The quest for clean and healthy indoor environments has become increasingly critical in our urbanised world. Considering that we spend a significant period of our lives indoors, the quality of the air we breathe plays a pivotal role in our overall health and wellbeing. However, we seem to be overly reliant on air filtration as a band-aid and sole solution to all Indoor Air Quality (IAQ) issues without addressing the root causes of pollution.

Achieving optimal IAQ sustainably is complex, particularly when integrating HVAC and filtration performances in a single unit. Inconsistencies and deviations in filter performance, coupled with the influence of operational conditions, such as flow rate in the air-handling unit (AHU) within which they operate, add to the complexity of attaining the desired IAQ outcomes. Additionally, any vibrations or imbalances affecting the AHU’s frame – be they owing to a performance degradation of the fan/motor mounting assembly or external factors – can be transmitted to the filter frame and media structure. This can result in the re-entrainment of previously captured particles, thereby compromising the overall efficiency of installed filters. Furthermore, improper installation of filters can allow particles to bypass the filtration process. All these issues defeat the intended purpose of installing a filter in the first place.

It is critical to minimise such performance deviations, since many factors impacting filter performance act as moving targets to raise the bar of IAQ. Ideally, the filtration process can be better predicted when the filter media is homogeneous and particles are large, spherical and uniformly sized. However, while all these pre-conditions seem improbable, filtration pioneers need to engineer their filter design and media solutions to make them fit for purpose. Air filter performance can deteriorate, and clogging may accelerate deterioration when the filter is exposed to various outdoor and indoor pollutants with different physical characteristics and chemical compositions, particularly those emitted from sand storms, wildfires and volcanoes.

Urban air pollution

Outdoor and indoor air may contain a wide variety of contaminants, including particulate matter, gases, radon, biological contaminants (mould, bacteria, fungi, dust mites, spores and pollen) and multiple organic and inorganic chemical compounds, each with their own associated health risks. The World Health Organization estimates that 5.13 excess deaths per year globally are attributable to ambient air pollution from fossil fuel emission^[1,2]. This is a stark reminder of our addiction to fossil fuel combustion and the lives that could be saved by phasing out fossil fuels.

One should reflect on the nature and trajectory of our current growth as we continue deforestation and urbanisation, spending on filtration technologies to champion the clean air cause in the buildings we occupy. Therefore, before rushing to install countless filters to capture pollutants of various physical and chemical characteristics, we must establish the root causes of our anthropogenic emissions. This means critically examining the sources of pollution, such as industrial processes, transportation and power generation, and implementing strategies to reduce emissions at their source.

We cannot allow our planet to become a never-ending landfill overflowing with discarded filters and overwhelming our recycling systems. Just as an overflowing bathtub cannot manage excess water, the Earth struggles to cope with the immense air pollution and waste. The current situation has transformed what once was a manageable lifecycle of a planet capable of self-regeneration into an environmental crisis. This crisis stems from overproduction and overconsumption, which disrupt the natural balance. This disruption is highlighted by the concept of the bathtub surge, as proposed by Kenneth Boulding^[3].

Focusing solely on air filtration leads to a false sense of security, potentially diverting attention and resources from addressing the underlying drivers of pollution, akin to treating the symptoms of a disease or disorder without understanding the cause. While filters can temporarily alleviate the symptoms of poor air quality, they do not address the systemic issues that contribute to pollution in the first place. Moreover, it is crucial to recognise the non-linear relationship between pollution and air filtration. Adding more filtration stages is analogous to adding excessive fertilisers to our plants to accelerate growth and harvest; it does not necessarily translate to a proportional increase in overall efficiency. Each additional stage adds performance complexity, increases energy consumption, and may lead to diminishing returns in terms of air quality improvement. This is especially true in the absence of a detailed account of the pollutant’s characteristics challenging installed air filters and what they are designed to tackle.

For instance, while a single filter might capture 50% of a specific airborne particle size, adding another identical filter does not necessarily double the particle capture efficiency or perfect its performance. A simplistic assumption of that nature neglects performance parameters impacting filter loading conditions, most of which are transient and impact particle deposition on or within filter media. This highlights the importance of optimising filtration systems based on specific needs and considering factors like filter design, airflow, particle interactions with filter fibres, and their physical and chemical characteristics.

