Posts baselessly link influenza symptoms to 'toxic' fog

People across social media reported experiencing "flu-like" symptoms in February, linking their illnesses to a "toxic" fog in their area.

The claims first gained traction at the end of 2024 under the hashtag "fogvid" (archived here) -- and resurfaced in February.

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A screenshot of an Instagram post taken on February 28, 2025

Recent posts went as far as claiming a lab in Florida tested air samples and found traces of Serratia marcescens, a bacteria reportedly tested during a US Navy experiment (archived here) on the San Francisco coast in 1950.

But no evidence has been provided to support the claims.

"Florida has one of the nation's most comprehensive air quality monitoring networks, designed to provide the public with accurate data on air pollutants regulated under the Clean Air Act," said Alexandra Kucht, communications director for the state's Department of Environmental Protection, on February 27.

"We are not aware of any air quality data supporting" claims shared on social media, she told AFP.

'High severity' flu season

Fog most often forms (archived here) when the air is stagnant and cold. 

"Fog is simply a shallow cloud made up of condensed water vapor," UCLA climate scientist, Daniel Swain told AFP on February 27 (archived here).

It regularly forms in Florida and other parts of the United States.

Some posts suggested the fog was not natural and bore signs of weather manipulation. But Swain said that much of the dense fogs seen recently are formed under "exactly the kinds of ambient atmospheric conditions that would be most likely to produce them."

For people with conditions such as asthma or heart disease, additional particles trapped in the air by fog can have health impacts, but Swain said: "There is no biological/physiological mechanism by which ordinary fog could cause flu-like illness."

In contrast, the United States did see high flu case counts in February (archived here). The current flu season has been classified as "high severity" -- the first since 2017-2018 -- by the Centers for Disease Control and Prevention (CDC), impacting all age groups (archived here).

Most patients show symptoms such as fever, cough, sore throat, and body aches, mirroring what the social media posts describe (archived here).

The rise in seasonal flu cases is "likely due to the circulation of common influenza viruses, which tend to vary in prevalence and severity each year," Edwin Michael (archived here), professor at the Center for Global Health Infectious Disease Research for the University of South Florida, told AFP on February 27.

Florida has seen a notable increase in flu cases compared to previous seasons, he said.

Additionally, five states reported a growing number of Covid-19 infections in February (archived here) and reports of respiratory syncytial virus, which also leads to congestion, peaked in February (archived here).

Unusual smell

The posts further complain about the smell of the fog (archived here). Experts AFP talked to said that while fog itself does not have a noticeable scent, it can carry airborne particles -- sometimes visible to the naked eye -- such as sea salt, pollen, pollution, or industrial emissions, which in turn give the air an odor.

The high humidity and condensed water droplets within the fog can also intensify other ambient smells, such as smoke.

There are other situations where fog can trap pollutants, said Amy Stuart, an environmental health and science professor at the University of South Florida (archived here). 

A historical example of this phenomenon is the Great Smog of London in 1952 (archived here), when an extended period of cold, stable weather combined with very high pollution from factories, homes, and vehicles yielded what was described as a "toxic fog."

In this case, a normal fog turned deadly due to the pollution of millions of coal heaters, which produced acidic water droplets in the air and impacted people with already compromised respiratory systems, resulting in thousands of deaths and illnesses in the span of a few days.

"The combination of coal smoke and fog in that event gave rise to the word 'smog,'" said Jon Martin (archived here) as atmospheric and oceanic sciences professor from University of Wisconsin-Madison told AFP on January 2. However, the recent claims of a "heavy metal" fog are "ludicrous," he said.

In 2024, a fire at a chemical plant in the state of Georgia caused the release of a toxic plume that, when mixed with high humidity in the air, created a fog that affected the health of local residents for months (archived here).

However, there is no evidence of any similar industrial accidents occurring recently.

AFP has debunked other social media posts baselessly alleging that toxic chemicals and biological weapons are released in US skies.