Historically speaking, cities were perceived as unnatural for humanity and a culmination of our exile from rural life, where clean air and resources are abundant. In describing the isolation that falls on a soul in a city, Jean-Jacques Rousseau would declare towns as “the abyss of the human species” while dismissing cities as “pestilential to man’s morals, health and liberties”. As countries strive to depart from poverty to prosperity through building cities, the unprecedented scale, speed and scope of urbanisation spark doubts about its sustainable growth. 

Cities expanded, attracting inhabitants through their novelty as hubs of commerce, culture, education and economic opportunity. Urban population went from five per cent of the total population in the early 1800s to 54%, today[1]. Recent projections show that the global urban population in cities could reach 68% by 2050, adding 2.5 billion people to the urban environment. Although cities can represent a low-carbon way to live, where 80% of economic activity occurs[2], they consume 78% of the global energy generated and produce 60% of greenhouse gas emissions. However, they only account for two per cent of the Earth’s surface[3]. 

There may be nothing wrong with inviting people to move to the city and enhance their standard of living. There is nothing naive in intending to break the cycle of poverty by getting a decent education and attaining economic empowerment for dignified earnings and retirement. However, we cannot get people out of poverty and cluster them into polluted cities, limiting them to a mutually exclusive choice of being poor or polluted. Furthermore, clustering people in cities renders them a catalyst for virus transmission, impacting more inhabitants and sparking additional environmental, economic and social complexities. Why can’t we grow without polluting? If the data is correct and the science is precise, then inducing positive change requires that we grant our planet and its ecosystems a chance to regenerate. 

During the pandemic, air quality came under scrutiny, revealing the long-standing weaknesses in the installed air filters in HVAC systems that compromise the fitness of the built environment design and its building envelope for human occupancy. Cutting corners when selecting air filters can quickly result in frequent maintenance issues, deteriorate IAQ and accelerate the ageing of HVAC equipment. Although retrofitting has been the hype in recent years, alongside gentrification initiatives, their mere focus is land and property valuation rather than establishing infrastructure for air quality monitoring and enhancement. Furthermore, as building owners and decision-makers take a laissez-faire approach to air quality enhancement and inclusion, they exempt themselves from exposing human occupants to polluted air. They think providing their maintenance teams with primitive tools, such as screwdrivers and safety shoes, will position them to succeed. 

Going the extra mile 

Ambient conditions impact the performance of HVAC and filtration systems differently, as pollutant types, concentrations, size distributions, filter loading conditions, heat and moisture levels may vary simultaneously. Today, the premise of comprehensive air filtration solutions goes beyond particle capture to include gaseous and bioaerosols. Although air monitoring and filtration technologies may be ready to be enablers of better IAQ, they cannot be the only drivers of a healthy environment. Furthermore, capturing all pollutant types is not only impractical but also improbable. This is partly due to squeezing air filters in limited spaces within air-handling units focusing on particle capture only. Ultimately, such limitations impede any attempt to provide sustainable filter performance to enhance air quality by attaching higher filtration efficiency against all airborne pollutants. Therefore, continuous air quality monitoring 

characterises contaminants of interest and sets the filtration stages to mitigate their concentrations appropriately. Going the extra mile could mean employing online measurement of air filter performance during operation, which can prove invaluable in monitoring its efficiency, not just the rise in pressure drop. That would facilitate data relevant to the entire filter performance during its operative lifetime. Equally important is identifying the pollutant sources and realising that they can also be emitted from indoor sources, such as occupants, their activities, furniture and construction materials. 

Thriving for a pandemic-proof city 

What is the cost of enhancing IAQ? Why is it not worth investing in? How is the loss of IAQ calculated in the factoring of profit? To answer these questions, we need to understand the magnitude of our overwhelming losses incurred in the recent pandemic due to seven million deaths, with over two million of them in the United States, Brazil and India alone[4]. Going beyond lives lost, the devasting bill that came due to lockdowns, imposed to slow the spread of the disease and ease pressure on healthcare systems, triggered an unprecedented global recession. Therefore, drawing commitments from such losses necessitates a new approach to reduce anthropogenic emissions, as we continue to unsustainably replace forests with concrete and asphalt. 

The pandemic revealed and amplified decades of epic errors, centuries of bad habits and negligence in air quality practices. Scant and fluctuating global attention on air quality further twists the diction. The pandemic was also a compendium of poorly implemented standards, maintenance hacks, inappropriate filter selection and quick-fix installation. But filtration alone cannot render our building envelopes safe to occupy, irrespective of our outdoor air pollutants. While filtration technologies can remove various pollutant types, their availability does not justify the pace of current anthropogenic emissions. Meanwhile, widely used practices, such as cleaning diffusers, ducts and coils give an ominous glimpse of the consequences of adhering to old maintenance hacks. Furthermore, disposing of loaded filters remains a pressing waste issue that hardly adheres to the circular economy metrics. Ultimately, engineering the filtration requirements and selections is emphasised to extend the filter life cycle through sustainable performance and operation. 

