Study: Temperature and Humidity May Indicate COVID-19 Transmission Risk

Depending on what season it is, you could be seeing higher peaks of COVID-19 cases, according to a new study.

Researchers found that COVID-19 transmission may be tied to temperature and humidity. Specifically, warmer regions like southern United States may run into spikes of COVID-19 cases in the summer months, while colder regions like the Northeast could see more cases in the winter. 

“On a cold day in New York, people like to stay in an indoor environment with the heater on. Similarly, on a very hot day in Florida, we hide in air-conditioned rooms,” Chang-Yu Wu, PhD, coauthor and aerosol researcher at the University of Florida told Verywell, adding that poor indoor ventilation is the culprit for the spread of viral particles.

Wu and his colleagues used data from various COVID-19 epicenters including the U.S., India, China, and Germany. They found that cases surged at a certain temperature and humidity thresholds based on human behavior and how the virus spreads as an aerosol and droplet.

The authors wrote the virus was able to linger longer in dry, low-moisture environments—notably at dew point temperatures below 32 degrees. Cases also spiked when air temperatures went above 75 degrees or dipped below 62 degrees.

In environments where air temperature ranged from 62 to 75 degrees, which is considered more manageable for people to stay outdoors, virus transmission appeared to decrease.

Humidity and Temperature Play Important Roles in Virus Concentration

People release respiratory droplets when they speak, sing, cough or sneeze. These very small droplets and aerosol particles can carry and spread the COVID-19 virus in the air. They may vary in size from visible to microscopic. While larger droplets and particles drop to the ground within seconds or minutes of their release, smaller ones can remain in the air for minutes to hours, according to the Centers for Disease Control and Prevention (CDC).

Wu and his colleagues wrote that humidity and temperature influence the size of virus particles and affect how long they would remain in the air. In a dry environment such as a heated room in the winter or an air-conditioned room in the summer, water evaporates from respiratory droplets quickly, shrinking their size and allowing them to float in the atmosphere for longer.

“If the droplets settle on the ground quickly, their chance of getting into the lungs by inhalation is much lower than if they were floating tiny particles,” Wu said.

Jose-Luis Jimenez, PhD, an aerosol expert and a professor at the University of Colorado Boulder, told Verywell that people often release more aerosols than droplets when they speak, sing, and cough.

“In most cases, you are expelling 1,000 more aerosols than droplets,” Jimenez said. “Droplets drop, and aerosols are the ones that float. They behave like cigarette smoke. They float in the air and they follow a current.”

Aerosol particles are smaller and they can suspend in the air longer compared to droplets, but they are easily dispersed outdoors by winds and air movements, Jimenez added.

“When we’re indoors, it’s like we’re in a box. The walls and ceiling trap the air which means aerosol particles can remain in the air—depending on how big the place is—for about an hour or a couple of hours,” he said.

The authors of the study also noted that extremely low or high temperatures indicate an upward trend of COVID-19 cases. People often move inside to avoid extreme heat in warmer regions, and those who live in wintry regions move indoors to avoid the cold. In both cases, people are shifting indoors, where they’re exposed to recirculated air.

Wu said that moving indoors leads to a higher risk of inhaling air that carries droplets and aerosols with the virus, resulting in more COVID-19 cases.

What Happens to Virus Particles When You’re Outside?

When you are spending time outdoors, fresh air is constantly moving, dispersing droplets and aerosol particles.

According to Wu, if someone has the virus and spends time outdoors, the aerosol particles and droplets they are expelling will be dispersed and diluted very quickly.

“The concentration of virus particles is going to be low outdoors unless you are really close to someone who is shedding the virus,” said Wu. “If you’re talking face to face with a very short distance outside then that risk can still be higher.”

Jimenez explained virus particles don’t last long outdoors compared to indoors because they have to survive wind, sunlight, humidity, and other forces that work to decay and disperse them. People are less likely to breathe in enough of the respiratory droplets containing the virus that causes them to become infected with COVID-19.

“Transmission is 20 times less outdoors than it is when we’re indoors. Virus particles are not going to accumulate because it’s going to rise and in most outdoor situations, it will dissipate.”

As recommended by the CDC, spending time outdoors with other people is a safer choice. You are also less likely to be exposed to COVID-19 during outdoor activities, even without a mask.

Preventive Measures

While the study shed more light on the seasonality of COVID-19, climate and weather alone aren’t sufficient to predict future outbreaks, according to the researchers. Prevention tools are still needed. Investing in proper ventilation and filtration, for example, is crucial for preventing indoor transmission.

“If you have a good ventilation rate, the air changes constantly helping to reduce the concentration of any viral particles,” Wu said.

According to Wu and Jimenez, other preventive measures include wearing a mask, opening the windows, keeping a safe distance from people. Jimenez added that a window air-conditioning unit or a fan does not ventilate a space, but it only cools and mixes the air, rather than allowing fresh air to cycle.

“Face masks or respirators are personal filters that lower the amount of airborne virus getting into your respiratory system,” Wu said. “All these things are good tools to help lower the risk of transmission in the indoor environment.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.