Following the COVID-19 pandemic, the American society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) put together the COVID-19 taskforce to address indoor propagation of the vector of this disease (SARS-CoV-2 virus) and propose recommendations to reduce this risk. This is how the ASHRAE standard 241 – Control of Infectious aerosol came to be. This standard has been the result of discussions between ASHRAE and the White House COVID-19 response team. It highlights the needs for improving indoor air quality (IAQ) as a critical way of reducing transmission of respiratory diseases known to be spread by small airborne aerosols, such as viruses (influenza, cold, RSV, coronaviruses) (Wang et al., 2021).
ASHRAE standard 241 stresses the importance of providing clean air, designated as equivalent clean air (eCAi) to building occupants. In that optic, different solutions and methods have been proposed to attain and comply to eCAi requirements. The most direct approach to improving air quality and eCAi is through improving ventilation and fresh air intake. However, geographical localisation of built environments can limit the amount of fresh air, due to energy efficiency considerations. Improving filtration has also been proposed as a method to control bioaerosols. Following the publication of Standard 241, many have recommended to upgrade filters to MERV13. Filtration certainly has a key role to play in the IAQ puzzle but might not be the most energy efficient solution when it comes to addressing the biological aspect of IAQ. Bioaerosols comprise many different biological organisms, ranging from larger mold and mold spores to very small viruses or virus-laden aerosols. Filters that are efficient at rapidly removing very small particles (think HEPA filters) are also very restrictive on the airflow, thus preventing their widespread use.
Lastly, ASHRAE standard 241 also proposes that eCAi can be provided through methods that would clean the air of bioaerosol. One such method is ultraviolet germicidal irradiation (UVGI) of the air. UVGI has been shown to be able to disinfect biological contaminants by inducing fatal alterations of the genetic sequence (DNA or RNA), which is at the core of all biological contaminants (viruses, bacteria, mold). Essentially, organisms are sterilized, as their DNA/RNA is modified to the point of preventing them from reproducing. Thus, using optimized UVGI system, either in the ductwork, or in standalone units can provide eCAi by effectively disinfecting the air of biological contaminants. To that end, standards have been devised to be able to evaluate the effectiveness of such systems, and provide compliance with ASHRAE standard 241, in terms of eCAi delivery, such as ISO Standard 15714 and ASHRAE standard 185.1.
UVGI systems are probably one of the most energy efficient way to address the biological aspect of indoor air quality; the impact on the airflow is negligible, the operating cost of a typical UVGI system is similar to lighting costs (as most UV systems using low-pressure mercury vapour lamp), and maintenance costs consist of changing the lamps every one or two years (depending on the type of lamp used).
Cooling coils – Using UVGI to improve IAQ, energy efficiency and reduce maintenance costs.
UVGI has also shown its dual role in improving energy efficiency as well as improving indoor air quality through installation of UV systems in front of the cooling coils. A double-blind study evaluating the impact of UVC lamps installed for cleaning the cooling coils of the HVAC systems was conducted in an office building in Montreal, Canada, over the span of 48 weeks (Menzies et al., The Lancet Medical Journal, 2003). Lamps were turned off and on, alternatively, for periods of 12 weeks and 4 weeks respectively. The surface of the coils, as well as different components of the HVAC systems were sampled to measure microbial growth, during periods where the UV lamps were on and off. Also, 771 office workers were recruited to the study and were asked to fill questionnaires about potential health symptoms that they experienced throughout the day, either before or after their arrival at the office.
Overall, what the study has shown is two-fold; UVGI indeed keeps the coil free of microbial growth when it is turned on, but also reduces contamination on other parts of the HVAC system, some of which were not exposed to UVC, such as the filters located upstream of the coils. This could indicate that cleaning the coils prevents resuspension in the air of bacteria and mold that could travel back to the filters.
The second interesting aspect of the study is that the office workers reported less health outcomes in the workplace during the periods when the UV system was turned on. As some of the microbial species measured on the coil are associated with negative health effects, it follows that cleaning the coils would improve people’s health.
Furthermore, reducing bacteria and mold on the coil have also been shown to improve energy efficiency of buildings. The microbial growth on the cooling coil (or ‘’biofilm’’) is known to reduce static pressure loss in the HVAC system, which can negatively impact the HVAC system (Luong et al., 2016) (Firantello et al., 2018). Additionally, an ASHRAE field study has concluded that using UVGI for cleaning cooling coils improved heat transfer by 14.55% whilst also showing a 10% decrease in pressure drop (Firantello et al., 2016).
Lastly, a properly sized UVC system requires minimal maintenance; most UVC lamps on the market have a recommended lifetime efficiency of 9000-17000 hours of continuous use. Thus, maintenance consists about changing the lamp after that period. This reduces the cost of coil maintenance, as opposed to the more ‘’traditional’’ methods of coil cleaning. The system also always prevents any microbial growth, so long as the lamps are on, as opposed to rapid recolonization of the coil following physical/chemical cleaning of the coil, in a matter of weeks.
The role of IAQ and disease transmission is a very complex and multifaceted problem that currently affects built environments. Nonetheless, utilising all the available effective technologies to improve IAQ, whilst also keeping in mind the ever-growing aspect of energy efficiency, clearly shows the key roles of ventilation, filtration and UVGI disinfection as a trifecta for efficient and optimal reduction of airborne pathogens in indoor spaces.
Chia C. Wang et al.,Airborne transmission of respiratory viruses. Science 373, eabd9149(2021). DOI:10.1126/science.abd9149
Duval D, Palmer J C, Tudge I, Pearce-Smith N, O’Connell E, Bennett A et al. Long distance airborne transmission of SARS-CoV-2: rapid systematic review BMJ 2022; 377 :e068743 doi:10.1136/bmj-2021-068743
Firrantello, Joseph & Bahnfleth, William & Montgomery, Ross & Kremer, Paul. (2016). Field Study of Energy Use-Related Effects of Ultraviolet Germicidal Irradiation of a Cooling Coil.
Joseph Firrantello & William Bahnfleth (2018) Field measurement and modeling of UVC cooling coil irradiation for heating, ventilating, and air conditioning energy use reduction (RP-1738)—Part 2: Energy, indoor air quality, and economic modeling, Science and Technology for the Built Environment, 24:6, 600-611, DOI: 10.1080/23744731.2017.1383821
Luongo, Julia & Brownstein, Jason & Miller, Shelly. (2016). Ultraviolet germicidal coil cleaning: Impact on heat transfer effectiveness and static pressure drop. Building and Environment. 112. 10.1016/j.buildenv.2016.11.022.
Menzies D, Popa J, Hanley JA, Rand T, Milton DK. Effect of ultraviolet germicidal lights installed in office ventilation systems on workers' health and wellbeing: double-blind multiple crossover trial. Lancet. 2003 Nov 29;362(9398):1785-91. doi: 10.1016/S0140-6736(03)14897-0. PMID: 14654316.