Humidity and virus spread: The importance of physics in the pandemic

The virus is an inactive dust-like particle that cannot walk or fly. Image courtesy: Lightspring/Shutterstock

While attempting to explain the uneven spread of the SARS-CoV2 virus around the world, trendy topics such as mutations, variants and vaccines, sometimes even politics - are discussed.

The objective of this article is to explore the environmental factors, specifically on why respiratory viruses apparently spread more in high humidity conditions. Understanding these factors will help us gain better control of the pandemic in the months and years ahead.

Back to basics.

We know that the SARS-COV2 virus spreads through respiratory droplets by air. We also know that the virus is somewhat seasonal, appearing in waves - the reasons for which have not yet been fully elucidated.

While discussing the spread of respiratory viruses, there are two fundamental questions: (see diagram)

  1. How far, and for how long can the droplets travel?

  2. How well can the virus survive the ride? (stability of virus)

This impacts how many people get infected.

Back to Basics: Epidemiological triad or triangle

The three components of the epidemiological triangle are agent, host and environment.

Of the three, a disproportionate amount of time has been spent discussing agent and host. On the other hand, apart from a few suggestions on improving ventilation and airflow, not much has been said about the role of the environment.

The agent in this case is the SARS-CoV2 virus. Human beings are the host, and environment refers to what is in between the person who is infected and a person who is susceptible.

In the context of the pandemic, the environment mainly refers to the physical properties of air. Weather-related variables such as humidity, temperature, wind speed, and construction related parameters like ventilation and airflow are a few of these properties.

What are droplets? Why are they important in a pandemic?

The virus is an inactive dust-like particle that cannot walk or fly. To reach new hosts, it needs to hitch a ride from one person to another. Droplets are the vehicles used by the SARS-CoV2 virus for this purpose. Essentially these are like drops of fluid, only smaller and lighter. It is an established fact that respiratory viruses spread more during the monsoon season in tropical countries. This is due to more efficient droplet-based transmission occurring between people during wet humid conditions.

Droplets are generated when people talk, laugh and socialise. Some are generated while breathing. The smallest among these droplets are so light, they behave like invisible mist particles that hang suspended in the air. The tiniest droplets, typically less than 5 µ in diameter, are classified as aerosols. They circulate predominantly in closed spaces - where people spend time together. Poorly ventilated spaces - where the same air recirculates over a period of time, pose greater risk for spread of infection to anyone who shares that space.

The greater the number of people, the longer the time spent and the more the conversation, the more the droplets produced.

How does the virus travel?

SARS-CoV2 virus travels within these droplets of fluid, that remain suspended in the air for minutes to hours. When a person breathes these in, the virus enters the new host - and the process repeats.

From the viewpoint of the virus, the survival of the droplet is important while it travels from the patient to the host. Equally important is the survival of the virus within the droplet. These two topics require to be discussed separately.

To use a metaphor, when an army decides to cross a large turbulent river using small rafts, it is important that the rafts remain intact till the men reach the other shore. Equally important is that the men remain safe on the raft.

In the above example, the two shores of the river represent the patient and the host, the raft is the droplet, and the soldiers are the virus. External factors that threaten the raft or the soldiers can result in failure of the mission.

Likewise, virus transmission fails if either the droplet or the virus inside it is compromised.

  1. Survival or fate of the droplet

The survival of the droplets depends on evaporation rate, wind speed, temperature and humidity. The droplet is a spherical blob of fluid. Evaporation occurs at its surface. The greater the evaporation, the faster the reduction in size of this droplet. Droplets come in numerous sizes.

The largest droplets immediately fall on to the ground; contaminating surfaces. It is the medium and smallest size droplets that remain in the air, and are affected by environmental factors. The longer these droplets survive in the air, the more the virus transmission.

  1. Survival of the virus

The second factor is the survival of the virus within the droplet. If the virus is inactivated within the droplet, it is unable to infect another person even if inhaled. The virus’ survival depends on multiple factors such as solute concentration (osmolality) and surface tension. Evaporation increases osmolality by raising the solute concentration, which can inactivate the virus. Proteins located on the surface of the virus can suffer damage due to physical forces such as surface tension of the droplet. Very high temperatures are also considered unsuitable for virus survival.

This vulnerability is particularly true for enveloped viruses such as influenza and SARS-CoV2 virus. They have a fatty coat over a protein and nucleic acid core - which makes them vulnerable to environmental factors. Researchers have looked at survival of influenza virus.

What is humidity? How does it affect droplets?

An easy example to understand humidity is that clothes take longer to dry during rainy season. That is because evaporation is lower when humidity is high.

