Strategies for Improving Indoor Air Quality through Ventilation and Air Conditioning Systems

Since the first case was diagnosed in China in November 2019, as of December 24, 2022, COVID-19 has caused over 650 million confirmed cases and 6.5 million deaths worldwide. Due to the recurrent outbreaks of the SARS-CoV-2 (COVID-19) pandemic worldwide was primarily caused by infected individuals transmitting the virus to others indoors through droplets, there is an urgent need to design new indoor ventilation models. To prevent cross-contamination caused by viruses, bacteria, and other pollutants spreading through indoor air, it is necessary to redesign and add components to combat the SARS-CoV-2. Properly designed purification systems can effectively filter the air in the space, providing occupants with a safe and comfortable environment by utilizing air purification technology to eliminate pollutants from the environment.

Air conditioning systems play a significant role in preventing the COVID-19 pandemic, and the use of ventilation and air conditioning (HVAC) systems has become an urgent issue to address within the air conditioning industry. As of 2022, related institutions such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the Federation of European Heating, Ventilation and Air-Conditioning Associations (REHVA), and the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan (SHASE) have issued documents responding to the COVID-19 pandemic. With the worsening global pandemic, ASHRAE has published building operation guidelines on its website and updated its position on airborne transmission of diseases. REHVA also released a guide on HVAC systems for SARS-COV-2 and mentioned that the document will be updated as necessary, indicating the importance of indoor ventilation models in preventing the spread of COVID-19.

Recently, more scholars have pointed out that SARS-CoV-2 contains aerosols measured in microns and sub-microns. The Centers for Disease Control and Prevention (CDC) in the United States also stated that pathogen-carrying aerosols can accumulate in poorly ventilated indoor spaces such as classrooms, clinics, large hospitals, offices, restaurants, bars, and other community environments, which could lead to SARS-CoV-2 infections when inhaled.

Moreover, in the same indoor environment without external air introduction and filtration, toxic pollutant contamination may occur, and allergens such as pollen, dust, bacteria, or fungi can also be present. Commercially available portable air purifiers, such as high-efficiency particulate air (HEPA) purifiers, have been proven to be effective. They can efficiently filter pollutants without altering the existing air conditioning system design. If portable air purifiers are effectively used with existing air conditioning equipment to increase air exchange rates and filtration efficiency, this can significantly reduce the energy consumption and cost of operating air conditioning equipment while improving air quality.

The recurring global COVID-19 infections raise a significant question: how long will it take to completely eradicate this infection? SARS-CoV-2 can spread on such a large scale because it is transmitted through the air in enclosed indoor environments. Although there is still debate over the relative contribution of different transmission routes to the spread of SARS-CoV-2, the current evidence is sufficient to conclude that indoor airborne transmission is primary. Poor indoor ventilation is associated with increased transmission of respiratory infections. Researchers described the principles of respiratory virus transmission through aerosols and the factors affecting the transmission process in indoor environments. The suspended particles of three sizes, 100 μm, 5 μm, and 1 μm, take approximately 5 seconds, 33 minutes, and 12.2 hours respectively to fall from a height of 1.5 meters to the ground. Therefore, while short-range transmission (<1 m) through inhalation of "droplet nuclei" has been considered the main route, long-range transmission (>2 m) through inhalation of "aerosols" should not be ignored.

Figure 1: Theoretical Airborne Biology of Droplets and Small Particles Produced by Acute Infection Patients

Kumar and Morawska mentioned important preventive measures, including maintaining social distancing, wearing masks, frequently washing hands, disinfecting surfaces, increasing indoor air exchange rates, and using portable air purifiers. Blocken and other researchers' experiments concluded that using portable air purifiers combined with existing air conditioning systems can reduce aerosol particle concentrations by 80-90%, making this energy-saving method applicable to various indoor environments.

Additionally, the current air pollution is worsening worldwide. The World Health Organization warns that air pollution poses a health risk, causing approximately 7 million deaths annually. According to the 2014 Global Air Pollution Survey by the World Health Organization, Taiwan ranks 60th out of 90 countries in air quality. In addition to the outdoor PM2.5 particulate matter causing public panic, poor indoor air quality has led to Sick Building Syndrome (SBS) in many buildings. Poor indoor air quality (IAQ) pollution causes various discomfort symptoms for users. According to research, people spend approximately 80-90% of their time indoors daily (including at home, in offices, or other buildings). Indoor air quality directly affects work quality and efficiency. Therefore, in November 2012, the Environmental Protection Administration announced and implemented the Indoor Air Quality Management Act, setting regulatory standards for nine pollutants, including carbon dioxide, carbon monoxide, formaldehyde, bacteria, and fungi. Regulated venues must appoint dedicated air quality maintenance personnel, develop management plans, conduct regular inspections, and disclose the results.

