Indoor Air Cartoon Journal, February 2020, Volume 3, #99
Why is the majority of the COVID-19 clusters of human to human transmission of coronavirus reported globally related to air-conditioned indoor environments – cruise ships, places of worship, hospitals, etc? One possible explanation is air-conditioned indoor environments make their occupants vulnerable to being exposed to infectious viruses, even for several days after being generated. How?
The environmental conditions, like indoor temperature and relative humidity, the air-conditioned indoor environment creates makes it conducive for viruses to be viable for several days on indoor surfaces and the air. Thus, making viruses to be still effective in being transferred to the next human that gets in contact with them.
Activities like coughing, sneezing, and talking generates viruses of large particle size that pass through the air and get deposited on indoor surfaces or people in close contact with the index person with infectious virus. An activity like breathing generates virus particles small enough, 0.3µm or less, to linger in the air for several days. The higher the concentration of virus particles on the surfaces or in the air, the larger the concentration of virus particles that will be viable for several days because the indoor environmental conditions favour them.
Thus, increasing the vulnerability and risk of indoor occupants of being infected with the virus. It is reported in the literature that the lower the indoor air temperature and relative humidity (RH), the higher the period of infectious virus viability on indoor surfaces and in indoor air.
Evidence suggests that the viability of infectious viruses would be rapidly lost in a matter of hours at a higher temperature (>38oC) and higher RH (>90%). However, such a high RH would lead to generation or emission of biological (e.g., mold) and chemical (e.g., VOCs) pollutants in indoor environments. The temperature is not safe, either. On the other hand, the typical, conducive, and safe temperature (23oC -25oC) and RH (65-70%) levels in air-conditioned space are conducive for the viability of infectious viruses to be prolonged for days.
Furthermore, many indoor environments are poorly designed or operated to have adequate ventilation, pressurisation, and filtration strategies that can help reduce the concentration of viruses in indoor environments. The pragmatic solution lies with the individual. Good hand hygiene, i.e., washing hands with water and soap or sanitizers, and regular nose rinsing are reported in the literature to be effective in reducing the risk of getting infected with a virus.
While we clamour for building occupants to adopt good hygiene, the built environment professionals have a significant role to play in reducing the burden placed on building occupants by ensuring good ventilation, pressurisation, and filtration strategies for healthy and energy-efficient indoor environments.
Do you want to know more about this topic? Read Bloomfield et al. (2007), Casanova et al. (2010), Chan eta l. (2011), and Ramli et al. (2018) papers.