Filtration is not a buzzword

Today, air quality and filtration are not included in the cache of popular buzzwords like energy efficiency, sustainability, AI and digital transformation. Air quality should have been woven into the tapestry of city design and urban planning decades ago but was undoubtedly overshadowed by the hype of other priority tides that better serve the business model. Current overreliance on air filters suggests that not only are we overpolluting, but we are also overproducing them.

Bridging the filtration gaps

One of the primary hurdles in ensuring effective IAQ is the discrepancy between laboratory test results and real-world filter performance. Laboratory settings often provide idealised conditions, neglecting the dynamic nature of urban environments. Fluctuations in climate, ambient conditions, and filter manufacturing and loading scenarii significantly impact filter efficiency and longevity. For instance, a filter that demonstrates high efficiency in a controlled laboratory environment may exhibit reduced performance in a humid, heavily polluted urban setting with an increased particle concentration. This disparity underscores the need for a more nuanced understanding of filter behaviour under diverse operational and filter-loading conditions. Therefore, to effectively select air filters, it is essential to understand how they perform under various conditions, not just in the controlled settings of a testing laboratory.

The variability in filter loading scenari poses a significant challenge. Filters are designed to capture airborne particles, but the rate at which they accumulate these particles can vary widely. Subjecting air filters to lower particle concentrations than those used when loading filters in laboratories may lead to different efficiency levels, which are likely better; however, that is not guaranteed, as it requires the correlation of efficiency to particle size distribution. High-efficiency filters should be entertained if the vast majorirty of the particles are PM1 (particle ≤ 1μm). On the other hand, during sandstorms, in areas of high traffic, or during periods of intense pollution, filters may become overloaded, leading to a decline in efficiency and an increase in pressure drop. This compromises IAQ and increases energy consumption, as HVAC systems work harder to push air through clogged filters. Striking a balance between filtration efficiency and energy consumption is essential for sustainable IAQ management. This can be achieved by developing a permeable, high-efficiency advanced filter media providing minimal pressure drop. In addition, developing filters with robust loading capacities and self-monitoring capabilities is crucial for maintaining consistent and reliable performance through feedback loops to achieve desired IAQ outcomes.

The road ahead

While full of uphill environmental battles, the road ahead holds immense promise if it can shape the story of human progress towards sustainability. These objectives are achievable if we prioritise pollution-prevention strategies over sole reliance on air filtration. In other words, we ought to develop and implement sustainable practices in transportation, energy consumption and industrial processes to lower pollution. We should also consider the entire lifecycle environmental impact of air filtration solutions. Lastly, we should focus on

researching and developing more energy-efficient and adaptable filtration technologies to incorporate air quality considerations into the initial stages of city design and urban planning. As we progress towards the 2030 Challenge^[4], it is essential to embrace science-based filtration solutions that ensure sustainable performance throughout their entire lifecycle.

Breathe easy

Enhancing air quality demands more than just air filtration; it requires rethinking city design and promoting responsible lifestyles that reduce emissions. We can prioritise clean air in urban planning by incorporating smart building technologies, establishing legally binding IAQ standards, and fundamentally understanding real-time filter performance encompassing building design, construction and operation phases. The assiduous implementation of these steps can ensure that clean air becomes an inherent characteristic of buildings, not a chronic engineering challenge.

References:

[1] World Health Organization. WHO global air quality guidelines. WHO, 2021.

[2] European Environmental Agency (2020). Towards zero pollution in Europe. Luxembourg: Publications Office of the European Union, 2020.

[3] Valentinov, V. (2015). Kenneth Boulding’s Theories of Evolutionary Economics and Organizational Change: A Reconstruction. Journal of Economic Issues, 49(1), 71–88. https://doi.org/10.1080/00213624.2015.1013880

[4] United Nations (UN). (2015). Trasforming our world: The 2030 Agenda for sustainable development