Smog: Causes, Seasonal Changes, Health Impact, and Other FAQs

Fact checked by Olga Sadouskaya, MD
Clinical Pharmacologist, Chief Medical Officer

How does smog form? What is it made of? Is it dangerous? In this article, we answer your burning questions about smog. Let's dive in!

What is smog?

The word smog is a combination of the words smoke and fog. It refers to a kind of air pollution that’s largely caused by cars, factories, heating stoves, and other human inventions.

According to the World Health Organization, up to a third of deaths from stroke, lung cancer, and heart disease can be traced back to breathing polluted air. This makes air pollution as dangerous as smoking tobacco!

What is smog made of?

Smog is a mixture of particles and gases. Let’s take a look at its main components.

Ground-level ozone

Ozone is a gas that consists of three oxygen atoms. This gas can be both good and bad for us, depending on where it’s found.

The ozone that’s high up in the atmosphere is good. It makes up the ozone layer that protects our planet from harmful ultraviolet radiation.

But the ozone that’s near the ground is bad. It’s created when other pollutants (emitted by cars, factories, and so on) go through chemical reactions in the presence of sunlight. Breathing ground-level ozone can harm your lungs.

Particulate matter

Particulate matter refers to solid and liquid particles that are suspended in the air. Some of these particles come from fires, construction sites, unpaved roads, and factories. However, most particulate matter is a product of complex chemical reactions between various pollutants.

A city skyline during smog

The particles that make up particulate matter come in different sizes. Some are big enough to be seen as dust or smoke. Others are very small; in fact, fine inhalable particles are 30 times smaller than the diameter of a human hair!

These tiny particles are dangerous because they can get deep into your lungs and bloodstream.

Other chemicals

In addition to ground-level ozone and particulate matter, smog also contains carbon monoxide, nitrogen oxides, sulfur dioxide, volatile organic compounds, and other chemicals. These substances interact with each other and produce even more pollutants, including ground-level ozone.

The chemicals that make up smog also cause acid rain, which is very harmful to plants and wildlife.

What causes smog?

There are two kinds of smog: London smog and Los Angeles smog.

London smog, also called sulfurous smog, is caused by burning coal. In the 19th century, London was a major industrial hub. All those factories used a lot of coal, filling the air with sulfur particles and other pollutants.

The worst smog incident ever reported was the Great Smog of London.

It happened in December 1952, lasted for five days, and killed at least 4,000 people. It was caused by a combination of air pollution and specific weather conditions that trapped the poisonous air near the ground.

After that, the British government took steps to reduce the use of coal. However, in many countries, people still use coal to heat their homes and produce electricity, so sulfurous smog is not a thing of the past.

Los Angeles smog is also known as photochemical smog. It’s largely caused by the emissions from cars and trucks. Some of the chemicals that also contribute to photochemical smog come from paints, solvents, and pesticides.

In the presence of sunlight, various pollutants interact with each other and produce ground-level ozone.

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Photochemical smog is at its strongest on hot, sunny days. In 2020, a heatwave in Los Angeles caused the worst smog in 26 years!

Though the two smog types are named after specific cities, they can occur anywhere — not just in London or Los Angeles!

Where is smog very thick in the United States?

Los Angeles is the “smog capital” of the United States. The city’s crowded roads and sunny climate create the perfect conditions for photochemical smog. It shouldn’t come as a surprise that around 1.5 million residents of Los Angeles have some form of asthma.

Another city that’s notorious for its air pollution is Bakersfield, CA. Here, the smog is caused by the mining and refining of oil, intense industrial activity, and frequent wildfires.

However, smog doesn’t only affect big cities. For example, while air quality in Alaska is generally good, communities there that burn wood to heat their homes still experience smog.

Are smog and weather conditions connected?

Air quality depends on the same factors as weather in general: air pressure, humidity, and temperature.

Strong winds carry pollutants from one area to another. For example, coal burning in the Ohio Valley was shown to cause acid rain up to a thousand miles away.

On the other hand, high air pressure makes air stay in one place. Pollutants have nowhere to go, so their concentration increases and smog gets worse.

Does smog get thicker in winter? Smog in different seasons

Concentrations of air pollutants change throughout the year because of weather fluctuations and seasonal variations in human activity.

For example, large amounts of ground-level ozone are only produced when there’s lots of sunlight, usually in the summer months.

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On the other hand, cold weather makes car engines work harder and produce more emissions. In addition to this, in cold weather, people may start their cars and wait for them to warm up. An idling car wastes fuel and emits unnecessary pollutants into the air.

Winter is also the season when people burn wood and coal to heat their homes, leading to more gas emissions. Some people also burn their household trash, which releases a lot of toxic chemicals.

Another reason smog can get thicker in winter is temperature inversion. This is when air gets trapped near the ground and pollutants keep accumulating until weather conditions change.

How do smog and air pollution impact your health?

Some of the better known health effects of smog are decreased lung function and the development of chronic diseases such as asthma and allergies.

But research shows that air pollution can affect every organ in your body.

Studies have demonstrated links between air pollution and many other chronic conditions, from acne to cardiovascular disease.

In addition to this, the International Agency for Research on Cancer also considers air pollution to be one of the leading causes of cancer.

The exact effects of smog on your health depend on a number of factors, including:

  • The concentration of pollutants in the air you breathe
  • The types of pollutants prevalent where you live
  • Your age and chronic health conditions
  • How long you are exposed to smog

Smog is especially dangerous for seniors, children, and people with chronic diseases. But young, healthy people can also be affected if they spend a lot of time breathing polluted air.

Each component of smog can have a serious impact on your health. Ground-level ozone is known to irritate the airways and cause symptoms such as:

  • Coughing
  • Pain when taking a deep breath
  • Irritated throat
  • Trouble breathing during physical activity

Breathing ground-level ozone can also make asthma, bronchitis, and emphysema worse.

A young woman wearing a face mask to protect herself against smog

How to protect yourself from smog

The World Health Organization estimates that 90 percent of people worldwide breathe polluted air. This is why it’s good to know how to protect yourself from the harmful effects of smog.

Check air quality forecasts

Many weather reports also contain information about the air quality in your area. Make sure to check the forecast before going outside.

Stay indoors when necessary

To protect yourself from ground-level ozone, try to stay indoors on hot sunny days, when ozone levels are at their highest. If possible, schedule outdoor activities in the mornings and evenings.

Don’t exercise in high-traffic areas

While some people like jogging along busy city streets, this is not the best option if you want to breathe clean air. Cars emit a lot of pollutants that contribute to smog.

Wear a mask

Masks don’t just prevent airborne infections from spreading. A high-quality face mask with an N95 rating can filter up to 95 percent of fine particulate matter. To be effective, your mask needs to have a snug fit and adjustable straps.

Key takeaways

Smog is a mixture of air pollutants including ground-level ozone, particulate matter, volatile organic compounds, and various gases. It’s caused by emissions from cars, factories, heating stoves, etc.

Air quality depends on a variety of factors, including weather conditions. This is why it’s important to check air quality levels regularly and make sure you protect yourself.

Smog can be very harmful, so it’s best to avoid it as much as possible and stay in touch with your doctor if you have chronic diseases.

November 3, 2022