenty of generic advice but building owners are crying out for official and clear-cut guidance. The failure to distinguish between indoor mechanical or natural ventilation as part of this debate is also not surprising as we’re facing a complex and new challenge.
A breath of fresh air
In May this year, a global group of 39 leading scientists issued an urgent call in the journal Science to overhaul indoor air quality as a safeguard against the spread of contagions. Covid is spread via infectious aerosols which can stay airborne for long periods, particularly in poorly ventilated areas. A number of lab experiments have demonstrated that it can be aerosolised and survive in this form – up to 16 hours according to one study.
S&P’s European research and development centre has also conducted detailed research with the sole aim of shedding light on the difference mechanical ventilation can make over natural ventilation to improve air quality and curb the risk of spreading airborne diseases in indoors. A white paper produced by the centre concludes that mechanical ventilation systems reduce by half the risk of aerosol transmission compared to natural ventilation, such as opening windows and doors which has the added disadvantage of letting in noise, pollution and cold air in the winter.
Detailed scientific modelling and analysis using sophisticated software considered three scenarios: schools, offices, and bars/restaurants.
Let’s go back to school. The study found that after four consecutive days of an infected teacher leading a class of 24 masked students, the accumulated probability of infection stands at 21% with natural ventilation – resulting in five pupils infected. With mechanical ventilation, this falls to 9% – or two pupils infected. With no ventilation at all, seven pupils would contract Covid. In the event of a student being positive rather than the teacher, the numbers are halved.
How about the winter?
Opening windows for 10 minutes every hour results in a 4.8% infection rate, meaning four cases after four days – twice as high as mechanical ventilation.
Meanwhile, in a restaurant with 35 unmasked diners but a masked infected waiter, with natural ventilation, two customers are infected in the first two hours, and two more are infected over the second two-hour period, giving a total of four cases. With mechanical ventilation, again it is halved to one infected customer in each two-hour period, or a total of two infections. With an infected customer instead of the waiter, with open windows, three customers contract Covid by the end of evening. With mechanical ventilation, this is reduced to one.
There are long-held concerns that in most offices air-conditioning units only recirculate existing air, rather than pumping in fresh air and extracting stale air using mechanical ventilation systems. In the commercial world, S&P’s study found that in an open plan office with 40 masked workers where one is infected, there are four infected employees after four days with natural ventilation, and two with mechanical. With no masks, it rises to 13 infections with open windows but just eight infections with mechanical ventilation.
Our authors, computational fluid dynamics engineers, Albert Carrasquer and Jordi Fabregat, concluded: “Mechanical ventilation can reduce by more than twofold the infection risk when compared to open windows.”
Raising the bar
Governments around the world have invested heavily in food safety, sanitation and drinking water for generations. But what about air quality and ventilation? And the effects – the consequences – of building designs, regulations and standards? We expect the air we breathe in a restaurant to be as clean as the glass of water that the waiter brings. We expect the water cooler in the office to be as healthy as the debate over the day’s national talking point. So why shouldn’t we expect, or demand, higher standards for ventilation? In the 19th century, a paradigm shift eradicated the notion that we must simply live with dirty drinking water. Can we use Covid to revolutionise indoor air quality in the 21st century?
Researchers further stated: “If a lower infection risk is desired, the installation of indoor ventilation systems must deliver higher ventilation rates than those defined by British standards. However, it is important to note that these standards were not devised in the context of a global pandemic with the associated risk of airborne transmission.”
Meanwhile, the Building Engineering Services Association (BESA) has called for a landmark coroner’s report to form a key part of new air quality laws. It follows the tragic case of nine-year-old Ella Kissi-Debrah who became the first person in the world to have air pollution listed on a death certificate. The Lewisham child had been exposed to nitrogen dioxide and particulate matter (PM) pollution well above World Health Organisation guidelines – primarily from traffic close to her home and school. Ella's Law is hoped to prevent around 40,000 excess deaths in the UK every year.
With stricter hygiene measures likely to remain for the foreseeable future, there are also low-cost, short-term and efficient solutions to enhance indoor air quality and help the economy to reopen safely and with confidence. Take air purifiers, for instance. There are plenty of highly-efficient units which can improve indoor environments. Most feature a filtration efficiency of over 99.5%, meaning they’re very capable of trapping viruses and bacteria to provide optimal levels of indoor air quality.
Elsewhere, the Scientific Advisory Group for Emergencies (SAGE) has said building ventilation should be “integral to the Covid risk mitigation strategy for all multi-occupant public buildings and workplaces”.
Professor Stephen Reicher, of SAGE, recently raised the prospect of ventilation grants to help businesses with the costs.
“In the same way we used to have insulation grants, how about we have ventilation grants – both for private homes and businesses, to make sure the spaces we go into aren’t ones where Covid can accumulate, can hang around and affect us”, he told BBC News.
If there are any silver linings to the past 18 months, then underscoring the urgent need for better indoor air quality management must be one of them.
Authors: Diaz Carrasquer, Albert J, CFD Engineer – Aerodynamics and Acoustics Lab., IDI, S&P Research SLU, Soler & Palau Ventilation Group and Lanuza Fabregat, Jordi CFD Engineer – Aerodynamics and Acoustics Lab., IDI S&P Research SLU, Soler & Palau Ventilation Group