Relative humidity could play a role in transmission of SARS-CoV-2 indoors

By Dr. Ananya Mandal, MDAug 24 2020

A new study has explored the role of relative humidity in the transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from person to person indoors. The study titled, “An Overview on the Role of Relative Humidity in Airborne Transmission of SARS-CoV-2 in Indoor Environments,” is published in the latest issue of the journal Aerosol and Air Quality Research.

Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID

Highlights of the study

This work is a collaboration between researchers at Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany, and CSIR-National Physical Laboratory, New Delhi, India. The team explored the role of relative humidity (RH) in the aerosol-based transmission of the SARS-CoV-2 virus. The team found that the risk of spread indoors was more significant when it was drier compared to rooms with higher humidity. The optimum humidity for maintaining human health was found to be between 40 and 60 percent, and it was essential to set standards for RH indoors to prevent transmission of the infection.

Dr. Ajit Ahlawat from TROPOS, one of the authors, said, “In aerosol research, it has long been known that air humidity plays a major role: The more humid the air is, the more water adheres to the particles and so they can grow faster. So, we were curious: what studies have already been conducted on this.”

Airborne transmission of viruses

The team wrote that certain factors could aid in the transmission of the novel coronavirus. These include entry of the virus particles into the indoor environments from the sources of emissions helped by other factors such as “relative humidity, wind speed, temperature.”

They explained that respiratory infections such as COVID 19 disease could spread via aerosols that are of size less than 5 µm and viral droplets that are of size between 5 and 10 µm. An infected person usually exhales these. While the more significant drops fall to the ground due to their weight, smaller droplets lose mass due to evaporation and remain suspended in the air for more extended periods of time. The team wrote, “small droplets of radius approx. 5 µm will take 9 minutes to reach the ground when produced at the height of 160 cm, i.e., average speaking height.” These droplets, they wrote are important in the transmission of SARS-CoV-2.

Indoor environments and viral transmission

The team explained that in regions with poor ventilation, there is a recirculation of air within the rooms. They added, “In cold and temperate climates, within an indoor environment, the RH values are typically low.” Due to dry air and low relative humidity, there is more rapid evaporation, and the larger droplets get converted to smaller droplets that can help transmission of the virus.

However, in regions of high humidity, the droplet size of the virus increases. Thus it falls faster, and there is less risk of other people in the vicinity to breathe in the infectious droplets. The team wrote that there could be three effects of RH on viral transmission. These include;

• “Fate of microorganisms inside the viral droplets” – The viability of the viruses was low between 33 and 100 percent RH. Below and above that, the viruses survived longer.
• “Survival or inactivation of the virus on surfaces” – Lower temperatures and lower humidity prolonged survival of the viruses on the surfaces
• “Role of dry indoor air in airborne transmission of viruses” – At RH lower than 40 percent, the risk of viral respiratory infections is grater say studies.

Comparing indoor and outdoor RH and COVID-19 risk

This revealed that dry indoors with RH less than 40 percent raised the risk of airborne transmission of COVID-19. The risk was lowered when RH was raised from 23.33 percent to 82.67 percent, said one study.

When looking at outdoor humidity, they noted that absolute humidity (AH) had a direct correlation with COVID-19 spread risk. One study showed that 73 percent of the confirmed cases were from regions with AH in the range 3–10 g m^–3.

Conclusions

Dr. Ajit Ahlawat summarized the findings saying, “If the relative humidity of indoor air is below 40 percent, the particles emitted by infected people absorb less water, remain lighter, fly further through the room and are more likely to be inhaled by healthy people. In addition, dry air also makes the mucous membranes in our noses dry and more permeable to viruses.”

Prof. Alfred Wiedensohler of TROPOS added, “Heating the fresh air also ensures that it dries. In cold and temperate climate zones, therefore, the indoor climate is usually very dry during the heating season. This could encourage the spread of coronaviruses.”

Dr. Sumit Kumar Mishra of CSIR - National Physical Laboratory in New Delhi said, “A humidity level of at least 40 percent in public buildings and local transport would therefore not only reduce the effects of COVID-19, but also of other viral diseases such as seasonal flu. Authorities should include the humidity factor in future indoor guidelines.”

Recommendations

The team wrote that there are no policies yet to regulate the temperature and RH indoors to prevent transmission of SARS CoV-2. They recommended, “Based on research findings, for future scenarios, setting a minimum RH standard of 40% for public buildings will not only reduce the impact of COVID-19, but it will also reduce the impact of further viral outbreaks, both seasonal and novel.” They also emphasized other measures of infection control such as increasing natural ventilation using open windows, the use of face masks and shields, and avoiding close contact of persons infected with the virus.