The past year has seen a lot of extreme weather threats, from the California wildfires to Hurricane Helene. One surprisingly dangerous weather event that tends to escape notice - Mysterious fog.
Individual YouTube users uploaded videos seeming to show dense, grainy fog throughout the early parts of 2025. This sparked fears of "toxic fog."
Furthermore, this mysterious fog led many people across the world to complain of respiratory symptoms and even blame recent hospitalizations on the fog. Others thought the fog had a distinct “chemical” smell to it, leading to fears that industrial pollutants might be contaminating the fog. Winter 2024 & 2025 "Strange Fog" Natural Fog Phenomena Can Chemicals Hide Inside Fog? Toxic Fog around the World Fog in Folklore, Myth, and Spirituality Dangers of Severe Fog
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Is there anything behind these fears? When do we need to get concerned about fog days, or are they just a mysterious weather phenomenon? And is fog permanently linked to a subconscious fear that humans have of the unknown and mysterious? We'll break down the research.
Source: Patrick Hendry on Unsplash
Winter 2024 & 2025 "Myserious Fog"
Despite the social media influencers panicking the world on TikTok & YouTube, the fog has natural causes, say meteorologists from the United States and Canada.
While there's no evidence to support outlandish claims about "human-made fog," many of the comments being made on social media about a "strange chemical smell" are rooted in fact. To understand this story, we'll share the differences between fog and smog.
Fog versus Smog
Fog may look creepy, but it comes from a very natural - but complex - relationship between the air and water in the environment.
By contrast, smog is the result of particulate matter suspended in the air. It's named smog because it looks like a cloud hovering above the area. Instead of water vapor, however, it's a cocktail of toxic pollutants that are hazardous to breathe.
Much of the current confusion arises from specific fog events interacting with chronic smog and low air quality.
Thinking about Air Quality
People are much more likely to think about the air they breathe when there is a visual prompt, like unusually dense fog, on their daily commute. Once they start considering it, they may notice that the air in their region has a strange smell or taste. Unfortunately, they're probably right, but this doesn't have anything to do with the fog. Instead, it's a reflection of the amount of particulate matter in the air at any given time.
Define particulate matter: a complex mixture of tiny particles, including dust, smoke, chemicals, and minerals, suspended in the air.
PM 2.5 refers to particulates that are less than 2.5 microns. It is the matter most likely to be breathable by humans and animals. It poses the most respiratory health risks.
READ NOW: From Forest Fires to “Smoke Brain”: The Hidden Hazards of Smoke Inhalation
How does fog form?
Fog forms when water vapor condenses into tiny water droplets that are suspended in the air. This hovers near the ground, causing that familiar murky miasma that's a favorite of horror movies.
In fact, it's the same phenomenon that creates clouds.
To form fog, the air must cool enough to reach its dew point.
Define Dew Point: This is the point at which the air cannot hold any more water. This causes the water to condense into water droplets and form precipitation or fog.
This is why we often see localized fog after a rainstorm or sudden cooling of the air. Water-filled air rapidly cools, reaching its dew point, and then condensation occurs.
Source: Sofa Tutor
While this is the basic description of how all fog forms, there are actually many different kinds of fog, and some of them can have surprising side effects.
Natural Fog Phenomena
No matter how hard scientists try, it's still not possible to entirely predict the weather. Most of us rely on local news reporting that can only deliver weather updates once the patterns are recognized.
According to some meteorologists, fog is one of the most difficult phenomena to predict. This is because it involves topography, precipitation, temperature, and more.
In fact, even the most sophisticated weather computer models (numerical weather prediction) currently in existence struggle to predict fog.
Radiation Fog Formation
No, not radiation like from a bomb. This is a type of fog that occurs in low-wind scenarios, often near a large body of water or in a valley. As the wet air cools off near the ground overnight, the dew point is reached, and fog forms.
