In March 2020, the World Health Organization (WHO) announced a global pandemic known as the coronavirus disease 2019 (COVID-19) pandemic. COVID-19 has been caused by the rapid spread of a new coronavirus, namely severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Study: The Impact of Heating, Ventilation, and Air Conditioning (HVAC) Design Features on Virus Transmission, Including the 2019 Novel Coronavirus (COVID-19): A Systematic Review of Humidity. Image Credit: Africa Studio/Shutterstock
Background
Scientists and public health authorities around the world have analyzed the transmission routes of the virus to formulate effective measures to stop the spread of the virus. A recent PLoS ONE study reviewed the impact of humidity on virus transmission and viability.
Three electronic databases, ie Ovid MEDLINE, Web of Science Core and Compendex, were used to obtain relevant data. Of 12,177 unique citations, 568 studies were used because they met the inclusion criteria. Although most of the studies were based on in vitro experiments, some also involved in vivo animal studies, along with observational and modeling studies.
Routes of viral transmission
Many viruses are transmitted through the aerosol route. Virus-laden aerosols are formed when an infected person talks, coughs, or sneezes, and these aerosols remain in the air for a long period of time. Viruses, such as SARS-CoV-2, spread rapidly in the poorly ventilated indoor environment through airborne transmission.
Coronaviruses, such as SARS-CoV-2, Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV, are associated with airborne transmission.
Very few studies are available related to the viral transmission routes of coronaviruses; however, substantial studies on influenza were found. The current study included influenza (both A and B strains) due to its similarities to coronaviruses. For example, both influenza and coronaviruses are lipid-enveloped single-stranded RNA (ssRNA) viruses.
The impact of humidity levels on viral transmission and viability
HVAC systems can be used to reduce airborne transmission of viruses by removing or diluting contaminated air in an indoor environment. Temperature and humidity influence the transmission of the virus. For example, experimental and modeling studies have shown that low relative humidity (RH) plays an important role in virus viability and transmission.
The authors grouped HR levels as low (<40% RH), mid (40–60% RH), and high (>60%RH RH). An increase in humidity from low RH levels to around 50% RH decreased viral transmission.
Conflicting results were found related to the impact of humidity on SARS-CoV-2. Some studies indicated that increasing humidity from medium to high RH improved virus survival in both tissue culture medium (TCM) and artificial saliva (AS). Virus survival in TCM was significantly higher compared to AS at the same HR. Other research emphasized that HR alone did not affect viral survival. Interestingly, increasing RH with subsequent reduction in temperature was found to prolong viral survival.
Enhancement of HR from low to medium HR resulted in elevated viral survival for the common cold virus (hCoV-229E). Similarly, an increase in HR from medium to high decreased hCoV-229E survival. For MERS-CoV, high humidity was associated with minimal viral survival. In contrast, another study found that median HR was associated with minimal viral survival, while maximal survival was found with high HR values.
For influenza virus, increasing humidity from low to medium RH was associated with reduced survival and infectivity. However, a further increase in humidity from medium to high RH decreased viability and infection rates. Only one study contradicted this observation, stating that increasing humidity from medium to high RH improved viral survival.
Importantly, most influenza aerosols were associated with the highest survival, viability, and infectivity at low RH. Also, both medium and high HR were associated with minimal survival and viability.
Modeling studies were performed to determine the optimal temperature/RH pairings for viral inactivation. Compared to humidity, specific enthalpy was found to be a better indicator of virus control. Models based on HCoV-EMC, MERS-CoV, SARS-CoV-1, and SARS-CoV-2 revealed that a target value of 55 kJ/kg was optimal. However, this value was well above typical set points for mechanical systems, i.e. around 93% indoor RH at 20°C in the HVAC system, to achieve a specific enthalpy of 55 kJ/kg .
Study Implications
There is a need for statistical analysis to properly interpret the influence of humidity, temperature, and exposure time on viral transmission and viability. Also, there is a lack of standardized testing procedures, which needs to be corrected in the future. The test medium was found to influence the outcome of experiments used to study the role of moisture in viruses. HBE ECM did not show a significant effect of humidity on viruses, whereas in AS medium, increasing humidity from medium to high RH improved viral survival.
Influenza transmission was significantly reduced by maintaining an indoor temperature of 20°C at intermediate (50%) or high (80%) RH. The current systematic review highlighted the role of humidity as an HVAC intervention and its effect on viral transmission and viability. Aerosol medium, temperature, and exposure time were found to influence the role of RH in viral transmission and viability.
Source: news.google.com