Currently, the types of air pollutants are becoming increasingly complex. Before discussing the types of air pollutants, we must first have a clear definition of air pollutants. The 2018 amendment of the Air Pollution Control Act states the legislative purpose is "to control air pollution, maintain the living environment, and protect public health to improve quality of life." Broadly speaking, any substance in the air that has negative effects on humans, animals, and the surrounding environment can be considered an air pollutant. Generally, air pollutants can be categorized into "gaseous pollutants" and "particulate pollutants," with the latter further divided into liquid and solid particles. The 2003 revision further subdivides air pollutants as follows:

Table 1: Types of Pollutants Announced by the Air Pollution Control Act Implementation Rules
(Source: Environmental Protection Administration)

The primary goal of air filtration is to clean and control pollutants entering the atmosphere of the workplace and living environment. The filtration systems that protect people include ventilation, air conditioning (HVAC), and air purification systems, these measurements can keep the living and working spaces free of bothersome air pollutants such as viruses, irritating dust, microbial allergens, and other volatile organic gases.

Reference:

1. https://news.campaign.yahoo.com.tw/2019-nCoV/index.php
2. J. Schoen, “Guidance for building operations during the COVID-19 pandemic, vol. 5, p.3, 2020.
3. J. Stewart et al., “ASHRAE position document on infectious aerosols,” ASHRAE Atlanta, GA, USA, 2020.
4. REHVA, “REHVA COVID-19 Guidance Document Version 4.1. How to Operate HVAC and Other Building Service Systems to Prevent the Spread of the Coronavirus (SARS-CoV-2) Disease (COVID-19) in Workplaces.” REHVA Brussels, 2021.
5. K. Coleman et al., “Viral Load of SARS-CoV-2 in Respiratory Aerosols Emitted by COVID-19 Patients while Breathing, Talking, and Singing,” medRxiv, p. 2021.07.15.21260561, Jan.2021, doi: 10.1101/2021.07.15.21260561.
6. O.Adenaiye et al., “Infectious SARS-CoV-2 in Exhaled Aerosols and Efficacy of Masks During Early Mild Infection,” medRxiv, p. 2021.08.13.21261989, Jan.2021, doi: 10.1101/2021.08.13.21261989.
7. A. Workowski and S. M.Berman, “Centers for Disease Control and Prevention Sexually Transmitted Diseases Treatment Guidelines,” Clin. Infect. Dis., vol. 44, no. Supplement_3, pp. S73–S76, Apr. 2007, doi: 10.1086/511430.
8. Renyi, L. Yixin, Z. A. L., W. Yuan, and M. M.J., “Identifying airborne transmission as the dominant route for the spread of COVID-19,” Proc. Natl. Acad. Sci., vol. 117, no. 26, pp. 14857–14863, Jun.2020, doi: 10.1073/pnas.2009637117.
9. ECDC, “Heating, Ventilation and Air-Conditioning Systems In The Context of COVID-19,” Eur. Cent. Dis. Prev. Control., no. June, pp. 1–5, 2020, [Online]. Available: https://www.ecdc.europa.eu/sites/default/files/documents/Ventilation-in-the-context-of-COVID- 19.pdf.
10. C.C. et al., “Airborne transmission of respiratory viruses,” Science (80-. )., vol. 373, no. 6558, p. eabd9149, Jun.2022, doi: 10.1126/science.abd9149.
11. W. Tang, L. C. Marr, Y. Li, and S. J. Dancer, “Covid-19 has redefined airborne transmission,” BMJ, vol.373, p. n913, Apr.2021, doi: 10.1136/bmj.n913.
12. A. Stern et al., “Characterization of hospital airborne SARS-CoV-2,” Respir. Res., vol. 22, no. 1, p. 73, 2021, doi: 10.1186/s12931-021-01637-8.
13. Kumar and L. Morawska, “Could fighting airborne transmission be the next line of defence against COVID-19 spread?,” City Environ. Interact., vol. 4, p. 100033, 2019.
14. Blocken et al., “Ventilation and air cleaning to limit aerosol particle concentrations in a gym during the COVID-19 pandemic,” Build. Environ., vol. 193, p. 107659, 2021, doi: https://doi.org/10.1016/j.buildenv.2021.107659.
15. Cheng Ming Hui, 2023, “The Research on Droplet Cross Infection Prevention between Doctors and Patients, The Development and Performace Analysis of the Clean Anti-Cross Infection Shield, National Chin-Yi University of Technology”, a master's thesis.