If there is no wind, the fog tends to hover. When the sun is strong enough, the fog dissipates. This is the kind of fog we see hovering over lakes and bays. IN fact, these dense waves of fog can be so large that they can sometimes be seen from space!
Source: NASA Earth Observatory
Valley and Mountain Fog
Radiation fog gets even more complicated when mountains are involved. Valley or Mountain fog forms when cool air gathers in the low-lying areas between mountains. This can make travel extremely hazardous.
It's most common in the spring and autumn, where the warm daytime air cools rapidly near the ground. Since warm air rises, the air near the ground stays cooler. If it is full of moisture - as it often is during rainy times of year - fog can quickly form. This gives the majestic views of mountain tops soaring above "oceans" of fog.
Source: Met Service
Coastal and Sea Fog
When warm weather blows over the surface of cooler oceans, fog can quickly form over the surface of the water. Since this is very common along coastlines, we call this "coastal fog."
This can be dangerous to boaters and to drivers near the coast. It spreads rapidly and is often unpredictable.
Thanks to the complicated interactions between sea currents, layers of air, pressure, and more, sea fog is the most complicated form to study. Scientists in California, for example, are currently studying historical weather reports and satellite images to determine if fog patterns are changing across the West Coast - and what might be causing it.
Can Chemicals Hide Inside Fog?
So far, we've covered some of the natural causes of fog. We've also discussed how smog is a product of toxic particles in the air, from sources like pollution or volcanoes.
Some of the recent winter-time fog reports might have been the result of a phenomenon called "inversion”. This is when warmer air traps the cooler, fog-producing air underneath it. This keeps the air from circulating, trapping pollutants underneath the blanket of warmer air.
So if you're worried about a foggy day, a good rule of thumb is to check your local AQI - air quality index. You can do this through your regional weather channel or by using the AirNow interactive map published by the Environmental Protection Agency.
If your area has relatively clear air, you probably don't have anything to worry about from some fog - other than the potentially deadly low visibility conditions, of course.
If your AQI is high - above 100, then that "cloud" hanging in the air might contain more than just water particles. Please take whatever precautions you normally would for high levels of pollution.
Source: Environmental Initiative
Toxic Fog around the World
You might be surprised that there are historical examples of toxins getting trapped in fog.
Case Study: Great Smog of 1952
If you've visited a major city - like LA, London, or Dubai - you're familiar with smog. Even a few hours in the heavily polluted air can make you feel sick.
The poisonous effects of smog are well-known today. Back in 1952, however, it wasn't known how dangerous particulates suspended in the air could be. Smog settled over London, which was full of sulfur dioxide. It's estimated that over 12,000 people died as a result of the smog.
READ NOW: Understanding AQI Meaning: How Climate Change Affects Air Quality
Case Study: Beijing "Killer Smog"
In 2005, Beijing had air quality levels so high that they were off the AQI charts.
The crisis was so extensive that schoolchildren had to stay inside, construction projects were put on hold, and traffic restrictions were placed. The smog didn't dissipate until a steady rain helped wash away the pollutants.
Today, Beijing's response to the smog process is being used as an example of how other heavily polluted cities can deal with their own air quality problems.
Fog in Folklore, Myth, and Spirituality
Thanks to modern pollution, most of us are aware that a "cloud" in the air might be full of unfriendly, barely visible contaminants. This suspicion towards “mysterious fog” isn't anything new, however. In fact, it has ancient roots.
Fog holds a powerful place in myth and folklore. Since it's neither quite water or air, but a mixture of both, it tends to symbolize mystery, transformation, and spirituality for a variety of cultures.
Celtic Fog Myths
In Irish legend, the Féth fíada is an otherworldly mist that weather gods use to hide themselves as they interfere in the mortal world.
In Scotland, the Cù Sìth is a demonic black dog that haunts the mists and storms. Spotting one while out late on the moors is said to portend death or misfortune.
It's not surprising that Great Britain is chock-full of bog and fog myths, including will o' the wisps, fairy rings, and other mysterious creatures.
