Researchers Explain Link Between Humidity and COVID-19 Transmission
Sun, April 18, 2021

Researchers Explain Link Between Humidity and COVID-19 Transmission


Low humidity can be a factor in the increased risk of COVID-19, according to a new study. As such, researchers recommend wearing masks to lower the risk.

The link of low humidity to COVID-19 risk was revealed by researchers at the University of Sydney, an Australian public research university. Their study showed that low humidity could raise the transmission risk of SARS-CoV-2. This was a follow-up on a previous study suggesting the risk in the early stages of the pandemic. The risk would go higher in low humidity due to aerosols being smaller. Smaller aerosolized particles could stay longer in the air, which increases the exposure risk of others. They published their findings in the journal Transboundary and Emerging Diseases.

The Role of Humidity in Diseases

Environmental effects are notable factors in the growth of microbes and the spread of diseases. When the air is humid, water condenses on different surfaces, such as walls and windows. This causes certain microorganisms to grow and thrive. Both bacteria and fungi prefer moist places because of water. So, high humidity often results in moldy bathrooms, musty odor, and quick food spoilage at room temperature.

Lowering humidity fixes the problem with bacterial and fungal growth in a room. But doing it gives an advantage to another microbe: viruses. While viruses are not living organisms, their ability to infect and spread is affected by humidity as well. This is recognized in the transmission of influenza during flu seasons, which is often high in colder months. Colder months usually have dry, cold air that weakens the human respiratory system.

According to a 2019 study by Yale University, an American private research university, the reason why more people contract influenza during winter months is because of low humidity. Scientists explored this angle using genetically modified mice models. The animals were housed in chambers at the same temperatures, but with either low or normal humidity. Next, they exposed the mice to influenza A virus and observed their susceptibility to the pathogen.

Mice in low humid chambers got sicker compared to those in normal humid chambers. They examined the mice and found three reasons how humidity affected the immune system. First, low humidity prevented the hair-like structures called cilia to effectively remove mucus and viral particles. Second, low humidity restricted the ability of airway cells to repair damage induced by influenza. And finally, low humidity interfered with interferon mechanisms, in which the innate immunity failed to signal a viral infection.

Scientists concluded that increasing humidity levels to normal range could decrease the risk of influenza.



Low Humidity and COVID-19

When the COVID-19 pandemic started, environmental scientists expressed concern about humidity's role in spreading the novel coronavirus. Being aware of what the climatic factor could do in influenza transmission, they believed that the same could occur in SARS-CoV-2 transmission. Back in May 2020, Australian researchers published a study exploring the role of climate in the pandemic. They used time series analysis to find the connection.

A total of 749 cases of local COVID-19 infections were analyzed. These cases were compared to daily rainfall, temperature, and relative humidity from 9am to 3pm. Lower relative humidity at 9am was linked to increased cases of COVID-19, but not temperature and rainfall. If relative humidity was lowered by 1%, the predicted increase in COVID-19 cases was 6.11%. If humidity was lowered by 2%, the increase in cases would be 12.22% using that example. They concluded that a decrease in relative humidity should serve as a warning of increased COVID-19 cases for the healthcare sector.

Recently, the same Australian research team made a follow-up study to determine the consistency of relative humidity in COVID-19 cases. They investigated how the climatic factor spiked new cases and what could be done by the public to reduce the risk. Overall, relative humidity could promote aerosolization of liquid droplets emitted via coughing or sneezing. This promotion could significantly support the spread of SARS-CoV-2 to unsuspecting individuals.

"Dry air appears to favor the spread of COVID-19, meaning time and place become important. Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks," said Michael P. Ward, the first author of the study and an epidemiologist at the University of Sydney.

Similar to the previous study, they analyzed data of rainfall, relative humidity, and temperature between 9am and 3pm. A total of 1,203 local cases of COVID-19 in New South Wales were studied. The analyses of climatic factors and cases reflected the exponential and descending epidemic phases. In the study period of 75 days of the Nepean Blue Mountains public health unit, 28 cases were reported at a relative humidity of 86.73. At 75.96 relative humidity, 72 cases were confirmed.



In the study period of 85 days of the Northern Sydney public health unit, 237 cases were reported at 84.86 relative humidity while 64 cases at 74.46 relative humidity. Other public health units investigated were in South Eastern Sydney, South Western Sydney, Sydney, and Western Sydney. At first, the numbers do not seem to show the trend. But when temperature association was assessed, the trend with relative humidity appeared.

Researchers noted that a negative connection between relative humidity and 19 COVID-19 cases in both phases was identified. It had no connection to temperature. A 1% decrease in humidity was linked to a 7.7% increase in COVID-19 cases in the exponential phase and 6.8% in the descending phase. In simple terms, a 10% drop in relative humidity could result in a two-fold increase in new cases.

The explanation for that is better aerosolization. The moisture in the air affects the size of aerosols. Dry air has a lower amount of moisture than wet air. This causes aerosols to become smaller than their typical size. Smaller aerosols can suspend longer in the air and travel at greater distances, compared to bigger aerosols.

When a person coughs or sneezes openly, the liquid droplets are aerosolized into extremely tiny particles in the air. If another person goes to where these extremely tiny particles are, their chances of exposure are very high. If the aerosols contain SARS-CoV-2, their risk of COVID-19 is alarming. But if they are wearing masks, especially medical-grade ones, their risk will drop due to the filter. If the person who coughs or sneezes wears a mask, their chances of spread the disease will go down. And if the person who coughs or sneezes wears a medical-grade mask, they are helping protect others from infectious diseases.