Study Finds Millions of Bacteria Live Inside Pennsylvania Fog Droplets

A research team from Susquehanna University and Arizona State University sampled fog droplets from 32 weather events in Pennsylvania over two years and found that millions of bacteria live inside them. Fewer than 1% of individual fog droplets contain bacteria, yet a thimble’s worth of fog water holds about 10 million bacteria.

When averaged across all droplets, the concentration matches levels found in the ocean, according to Ferran Garcia-Pichel, a study co-author from Arizona State University.

One bacterial group, methylobacteria, stood out in the samples. These organisms consume simple carbon compounds, including formaldehyde, a pollutant that contributes to ozone smog and harms human health. Garcia-Pichel said in an email to USA Today that the bacteria are not only present but actively consuming atmospheric pollutants and likely growing within the droplets.

“Fog is a habitat,” he said. The study focused on radiation fog, which forms overnight when air near the ground cools and stabilizes under stagnant conditions. This type of fog is common across the United States and occurs most often in fall and winter, according to the National Weather Service.

Thi Thuong Thuong Cao, the paper’s lead researcher and a cloud researcher, noted that knowledge about bacteria in fog remains limited because fog sits at ground level like a cloud. ” He added that fog affects visibility for transportation, supplies water to ecosystems and some communities, and enables atmospheric chemical processes.

The researchers cautioned that harvesting fog for water could remove these bacteria, though the overall impact is unknown.