The British Isles have a complex relationship with water, unique weathers patterns, and decomposing biological materials. This interplay of elements means that there is a high chance of conditions that produce all the kinds of fog discussed above - radiation fog, sea fog, and valley fog. These mists are so common in this region that the culture's folklore is permanently tied to "mysteries in the fog."
Source: Ricardo Gomez Angel on Unsplash
Fog Symbolism Globally
The British aren't the only culture that has developed fog mythology. The Norse believed in Niflheim, a frozen realm of monsters. In Niflheim, a dense fog constantly covered the land, concealing the beasts hiding there.
Legends in Cameroon held that the mist-shrouded lakes were places of danger, that must be treated with respect. This ancient memory was revealed to hold truth in 1986, when the Lake Nyos exploded, releasing years of pent-up carbon dioxide produced by complex subterranean volcanic activity. It instantly killed over 1,000 local villagers.
Dangers of Severe Fog
Fog is a common occurrence across the planet. Any time air near the ground condenses to its dew point, we see fog.
Despite how common this weather phenomenon is, people seem consistently unable to prepare for its risks. Since it is such an ordinary event, fog dangers tend to be underestimated. Some of the fear surrounding mysterious fog might even be tied to the very real threats that come from reduced visibility and dangers to people with weak lungs. Mysterious fog or not, to protect ourselves from the dangerous health effects of severe fog, we need to wear good quality respirators and gas masks with proper gas mask filters.
Increased traffic accidents
According to the U.S. Department of Transportation, over 38,700 vehicles are involved in fog-related crashes annually. These crashes result in over 600 deaths each year.
The reason for this is simple - drivers don't want to slow down. It's simply not possible to drive at your accustomed speed during an extreme fog situation. Drivers are less likely to drive at a consistent speed or maintain an assured clear distance between themselves and the vehicle in front of them.
So tell your family and friends - when it's foggy, slow down!
Respiratory dangers
Fog itself may not seem like a respiratory risk. In fact, personal use humidifiers are often recommended to people with colds and allergies to soothe irritated nasal passages.
The real health threats comes from the other substances that can be carried along with the moisture in the air. Common pollutants, like sulfur dioxide, can contaminate the water that is then suspended in the air. Recent scholarship concludes that it's possible that the liquid might make the contaminants slightly less damaging to the airways. On the other hand, it might allow the contaminants to spread over a larger distance.
People with asthma or COPD often find that heavy, wet air makes it harder for them to draw in a deep breath.
Source: Olga Govorun on Unsplash
Lower oxygen levels
Some studies report that extended periods of fog can cause decreased concentrations of oxygen in the air, thanks to decreased photosynthesis in plants. Again, naturally occurring, water-based fog is not the problem here. The particulate matter that is carried throughout smog is the issue, especially in regions where there is a chronic covering of the material.
Dubai is one example, often seeing AQI readings exceeding 150, meaning very unhealthy air.
Frequently Asked Questions
Sources:
“Low Visibility,” U.S. Department of Transportation
“Potential risks to human respiratory health from "acid fog": evidence from experimental studies of volunteers,” Environmental Health Perspectives
“Daily lifestyles in the fog and haze weather,” Journal of Thoracic Disease
“Dubai’s Air Quality Crisis – What It Means for You & Your Health!” UAE Stories
“‘Strange’ fog in parts of U.S. and Canada has meteorological explanation, experts say,” The Canadian Press
“Widespread Fog Sparks Wild Theories About Secret Tests And Alien Activity,” Forbes
“Beware the Fog,” Science Magazine
“Radiation Fog,” National Weather Service
“Particulate Matter (PM) Basics,” Environmental Protection Agency
“Researchers Shed Light on How Mountain Fog Is Formed,” Technology Networks
“Numerical Weather Prediction,” National Centers for Environmental Information
“The Pacific Coastal Fog Project,” The United States Geological Survey
“When the smoke clears: The legacy of killer smogs,” The American Association for the Advancement of Science