Author: González López De Aspe Enrique
It is common knowledge that atmospheric pollution has a significant impact on health due to the wide range of harmful particles that may be present in the air we breathe. However, there is a widespread lack of awareness regarding exposure to air pollutants, as this issue is usually linked to outdoor spaces, overlooking the importance of indoor air quality and the fact that people spend most of their time indoors.
In fact, it has been proved that we spend around 90% of our time in closed spaces, where certain pollutants can be 2 to 5 times higher compared to typical outdoor concentrations [1]. Therefore, the WHO has focused on raising awareness of this risk by publishing indoor air quality guidelines, including a comprehensive review of the most relevant pollutants, their sources and their health effects [2].
Figure 1: WHO guidelines for indoor air quality. Source: World Health Organization
Concerning strategies, indoor air quality can be improved by controlling sources of pollutants, designing ventilation systems to dilute contaminated air, or cleaning the air [3]. This article will focus on the third strategy.
Introduction to air filters
Since the appearance of the first models at the beginning of the twentieth century, air filters have undergone remarkable development, leading to the introduction of the latest generation of systems. The most relevant technologies developed during these 100 years are glass fiber air filters, electrostatic filters, activated carbon filters (ACF), high-efficiency particulate air filters (HEPA), ultra-low penetration air filters (ULPA), plasma filters and photocatalytic filters.
Today, most HVAC systems use fiberglass or electrostatic filters which have limited effectiveness, as they are designed to retain dust and dirt. Typically, more advanced filters are used when the removal of extremely small particles is required such as in high-security installations or critical ventilation systems, the most common case being hospitals. Concerning these advanced types of air filters, the following classification is adopted:
- Established systems: HEPA and UHLA filters whose effectiveness is based on their porous structure.
- Emerging systems: photocatalytic and cold plasma filters whose effectiveness comes from complex processes.
Photocatalytic and cold plasma filters emerged in the 1990s as innovative alternatives to conventional filters. In the last decade, these systems have developed remarkably and become one of the most important lines of research for removing air pollutants. The main feature of these systems is that, unlike conventional filters, they do not remove pollutants by retention, but use chemical and physical processes to transform them into less harmful compounds.
Photocatalytic filters
As mentioned above, pollutants passing through photocatalytic filters are not retained but degraded due to photocatalysis, a chemical process based on the absorption of radiant energy (light) by a heterogeneous photocatalyst (usually TiO2). This leads to the formation of electron-hole pairs that react with oxygen and water and give rise to highly oxidizing species, capable of degrading microbiological pollutants by destroying the cell wall or oxidizing internal elements, causing cell death. As a result, less harmful products such as CO2 or water are obtained [4].
Regarding this statement, it should be noted that CO2 is currently recognized as a pollutant in its own right, meaning that products resulting from photocatalysis mat not be completely innocuous.
Figure 2: Photocatalysis process. Source: El-seesy et al (2011).
Some advantages of photocatalytic filtration are the wide variety of contaminants it can remove, the low energy consumption required for its operation and the photocatalysts’ durability. Consequently, it is considered a promising technology for improving indoor air quality, although it is ineffective against suspended particulates. Considering this limitation and the inability of HEPA filters to remove gases and odors, both technologies are frequently used together, resulting in an extremely efficient system capable of removing suspended particles, gases, odors, bacteria, viruses and volatile organic compounds (VOCs).
Cold plasma filtration
Plasma is generated by ionizing certain gases using an external energy source. Specifically, cold plasma technology uses a low-temperature ionized gas formed by several electrically charged particles. When these particles collide, they can generate species capable of decomposing certain air pollutants into harmless products through an oxidation process [5]. Therefore, it is a technology whose effectiveness lies in degradation rather than retention.
Recent investigations have demonstrated the effectiveness of cold plasma in decomposing volatile organic compounds (VOCs), odors, bacteria, fungi, and viruses. It has been found that, even with a very low exposure time (0.06 s), high levels of decontamination (85%-98%) can be achieved [5]. Other advantages of this technology include the durability of the filters and the low energy consumption.
Cold plasma filters are one of the most effective technologies for improving indoor air quality. However, these systems have a significant drawback: they can generate undesirable chemical by-products, such as ozone and nitrogen oxides. Therefore, their application must be carefully evaluated and controlled to minimize these adverse effects. One of the most appropriate strategies to solve this problem involves using other technologies such as photocatalytic filters. Non-thermal plasma-photocatalysis is an integrated air purification technology that removes pollutants from the air using photocatalysis and non-thermal plasma processes. Combining these two systems increases air purification efficiency and minimizes detrimental effects on air quality [5].
Conclusion
Indoor air pollution is a major problem that has traditionally been overlooked even though people spend most of their time in closed spaces. However, international organizations are now making significant efforts to raise awareness of the harmful effects of permanent exposure to foul air. Additionally, there is an important investment being made in the development of new air filtration technologies.
The evolution of air filters has been extraordinary since the introduction of the first models in the early 20th century, leading to the development of high-efficiency systems such as cold plasma filters and photocatalytic filters. Unlike conventional filters, these technologies do not remove contaminants by retention but by means of complex processes through which pollutants are degraded. Although each technique has limitations, combining them leads to a highly effective system for removing air pollutants without generating harmful byproducts. Thus, non-thermal plasma-photocatalysis is considered one of the most promising filtration systems. This statement is reflected in the attention that this technology is currently receiving from researchers.
References
[1] United States Environmental Protection Agency (2023). Indoor air quality. Indoor Air Quality | US EPA
[2] World Health Organization (2023). Guidelines for indoor quality. Selected Pollutants. WHO guidelines for indoor air quality: selected pollutants
[3] Wang et al (2011). Characterization and performance evaluation of a full-scale activated carbon-based dynamic botanical air filtration system for improving indoor air quality. Characterization and performance evaluation of a full-scale activated carbon-based dynamic botanical air filtration system for improving indoor air quality – ScienceDirect
[4] Huang et al (2016). Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A Short Review and Prospect. Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A Short Review and Prospect – PubMed (nih.gov)
[5] Liu, G et al (2017). A review of air filtration technologies for sustainable and healthy building ventilation. A review of air filtration technologies for sustainable and healthy building ventilation (core.ac.uk)