Fog is a living ecosystem for bacteria, rather than just a sterile cloud of mist, and may provide an unexpected buffer against harmful pollution.
In a recent paper published in mBio, an Arizona State University team led by then PhD candidate Phi Thuong Thuong Cao, now a graduate, revealed that bacteria are present in fog that cleans the air at a surprising rate, but also poses a possible risk to drinking water collection.
While scientists have long known that bacteria exist in fog, the researchers were surprised to find that the bacteria are not merely present but form an active, if short-lived, ecosystem.
Investigating the Mysteries of Fog Bacteria
“There’s very limited knowledge about what kinds of bacteria are present in fogs, which are like clouds at the ground level,” Cao said.
To probe the mystery of fog bacteria further, Cao focused on two questions: what kinds of bacteria are present, and whether they are active. Discovering growing bacteria within fog droplets elevates fog from a simple atmospheric phenomenon to a biological habitat, a major shift in how researchers have traditionally viewed it.
Fog is composed of tiny droplets of water, each less than a tenth the width of a human hair and too small to fall to the ground. These minuscule droplets form around particles, which can occasionally be living microbes. Although fewer than 1% of droplets contain bacteria, that still represents a substantial population, with bacterial concentrations roughly comparable to those found in the ocean. About 10 million individual bacteria inhabit just a thimbleful of fog water.
A major challenge in studying fog is the wind. Sampling the same location before, during, and after a fog event can produce misleading results because airborne particles are constantly being transported and mixed. For this reason, Cao focused on radiation fog, which forms in still air near the ground as temperatures cool. This allowed the team to minimize complications associated with sample collection.
Standout Bacteria
In Cao’s investigation, one particular type of bacteria, methylobacteria, predominated within the fog droplets. Intriguingly, samples collected during fog events contained far more bacteria than samples of dry air gathered beforehand, suggesting that these microbes thrive in foggy conditions. Among the compounds methylobacteria consume are simple carbon-based molecules, including pollutants such as formaldehyde. In a laboratory setting, Cao was able to observe and experiment on samples collected in the field.
“We observed them under the microscope to see that, yes, the bacteria are getting bigger, and they’re dividing, so there is growth,” Cao explained. “We also found that they’re using the formaldehyde as food to support their growth.”
Even more surprising was the rate at which methylobacteria were breaking down formaldehyde—far faster than they could consume it. This led the team to conclude that because formaldehyde is toxic to the bacteria, they were converting much of it into carbon dioxide to make their environment safer, doing so even faster than they could use it as a food source.
Potential Hazards
The team’s research suggests that fog may play a more important role in Earth’s ecosystems than previously understood, serving as both a temporary aquatic habitat and a natural filter for harmful pollutants.
The implications of the findings are varied. Harvesting fog for drinking water and crop irrigation is already practiced in some regions of the world. This study suggests that such water may contain more bacteria than previously recognized, potentially requiring additional filtration. The risk to most people appears minimal; researchers say only a slight risk may exist for individuals with severely compromised immune systems who are exposed to fog. However, more work is needed to determine the potential benefits and drawbacks of consuming water collected from fog.
Harvesting fog for drinking water and crop sustenance is an ongoing practice. This work shows that water may be full of bacteria, requiring more filtering than previously understood. The risk to most people is very minimal; only a minor risk exists to people with heavily compromised immune systems from breathing in fog. However, the researchers say that more work is needed to determine the benefits and harms of drinking fog water.
“It’s relatively new that people are starting to look at biological activities in clouds, so there’s still a lot which we don’t understand,” said co-author Pierre Herckes. “At nighttime, for example, there isn’t that much atmospheric chemistry going on. Chemistry is largely driven by the sun and by light. But if the bacteria are still doing their thing even during the nighttime, they can be important.”
Many questions remain unanswered in the wake of this discovery. Cao’s team focused on a single type of fog, leaving open which bacteria thrive in other kinds of fog in other locations, what they eat, and how they impact air quality, all of which will require further study.
“The sky’s the limit,” co-author Ferran Garcia-Pichel says. “No pun intended.”
The paper, “Growth and Formaldehyde Degradation of Photoheterotrophic Methylobacterium within Radiation Fogs,” appeared in mBio on May 11, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.