The next time you walk through fog, consider what you're actually walking through. A study published this month in the journal mBio by researchers at Arizona State University has found that fog isn't sterile mist floating near the ground. It's a temporary living ecosystem — and the bacteria inside it are actively cleaning the air.
A thimbleful of fog water contains roughly 10 million bacterial cells. Most of those cells aren't just drifting. They're growing. They're dividing. And they're eating toxic chemicals out of the atmosphere while they do it.
What a PhD Student Found Sampling Fog Before Sunrise
Thi Thuong Thuong Cao started with a question that nobody had quite answered: are the bacteria found in fog actually alive and active there, or are they just being carried along passively the way dust moves through air?
To find out, she went to Pennsylvania before sunrise across 32 fog events — sampling radiation fog, the kind that forms close to the ground in still, cool air near rivers and fields. She brought the samples back to the lab at Arizona State University and watched them under a microscope.
"We observed them under the microscope to see that, yes, the bacteria are getting bigger and they're dividing," Cao says. The droplets were not transportation vehicles. They were habitat.
The Bacteria That Keeps Showing Up
One group dominated the fog samples across all 32 events: Methylobacterium, which made up an average of 29% of all bacteria detected. Methylobacterium are known for feeding on simple carbon compounds. One of the most significant compounds they consume is formaldehyde.
Formaldehyde is a common air pollutant — a byproduct of combustion, car exhaust, and industrial activity that contributes to ozone smog and has documented effects on human health. It appears naturally in fog water at measurable concentrations.
The fresh fog water in Cao's samples degraded formaldehyde at rates roughly 200 times faster than rates previously measured in cloud water. The researchers could see the bacteria consuming it so quickly that they initially suspected something else was happening. What they found was that at high concentrations, formaldehyde becomes toxic even to the bacteria eating it — so they've developed a second mechanism to break it down into carbon dioxide before it reaches harmful levels. A biological safety valve, built into fog.
What Fewer Than 1% Adds Up To
The numbers require a reframe. Fewer than 1% of individual fog droplets actually contain bacteria — which sounds small until you consider how many droplets are suspended in even a modest bank of fog. The atmosphere already contains thousands to millions of bacterial cells per cubic meter. Fog concentrates them into temporary water habitats where the conditions allow active growth. A thimble of the resulting water holds 10 million cells.
Cao's study examined radiation fog specifically — the ground-level variety — but the researchers note that fog is essentially a cloud at the surface. The finding raises a reasonable question about whether clouds, which cover roughly two-thirds of the Earth at any given time, are doing something biologically similar at altitude.
A Living System Nobody Knew to Look For
The study reframes fog from a passive weather phenomenon into something more biologically interesting: a temporary natural cleanup zone where microbes actively process pollutants before fog lifts and disperses.
Scientists have known for decades that bacteria travel through the atmosphere. What remained unresolved was whether the atmosphere was just moving them around or whether they were doing something while they were there. The answer, at least in fog, is both. They ride the air. They also work in it.
The next foggy morning, something invisible is happening at eye level — a temporary microbial ecosystem forming in billions of water droplets, eating the air clean before burning off in the sun. It's been doing this long before anyone thought to look.
