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 novel coronavirus, namely, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Scientists and public health authorities worldwide analyzed the virus transmission routes to formulate effective measures to stop the virus spread. A recent PLoS ONE study reviewed the impact of humidity on virus transmission and its viability.
Three electronic databases, i.e., Ovid MEDLINE, Web of Science Core, and Compendex, were used to obtain relevant data. Out of 12,177 unique citations, 568 studies were used as they met the inclusion criteria. Although the majority of the studies were based on in vitro experiments, some were also related to in vivo animal studies, along with observational and modeling studies.
Viral transmission routes
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 airborne for a prolonged period. Viruses, such as SARS-CoV-2, transmit rapidly in the ill-ventilated indoor environment via 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 related to the viral transmission routes of coronaviruses are available; however, substantial studies on influenza were found. The current study included influenza (both A and B strains) because of its similarities with coronaviruses. For instance, both influenza and coronaviruses are lipid-enveloped and 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 the contaminated air in an indoor environment. Temperature and humidity influence virus transmission. For instance, modeling and experimental studies have shown that low relative humidity (RH) plays a significant role in virus viability and transmission.
The authors grouped RH 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.
Contradictory results were found related to the impact of humidity on SARS-CoV-2. Some studies stated that increased humidity from mid to high RH enhanced the virus’s survival in both tissue culture medium (TCM) and artificial saliva (AS). The survival of the virus in TCM was significantly higher compared to AS at the same RH. Another piece of research emphasized that only RH did not affect viral survival. Interestingly, raising RH with subsequent reduction in temperature was found to prolong viral survival.
The enhancement of RH from low to mid-RH brought about elevated viral survival for the common cold virus (hCoV-229E). Similarly, an increment of RH from mid to high decreased hCoV-229E survival. In the case of MERS-CoV, high humidity was linked with minimal viral survival. In contrast, another study found that mid-RH was associated with minimum viral survival, while maximum survival was found at high values of RH.
For the Influenza virus, increasing humidity from low to mid-RH was associated with reduced survival and infectivity. However, a further humidity increase from mid to high RH decreased viability and infection rates. Only one study contradicted this observation, stating that increased humidity from mid to high RH enhanced viral survival.
Importantly, most aerosol influenza was associated with maximum survival, viability, and infectivity at low RH. Likewise, both mid and high RH was linked to minimal survival and viability.
Modeling studies were conducted to determine the optimal temperature/RH pairs for viral inactivation. Compared to humidity, specific enthalpy was found to be a better predictor for virus control. HCoV-EMC, MERS-CoV, SARS-CoV-1 and SARS-CoV-2-based models revealed that a target value of 55kJ/kg was optimal. Nevertheless, this value was well above typical set points for mechanical systems, i.e., an indoor RH of around 93% at 20°C in the HVAC system, to achieve 55kJ/kg specific enthalpy.
There is a need for statistical analysis to suitably interpret the influence of humidity, temperature, and exposure time on viral transmission and viability. Additionally, there is a lack of standardized testing procedures, which needs to be rectified in the future. It was observed that the testing medium influenced the outcome of experiments used to study the role of humidity on viruses. HBE ECM exhibited no significant effect of humidity on viruses, while in AS medium, increasing humidity from mid to high RH enhanced viral survival.
Influenza transmission was significantly reduced by maintaining an indoor temperature of 20°C with 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. It was observed that aerosol medium, temperature, and exposure time influenced the role of RH in viral transmission and viability.