The Causes and Effects of Smog

A smoggy view of the Shanghai Skyline
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Smog is a mixture of air pollutants—nitrogen oxides and volatile organic compounds—that combine with sunlight to form ozone.

Ozone can be beneficial or harmful, good or bad, depending on its location. Ozone in the stratosphere, high above the Earth, acts as a barrier that protects human health and the environment from excessive amounts of solar ultraviolet radiation. This is the "good kind" of ozone.

On the other hand, ground-level ozone, trapped near the ground by heat inversions or other weather conditions, is what causes the respiratory distress and burning eyes associated with smog.

How Did Smog Get Its Name?

The term "smog" was first used in London during the early 1900's to describe the combination of smoke and fog that often blanketed the city. According to several sources, the term was first coined by Dr. Henry Antoine des Voeux in his paper, “Fog and Smoke,” which he presented at a meeting of the Public Health Congress in July 1905.

The type of smog described by Dr. des Voeux was a combination of smoke and sulfur dioxide, which resulted from the heavy use of coal to heat homes and businesses and to run factories in Victorian England.

When we talk about smog today, we’re referring to a more complex mixture of various air pollutants—nitrogen oxides and other chemical compounds—that interact with sunlight to form ground-level ozone that hangs like a heavy haze over many cities in industrialized countries.

What Causes Smog?

Smog is produced by a set of complex photochemical reactions involving volatile organic compounds (VOCs), nitrogen oxides and sunlight, which form ground-level ozone.

Smog-forming pollutants come from many sources such as automobile exhaust, power plants, factories and many consumer products, including paint, hairspray, charcoal starter fluid, chemical solvents, and even plastic popcorn packaging.

In typical urban areas, at least half of the smog precursors come from cars, buses, trucks, and boats.

Major smog occurrences often are linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. Weather and geography affect the location and severity of smog. Because temperature regulates the length of time it takes for smog to form, smog can occur more quickly and be more severe on a hot, sunny day.

When temperature inversions occur (that is, when warm air stays near the ground instead of rising) and the wind is calm, smog may remain trapped over a city for days. As traffic and other sources add more pollutants to the air, the smog gets worse. This situation occurs frequently in Salt Lake City, Utah.

Ironically, smog is often more severe farther away from the sources of pollution, because the chemical reactions that cause smog take place in the atmosphere while pollutants are drifting on the wind.

Where Does Smog Occur?

Severe smog and ground-level ozone problems exist in many major cities around the world, from Mexico City to Beijing, and a recent, well-publicized event in Delhi, India. In the United States, smog affects much of California, from San Francisco to San Diego, the mid-Atlantic seaboard from Washington, DC, to southern Maine, and major cities in the South and Midwest.

To varying degrees, the majority of U.S. cities with populations of 250,000 or more have experienced problems with smog and ground-level ozone.

According to some studies, more than half of all U.S. residents live in areas where the smog is so bad that pollution levels routinely exceed safety standards set by the U.S. Environmental Protection Agency (EPA).

What Are the Effects of Smog?

Smog is made up of a combination of air pollutants that can compromise human health, harm the environment, and even cause property damage.

Smog can cause or aggravate health problems such as asthma, emphysema, chronic bronchitis and other respiratory problems as well as eye irritation and reduced resistance to colds and lung infections.

The ozone in smog also inhibits plant growth and can cause widespread damage to crops and forests.

Who Is Most at Risk from Smog?

Anyone who engages in strenuous outdoor activity—from jogging to manual labor—may suffer smog-related health effects. Physical activity causes people to breathe faster and more deeply, exposing their lungs to more ozone and other pollutants. Four groups of people are particularly sensitive to ozone and other air pollutants in smog:

  • Children—Active children run the highest risks from exposure to smog, as children spend a lot of time playing outside. As a group, children are also more prone to asthma—the most common chronic disease for children—and other respiratory ailments than adults.
  • Adults who are active outdoors—Healthy adults of any age who exercise or work outdoors are considered at higher risk from smog.
  • People with respiratory diseases—People with asthma or other chronic respiratory diseases are more sensitive and vulnerable to the effects of ozone. Typically, they will experience adverse effects sooner and at lower levels of exposure than those who are less sensitive.
  • People with unusual susceptibility to ozone—Some otherwise healthy people are simply more sensitive to the pollutants in smog than other people and may experience more adverse health effects from exposure.

Elderly people are often warned to stay indoors on heavy smog days. Elderly people are probably not at increased risk of adverse health effects from smog because of their age. Like any other adults, however, elderly people will be at higher risk from exposure to smog if they already suffer from respiratory diseases, are active outdoors, or are unusually susceptible to ozone.

How Can You Recognize or Detect Smog Where You Live? 

Generally speaking, you will know smog when you see it. Smog is a visible form of air pollution that often appears as a thick haze. Look toward the horizon during daylight hours, and you can see how much smog is in the air. High concentrations of nitrogen oxides will often give air a brownish tint.

In addition, most cities now measure the concentration of pollutants in the air and provide public reports—often published in newspapers and broadcast on local radio and television stations—when smog reaches potentially unsafe levels.

The EPA has developed the Air Quality Index (AQI) (formerly known as the Pollutant Standards Index) for reporting concentrations of ground-level ozone and other common air pollutants.

Air quality is measured by a nationwide monitoring system that records concentrations of ground-level ozone and several other air pollutants at more than a thousand locations across the United States. The EPA then interprets that data according to the standard AQI index, which ranges from zero to 500. The higher the AQI value for a specific pollutant, the greater the danger to public health and the environment.

Edited by Frederic Beaudry.