Researchers from the Massachusetts Institute of Technology have discovered that, in some rare cases, rain could potentially contribute to the spread of disease, a new study in the journal Nature Communications reports.
Almost everyone is familiar with the earthy smell that comes after a rain. However, few know it comes from a chemical known as geosmin — a byproduct of bacteria and fungi that get released when droplets hit the ground. Though scientists have studied this process before in both clouds and sea spray, they have never been able to figure out exactly how rain moves the bacteria.
Researchers in the study aimed to solve this by using high-speed cameras and fluorescent dye to film water droplets as they fell onto soil filled with bacteria. This allowed them to measure the speed of the falling raindrops as well as the type and temperature of the soil they impacted.
They found that when raindrops hit the ground at roughly three miles an hour, it traps tiny air bubbles — no wider than a human hair — beneath it. This then causes those bubbles to rise up to the surface, where they burst and push small jets of water up into the air.
“At this just-right speed, water wicks into the soil without splashing, but fast enough to trap air,” said Cullen Buie, an associate professor at the Massachusetts Institute of Technology, in a statement. “That trapped air gets released as bubbles that burst, releasing the aerosols. We found the relationship between the distribution of aerosol size and the number of bubbles bursting.”
The data showed that a single raindrop can create hundreds of droplets in a few microseconds, and each of those droplets could contain several thousand live bacteria. From there, the microbes are able to survive for at least an hour.
Three different types of bacteria — Corynebacterium glutamicum, Pseudomonas syringae, and Bacillus subtilis — were analyzed in the study. Though all of those are harmless, there is a chance pathogens can also be spread in this way, NPR reports.
Researchers started the study after British scientists noticed that a rare disease known as melioidosis — which is spread by soil-dwelling bacteria — became more prevalent in Southeast Asia and northern Australia during rainy seasons.
However, the team states that these new findings should not be cause for concern. Though the information may be useful for scientific purposes, it is highly unlikely that going outside in the rain would make you sick.
Scientists next plan to look at other features related to the droplets, such as how far rain can move bacteria, to get a better idea of how they spread. Such information could help scientists better understand pathogens and track the movement of certain diseases.
“Further investigation is required to narrow down the range of global emission of bacteria by rain, but aerosol generation by rain could be a major mechanism of bacteria transfer into the environment,” said lead author Youngsoo Joung, a researcher at the Massachusetts Institute of Technology. “Future work on these findings could provide new clues to trace soil-borne bacteria responsible for infections in humans, animals, and plants, as well as climate impacts due to cloud formation and ice nucleation.”