Industrial smog is a thick, dark haze caused by burning coal and oil in factories, power plants, and vehicles

Industrial smog forms when sulfur dioxide and nitrogen oxides from burning fossil fuels mix with moisture and particles in the air. Unlike the brown haze you might see over a city on a hot day, industrial smog is gray or black and often smells like rotten eggs or burnt rubber. It happens most often in winter, when cold air traps pollution close to the ground instead of letting it rise and disperse.

The term "industrial smog" comes from the combination of "smoke" and "fog." It was first named during the Great Smog of London in 1952, when the city was blanketed for days and thousands of people died from breathing problems. That event changed how governments think about air pollution and led to the first major clean air laws.

Key Takeaways

  • Industrial smog forms when coal and oil burning releases sulfur dioxide and nitrogen oxides that mix with moisture and particles in cold air.
  • The worst industrial smog happens in winter or in valleys where cold air gets trapped and pollution cannot escape upward.
  • Breathing industrial smog can cause when ready problems like coughing and wheezing, and long-term damage to lungs and heart health.
  • Coal-burning power plants and factories in older industrial regions still produce industrial smog, though cleaner fuels and regulations have reduced it in many places.

How industrial smog forms in cold weather

Industrial smog needs three things: a source of pollution, moisture in the air, and weather that traps the pollution close to the ground. When factories and power plants burn coal or heavy fuel oil, they release sulfur dioxide and nitrogen oxides into the air. On a cold day with high humidity, these gases mix with water droplets and particles like soot to form a thick haze.

Temperature inversions make industrial smog worse. Normally, warm air rises and carries pollution upward and away. But when a layer of warm air sits above a layer of cold air near the ground, the cold air cannot rise. Pollution gets trapped underneath, building up hour after hour. This happens most often in winter mornings in valleys or cities surrounded by hills, where cold air settles and has nowhere to go.

Industrial smog is different from photochemical smog, which forms on hot, sunny days when sunlight reacts with pollution from cars and creates brown ozone. Industrial smog is a winter problem; photochemical smog is a summer problem.

Where industrial smog still occurs

Industrial smog is most common in regions with heavy coal use and cold winters. Parts of China, India, and Eastern Europe experience severe industrial smog because coal-burning power plants and factories are concentrated there and regulations on emissions are less strict than in North America or Western Europe. Cities like Delhi and Beijing have reported industrial smog events that forced schools to close and hospitals to treat thousands of people for breathing problems.

In the United States and Europe, industrial smog is much less common than it was in the 1960s and 1970s, because coal use has declined and power plants now have pollution controls. However, older industrial cities in the Rust Belt and regions that still rely on coal-fired power still see industrial smog on cold winter days. Natural gas and renewable energy have replaced much of the coal burning that once caused thick smog.

Health effects of breathing industrial smog

Industrial smog irritates the lungs and airways. People exposed to it often cough, wheeze, and feel shortness of breath. The sulfur dioxide in industrial smog is especially irritating to people with asthma or chronic bronchitis, because it narrows the airways. Children, older adults, and people with heart or lung disease are at highest risk of serious effects.

Long-term exposure to industrial smog damages lung tissue and increases the risk of heart disease and early death. Studies of people who lived through the London smog of 1952 showed that survivors had reduced lung function for the rest of their lives. People who work in factories or power plants without proper respiratory protection face the highest risk.

During a smog event, people can reduce their exposure by staying indoors, keeping windows closed, and using air filters. People with asthma or heart conditions should have their medications on hand and avoid outdoor exercise when smog is heavy.

Pollution sources that create industrial smog

Coal-burning power plants are the largest source of industrial smog in regions where coal is still used for electricity. A single large power plant can release hundreds of tons of sulfur dioxide and nitrogen oxides per year. Steel mills, refineries, and chemical factories also burn fossil fuels and release these gases.

Vehicles contribute to industrial smog, especially in cities with heavy traffic and cold winters. Diesel trucks and buses release nitrogen oxides and soot. In regions with strict emissions standards, newer vehicles produce far less pollution, but older vehicles and those without proper maintenance still add to the problem.

Heating systems in buildings also contribute. In countries where coal or heavy oil is burned for home heating, winter smog is worse because millions of small sources add up. Natural gas heating produces less pollution, and electric heating produces none at the point of use.

Regulations and controls that reduce industrial smog

The Clean Air Act in the United States, passed in 1970 and strengthened in 1990, set limits on sulfur dioxide and nitrogen oxide emissions from power plants and factories. Similar laws exist in Europe, Canada, and other developed countries. These regulations require power plants to install scrubbers and other equipment that remove pollution before it enters the air.

Switching from coal to natural gas or renewable energy is the most effective way to prevent industrial smog. Natural gas produces far less sulfur dioxide, and wind and solar produce none. Many countries have set important date to phase out coal-fired power plants. China has closed thousands of small coal plants and is building more natural gas and renewable capacity, though coal still dominates its energy supply.

Fuel standards also help. Requiring lower-sulfur coal or oil reduces the amount of sulfur dioxide released. Some countries have banned the dirtiest fuels entirely. These changes take time because power plants and factories are expensive to build and operate for decades, but they have dramatically reduced industrial smog in places where they have been enforced.

The difference between industrial smog and other air pollution

Industrial smog and photochemical smog are both air pollution, but they form in different ways and at different times. Industrial smog is gray or black, forms in winter, and smells like sulfur. It comes from burning coal and oil. Photochemical smog is brown or orange, forms on hot sunny days, and comes from sunlight reacting with pollution from cars. Photochemical smog is the main air quality problem in Los Angeles, Phoenix, and other warm cities.

Haze is a third type of air pollution, caused by fine particles suspended in the air. Haze can be caused by industrial smog, dust, pollen, or smoke from fires. Acid rain is a separate problem caused when sulfur dioxide and nitrogen oxides from industrial smog mix with water in clouds and fall as acidic precipitation.

Frequently Asked Questions

Is industrial smog still a problem in the United States?

Industrial smog is much less common than it was in the 1970s because coal use has declined and power plants have pollution controls. However, on cold winter days in regions that still use coal for power, industrial smog can still form. Most severe industrial smog now occurs in Asia and parts of Eastern Europe.

Can I protect myself from industrial smog?

During a smog event, stay indoors with windows closed and use an air filter if you have one. N95 masks can reduce exposure if you must go outside, though they work best if fitted properly. People with asthma or heart disease should avoid outdoor exercise and have their medications ready.

Why does industrial smog smell like rotten eggs?

The rotten egg smell comes from sulfur dioxide, one of the main gases in industrial smog. Sulfur dioxide is released when coal and oil with high sulfur content are burned. The smell is a warning sign that pollution levels are high.

What is a temperature inversion?

A temperature inversion happens when a layer of warm air sits above a layer of cold air near the ground. Normally warm air rises, but during an inversion it cannot, so pollution gets trapped close to the ground. This is why industrial smog is worst on cold, still mornings in winter.