Likewise, evaporation rate affects the fate of droplets suspended in the air. Evaporation is influenced by the amount of water vapour present in the surrounding air (humidity), the surface area of the droplet, and wind speed. If there is more water vapour present in the air, the droplet cannot evaporate and shrink in size.

Humidity affects different droplets differently. For the smallest droplets, high humidity enables them to remain suspended for a long period of time. This means that more people could become infected during the lifetime of the droplet.

The relationship between humidity and evaporation rate also varies according to the temperature. As a result, there is a difference in the nature of this relationship among regions that have a temperate climate (lower temperature) and tropical regions (higher temperature).

The most efficient viral transmission occurs in dry low RH (relative humidity) conditions at low temperature and also in a wet, high RH environment. The general consensus is that such viruses spread least during intermediate humidity (40—60%) conditions. This has been called the U-shaped relationship, which means that the spread is more at either end of this range.

How is the pandemic being affected by humidity?

Kerala, Tripura, Mizoram and other north-eastern states have been reporting relatively more COVID-19 cases in late July-early August. These happen to be the areas of the country with the highest relative humidity (see map).

The attached chart shows the relative humidity (RED) in various parts of India during this period. It can be seen that the highest relative humidity is seen along coastal states along the west coast and in north-eastern states. These areas are also receiving substantial rainfall.

Colombo in Sri Lanka also has the same humidity as Trivandrum, and had a spike in cases, similar to what was seen in Kerala. In fact, Colombo’s COVID-19 situation and graph are near-identical to that of Kerala over this period (see pic), with a relative humidity of 87%.

In the US, greater number of cases in the US are being reported from the states of Florida and Louisiana. The maps reveal that these are the areas where the air has the greatest amount of water vapour in the air, as indicated by relative humidity and dew point.

Environmental factors: Enveloped viruses do better in high humidity.

In science, correlation is not always causation. Therefore, these observations must be cautiously interpreted. The hypothesis here is that these parts of the world are providing an optimal climate for the virus to stay alive longer in the air, in droplets, mostly indoors.

If humidity is important, why does the pandemic spread in places where humidity is low?

Environmental factors have a complex relationship with how droplets and viruses survive in the air. These relationships are not linear: for instance, the effect of humidity is also affected by the temperature. In other words, a single factor is insufficient to explain all the observations, and conflicting observations are common when such attempts are made.

Besides, droplets are not the only variable involved in the pandemic. Virus spread is also affected by agent fitness (often enhanced by mutations), ventilation, indoor air exchange rate, type of dwellings, population density and behaviour, migration trends as well as host immunity from prior infection or vaccination.

When it comes to diseases such as COVID-19, the spread and severity are multifactorial.

In summary, if we continue to focus only on the virus (mutation) and the host (vaccination, crowding, masks), we will miss the crucial third element of the epidemiological triad. More research on environmental factors is required, which will help customize preventive strategies in vulnerable areas, at appropriate times. The ‘sweet-spot’ or ideal spreading conditions for the SARS-CoV-2 virus needs to be better defined.

What is the current situation in Kerala, and what needs to be done in the months to come?

As mass gatherings have not been allowed, most of the virus transmission in Kerala now is happening indoors in social settings e.g. when people visit friends and family. Vaccinated individuals are also picking up the virus in their nose and throats, although they are not getting sick. With a shortened incubation period and a 1000-fold increase in viral load, the delta variant has clearly evolved for more efficient transmission. Non pharmaceutical interventions are less effective compared to a year ago, when we were dealing with a slower spreading version of the SARS0CoV2 virus. Continues adherence to social distancing and masks is required, while expediting vaccination.

Physics terms used in the article:

Humidity can be measured as absolute humidity, relative humidity or dew point.

Relative humidity is defined as the amount of water vapour present in the air expressed as a ratio to the maximum water vapour it can possibly hold at that temperature. The greater the water content, the higher the RH.

However, relative humidity is also dependent on temperature: air at higher temperatures can hold more water vapour. For example, when two regions of 15 C and 28 C report the same high relative humidity of 85%, the warmer region has greater amount of water vapour in the air.

Dew point is the temperature at which precipitation occurs (water vapour converts into liquid form), that is when the air can no longer hold water vapour in gaseous form. The higher the dew point, the more the water content in the air.

Further Reading

  1. How does the SARS-CoV-2 virus really spread? Article by Dr Rajeev Jayadevan on April 10, 2020

  1. The Epidemiological triangle,always%20sufficient%20to%20cause%20disease.

  1. A review of the multiple environmental factors that affect respiratory viruses is published by Prof. Akiko Iwasaki and team.

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