Fog is a natural cloud of water droplets sitting close to the ground. Smog is a mixture of air pollutants, sometimes combined with that same moisture, that creates a hazy, often discolored layer over cities and industrial areas. The two can look similar from a distance, but they form through completely different processes and have very different effects on your health.
How Fog Forms
Fog is simply a low-lying cloud. When air near the ground cools enough for its water vapor to condense into tiny suspended droplets, you get fog. It typically appears white or light gray in color, and it forms most readily in cool, humid conditions, often overnight or in the early morning when the ground radiates heat away quickly. Once the sun warms the air, the droplets evaporate and the fog lifts.
Natural fog is not inherently harmful. It reduces visibility and can make driving dangerous, but the droplets themselves are just water. The trouble starts when fog forms in a polluted area, because those water droplets act like tiny sponges. They absorb sulfur oxides, nitrogen oxides, and other polluting gases from the surrounding air. Once inside the droplets, those gases convert into sulfates, nitrates, and other particle pollutants faster than they would in dry air. In other words, fog in a polluted environment accelerates the creation of smog.
How Smog Forms
Smog comes in two main varieties, each tied to a different source of pollution and a different climate.
Sulfurous (London-type) smog forms in cool, humid environments where large amounts of high-sulfur coal are burned. Its main ingredients are soot, fly ash, and sulfur dioxide. This is the classic “pea-souper” that plagued industrial cities for centuries. It tends to look dark gray or black because of the heavy load of soot particles.
Photochemical (Los Angeles-type) smog develops in warmer, sunnier conditions. Vehicle exhaust releases nitrogen dioxide into the atmosphere, and ultraviolet radiation from the sun breaks that molecule apart, triggering a chain reaction that produces ground-level ozone. The result is a haze made primarily of ozone and nitrogen dioxide. Nitrogen dioxide is a deep orange-red gas, which is why photochemical smog often gives the sky a yellow-brown or reddish tint rather than the gray of industrial smog.
Why Smog Gets Trapped Over Cities
Both types of smog become especially dangerous during temperature inversions. Normally, warm air near the ground rises and carries pollutants upward, dispersing them. But during a high-pressure weather system, particularly in winter, the ground loses heat rapidly at night and the air closest to the surface becomes colder than the air above it. That warmer upper layer acts like a lid, trapping cold air and everything in it, including exhaust fumes, soot, and chemical pollutants, right at street level.
This continues until the weather pattern changes. During extended inversions, pollution accumulates day after day with no escape. The longer the inversion holds, the thicker and more hazardous the smog becomes. Cities surrounded by mountains or valleys are especially vulnerable because the terrain itself blocks horizontal airflow that might otherwise push the dirty air out.
The Great Smog of London
The most infamous example of trapped smog hit London from December 5 to December 9, 1952. An anticyclone settled over the city, creating a strong temperature inversion. Emissions from coal-burning factories and household fireplaces had nowhere to go. Water vapor stuck to the airborne particles, producing a thick, dark cloud that hung at ground level for five days and cut visibility to near zero in some areas. Thousands of people died from respiratory and cardiac complications, and the disaster ultimately led to the United Kingdom’s Clean Air Act of 1956.
London’s air quality problems actually stretched back centuries. Complaints about coal smoke appeared as early as the 1600s, and King James I attempted legislation to restrict coal burning. But the scale of the 1952 event finally forced meaningful policy change.
How to Tell Them Apart by Sight
Color is the fastest visual clue. Clean fog is white or very pale gray because it consists almost entirely of water droplets scattering light evenly. Industrial smog skews darker, toward gray or even black, because of soot and ash particles. Photochemical smog leans brown, orange, or yellowish, a color produced largely by nitrogen dioxide.
Location and timing also help. If a white haze rolls in overnight near a river, lake, or coast and burns off by mid-morning, that is almost certainly fog. If a brownish haze builds over a city during a hot afternoon and you can see a distinct dirty layer sitting against the skyline, that is photochemical smog. And if you are in a heavily industrialized area during cold, still weather and the air looks gray and smells acrid, you are likely dealing with sulfurous smog.
Health Effects of Smog vs. Fog
Pure fog poses no direct respiratory risk. Smog is a different story. The fine particles in smog, those smaller than 2.5 micrometers in diameter (about 30 times thinner than a human hair), are small enough to bypass your nose and throat and penetrate deep into the lungs, reaching the smallest airways and air sacs. Ultrafine particles, smaller than 0.1 micrometers, can even enter the bloodstream.
Exposure to these particles is linked to a wide range of respiratory problems: worsening asthma symptoms, increased emergency room visits, chronic obstructive pulmonary disease, pneumonia, lung scarring, and lung cancer. Ground-level ozone, the signature pollutant in photochemical smog, irritates the lining of the airways and can trigger breathing difficulties even in otherwise healthy people. Nitrogen dioxide exposure has been associated with flare-ups of pulmonary fibrosis.
People with existing lung or heart conditions, young children, and older adults are the most vulnerable during smog events. On high-pollution days, limiting time outdoors and keeping windows closed makes a measurable difference in your exposure.
Can Fog and Smog Exist Together?
Yes, and they frequently do. The word “smog” itself is a blend of “smoke” and “fog,” coined in the early 1900s to describe exactly this overlap. When fog forms in a polluted area, the water droplets scavenge gaseous pollutants from the air and accelerate their chemical transformation into harmful particles. The result is a hybrid: a fog that looks dirtier, lasts longer, and carries a much higher concentration of irritants than either clean fog or dry smog alone. NASA satellite imagery often captures these mixed events over industrial regions, where the haze appears gray rather than the bright white of clean maritime fog.
This blending also makes the fog itself more acidic. Urban fog samples collected in the Los Angeles area have shown significant concentrations of hydrogen ions, meaning the droplets are far more acidic than natural rainwater. When that acidic fog settles on crops, buildings, or your skin, it carries pollutants directly to the surface in a way that dry smog does not.