Study: Humidity May Dampen COVID-19 Transmission

July 11, 2022

Research by a Graduate Center biologist has implications for classroom and indoor safety.

a teacher stands in front of a classroom of students in desks
Professor John Dennehy and other scientists found that humidity reduced the spread of airborne virus particles inside classrooms, and the particles traveled well beyond 6 feet. (Credit: Getty Images)

The main route of infection by SARS CoV-2, the virus that causes COVID-19, is through exposure to airborne respiratory fluids, according to the Centers for Disease Control and Prevention. 

Now, a group of biologists has found evidence that humidity can significantly dampen the spread of airborne viruses in indoor spaces. 

“I think we were most surprised by the dramatic differences around 40% humidity,” said Professor John Dennehy (GC/Queens College, Biology), a corresponding author of a study published in Environmental Science & Technology Letters in June. “There seems to be some sort of inflection point.”

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The researchers tracked the spread of virus particles inside two New York City classrooms using agar petri dishes seeded with the bacteria Pseudomonas phaseolicola, which crowded the petri dishes until an aerosolized bacteriophagePseudomonas phage phi6, a virus that’s structurally similar SARS-CoV-2 — landed inside the dishes, killing the host bacteria and leaving behind blue blotches as signs of their handiwork. 

These detectors were placed at various distances from a nebulizer that spewed the virus over the course of an hour. Small, battery-powered humidifiers were placed 6 feet away. Subsequent experiments showed the levels of aerosolized virus decreased with an increase in relative humidity; in one experiment, the virus was reduced by eight-fold at a distance of just 3 feet. 

The decline was particularly steep at 40% relative humidity. Dennehy was asked what might have caused the shift. “We don't know exactly, but we speculate that the aerosol particles were gaining mass and falling to the ground faster,” he said.  

The biologists were surprised by how far the virus particles were able to travel in under 15 minutes. “We found that the exposure to aerosolized phage particles at 18 feet away from the source shows on average a modest 1.6 times reduction when compared to the exposure observed at 6 feet over the same durations,” the authors wrote. 

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John Dennehy

Moreover, the experiments were carried out in classrooms serviced by a modern HVAC system that delivered 15 air changes an hour. “Despite a state-of-the-art ventilation system, in a brand new building with all the bells and whistles, the virus continued to be spread,” Dennehy said. “In an hour, we saw high levels of virus 18 feet away.”

In fact, he said, the air conditioning appeared to stifle the humidifying effect on the virus particles. “Most ventilation systems, especially during the summer, will be removing moisture from the air,” Dennehy said. “So, you’re cooling the air but also making it easier for the virus to spread.” 

Dennehy said the study data runs counter to federal guidelines on social distancing indoors. “Current COVID guidelines imply that there is minimal risk of transmission between unmasked individuals who are more than 6 feet apart,” he explained. “Our data suggest that this may not be true.” 

The bottom line is that people may be more exposed than previously thought, Dennehy said, and ventilation alone is not enough protection from airborne viruses in indoor settings. “Don't assume remaining 6 feet apart means that you’re safe from infection,” said Dennehy. “You still need a mask.”

In November 2021, Dennehy and scientists at his lab detected the earliest-known evidence of the omicron variant in the U.S. in wastewater from New York City sewers. Some of this data was reported on in a paper published in Nature Communications in February. 

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