New year, new conspiracy theories.

Parts of the US have been blanketed by a “mysterious” fog, sparking concern from locals who claim it has a “chemical” smell and exposure can result in flu-like symptoms.

A Florida resident told the Daily Mail that they felt sick after being outside in the fog for just 10 minutes.

“Within about an hour, I kept sneezing over and over for about three hours, and my eyes were really puffy,” the unnamed Floridian claimed. “I got very warm and I felt like I had a fever, and my stomach was cramping.”

The Daily Mail reports that the occurrences have eerie echoes of “Operation Sea-Spray” — a secret biological warfare experiment conducted by the US Navy back in 1950.

At the time, serratia marcescens and bacillus globigii bacteria were sprayed over the San Francisco Bay area to determine how vulnerable a large American city may be to a bioweapon attack. At least one American was killed and 10 others were seriously sickened.

Conspiracy theories have run rampant on social media as people in multiple states have reported a similar phenomenon, which comes after drone sightings in some areas of the country.

Footage online shows a flurry of particles floating within the fog, sparking fears that it is no ordinary weather event but rather, some sort of bio-weapon, users allege.

“Described as a thick, lingering blanket, the fog has left people sick—many experiencing sudden cold or flu-like symptoms after only brief exposure,” one person explained on X of the “so-called ‘chemical fogs’.” “Dubbed ‘Fogvid-24,’ some victims have also reported an unexplained loss of energy.”

According to The Daily Mail, people on X have reported “strange smells” that burn their noses, speculating that the government “dumped a bunch of microbes on the country this week, in the form of fog.”

“It tastes and smells like after setting off a lot of fireworks. That sulfur smell,” one TikTok user wrote, adding that it is “freaking me out.”

Meanwhile, a Kansas resident also told the outlet that she saw “massive amounts of chemtrails” in her area preceding the onset of the fog.

However, there is a scientific reason for why fog forms — and it has nothing to do with biological weapons.

Fog, a cloud of water droplets or ice crystals that form close to the earth’s surface, occurs as a result of the air temperature reaching its dew point, which is its maximum water saturation.

Because it is comprised of tiny particles, shining a light into fog — as seen on social media — will make them visible.

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Not to mention, fog, because it is so close to the earth’s surface, absorbs pollutants and smells that could explain why residents notice a pungent odor, the Daily Mail reported. Fog also can cause respiratory issues due to the high moisture levels in the air, according to some experts.

Fall and winter months are the most common times of the year when fog forms, which may also explain the uptick in this phenomenon across the country.