Figure 1: Coil washing with chemical is a frequently practised measure to recover the heat transfer performance 

Figure 2: Mould growth and dust loading on a typical air diffuser 

Figure 3: Replacement of damaged cooling coil due to dust-loading and inappropriate maintenance 

If we cannot be inspired by the *miasma theory[5], perhaps we could be more intrigued to know that air pollution and dust challenges are not new. The problem of dust can be traced back to Roman times. Pliny the Elder (ca. A.D. 50) observed the toxic properties of lead oxide, sulphur dioxide, and mercury sulphide and described a rudimentary protective mask. Bernardino Ramazzini highlighted in his book (1713), Diseases of Workers that diseases could be correlated to occupation or environment types. At the time, face masks used for personal protection were made of animal bladders. In 1556, Georgius Agricola recommended ventilation and personal respirators to protect workers in mines in his book, De Re Metallica. In 1955, Professor Nikolai Fuchs published his book, The Mechanics of Aerosol that laid the foundation of aerosol science, when neither computers and lasers nor spectroscopy analytical tools were available. 

Celebrating all these early achievements lead us to infer that we should have defeated viruses and choked their transmissions in cities once and for all, given the technologies and innovations available at our fingertips. Historically, personal protection was sought as there was no control over the indoor environment. Today, well-maintained and protected central HVAC systems could provide more control of thermal comfort while aligning air quality targets to observe the application-sensitivity of a given indoor environment to render it safe, healthy and fit for purpose. Economic empowerment is also critical to the vitality of nations, where trade and commerce are key. However, history encourages us to learn from cities such as Venice. 

Venice, a gateway for trade into the continent, represents a great example of balancing economic priorities and a healthy city. In 1348, Venice was hit by the Black Death, which arrived on its shores on a merchant ship. The reliance of Venice on global trading and the continuous influx of infected travellers, exposed locals to the plague, which by 1352 had claimed the lives of around 60% of the city’s inhabitants. Venice was forced to impose its first quarantine, which required all ships from infected ports to sit at anchor for 40 days before landing[6]. 

The recent pandemic, wildfires and escalated air pollution have highlighted the critical role of governments in developing the capacity to prevent pandemics and to respond to air quality deterioration. Weak statutory underpinnings for air quality and the built environment have increased cities’ vulnerability to the pandemic. Vague or nonexistent legislation to govern and control the air quality human occupants inhale raises many questions and concerns. Addressing the spectre of virus transmission necessitates an in-depth analysis of existing city designs, coastal clustering urbanisation and street canyons. These pressing issues lead to traffic congestion and degrade urban air quality, further challenging the built environment. Therefore, embedding air quality legislation and governance as a function of the city’s topography and ecosystems is imperative before embarking on urbanisation. 

Urbanised but polluted 

Rising on a broken IAQ ladder suggests that cities can fall harder, faster and farther into the trap of future pandemics. Attaining and maintaining air quality in the built environment should be an explicit design priority and essential service similar to clean water, sanitation and electricity. The critical role of governments must be emphasised to lead the movement towards healthy, safe and sustainable cities. Air quality enhancement budgets can no longer split the difference with other HVAC-related maintenance endeavours. This ailing approach dismisses innovations and technologies, such as air quality monitoring and filtration, and regards them as expensive items with a peripheral function to building operations. The value of clean air lies in protecting the wellbeing of human occupants, rendering our indoor environment fit for healthy living and optimum productivity. Inducing change requires an architecture of innovation calling for a more engaged design of HVAC and filtration systems with adaptive performance to respond to variations in IAQ. Over-populating urban centres that are already polluted sets them up for defeat against virus transmission and challenges their ability to help their inhabitants prosper. As countries rapidly urbanise, they should embrace business models that decouple economic growth from increased emissions to turn the tide of anthropogenic emissions. As humanity faces monumental climate challenges, we all – novices and experts alike – have a role in shaping the future. 

*Miasma theory: 

An abandoned medical theory that believed diseases were caused by a noxious form of bad air, such as a cloud of foul-smelling vapour, like swamp gas. 

[1] Zinkina, J., Ilyin, I. and Korotayev, A., 2017. The Nineteenth-Century Urbanization Transition in the First World. Globalistics and globalization studies, pp.164-172. 

[2] Calthorpe, P., 2015. Urbanism in the age of climate change. In The city reader (pp. 555-568). Routledge. 

[3] MAISONNEUVE, C., 2023. Decarbonizing European Cities: How to Speed Up and Build Synergies? 

[4] Msemburi, W., Karlinsky, A., Knutson, V., Aleshin-Guendel, S., Chatterji, S. and Wakefield, J., 2023. The WHO estimates of excess mortality associated with the COVID-19 pandemic. Nature, 613 (7942), pp.130-137. 

[5] Bloom, B.L., 1965. The “medical model”, miasma theory, and community mental health. Community Mental Health Journal, 1, pp.333-338. 

[6] “Jedwab, R., et al., 2019, ‘Pandemics, places, and populations: Evidence from the Black Death’, CESifo Working Paper.” 

Pandemic, COVID-19, air filtration, IAQ, IEQ, Indoor Air Quality, urbanisation, airborne diseases, bioaerosols

Dr Iyad Al-Attar, an independent air filtration consultant, writes on specific science and technology issues relating to Indoor Air Quality, including airborne particles. He may be reached at iyadalattar@yahoo.com.