Image provided by Mohammad Rahman on Unsplash
TL;DR: Smog, a major type of air pollution, exists in two primary forms: sulfurous smog (from sulfur oxides and water vapor) and photochemical smog (from reactions between sunlight, VOCs, and nitrogen oxides resulting in ozone). Both harm health and the environment, causing respiratory issues, acid rain, and reduced visibility. While regulations have curbed some pollution sources, urban smog persists. Clarity’s air monitoring tools support efforts to track and reduce smog through real-time data.
Smog is considered one of the most pervasive forms of air pollution. The term “smog” was first coined in London as a combination of the words “smoke” and “fog.” In 2023, the American Lung Association found that nearly 120 million people are exposed to unhealthy levels of soot and smog within the United States alone, yet cities around the world suffer from this polluted haze. But what exactly is smog, and how is it formed?
What is smog?
Smog is a visible, polluted haze that often settles over urban areas. There are at least two different types of smog, known as sulfurous smog (aka industrial smog) and photochemical smog. Whereas the former was more common in the past, the latter is more prevalent now.
Sulfurous/Industrial smog
Sulfurous smog is also known as industrial smog or classic smog. This type of smog is considered to be one of the most serious types of air pollution. It is formed by a combination of sulfur oxide emissions, including sulfur dioxide, and water vapor, often from burning coal and sometimes oil. The sulfur mixes with the water vapor in the atmosphere to form sulfuric acid. In addition to being toxic to human health, sulfuric acid is also a component in acid deposition, which includes acid rain.
Characterized as brown, grey, or yellow, sulfurous smog is often associated with London before regulations were implemented following a particularly deadly smog event in 1952, known as the Great Smog of London. This event resulted in at least 4,000 and as many as 12,000 deaths. The Donora Smog in Pennsylvania is another notorious example of this kind of air pollution. Although it was considered synonymous with the Industrial Revolution, sulfurous smog continues today, with coal-burning power plants and factories being the leading sources. While cars also contribute to industrial smog, their contribution is much smaller.
Health and environmental effects of sulfurous smog
Sulfur dioxide and sulfuric acid, both integral to sulfuric smog, have negative effects on human health. Sulfur dioxide makes breathing difficult, with children and asthmatic individuals particularly vulnerable. Inhaling sulfur dioxide can also cause internal irritation and swelling. One of the most serious health concerns associated with this type of smog is bronchitis, caused by inhaling the polluted air. At high concentrations, these health issues become more severe and potentially even deadly.
Sulfur oxides also react with compounds in the atmosphere forming particulate matter air pollution. Small particulate matter particles (PM2.5) can enter the lungs and bloodstream and lead to lung disease, asthma attacks, bronchitis, and premature death. Sulfuric acid mist similarly irritates the eyes, nose, and throat while causing fluid buildup in the lungs at higher concentrations.
In terms of environmental effects, sulfur dioxide can harm vegetation, damaging foliage and decreasing plant growth. It also inhibits visibility and contributes to acid rain. Acid deposition, in turn, causes aluminum to be released into ecosystems, harms fish, strips soil of vital plant nutrients, and even damages man-made structures and monuments.
Fortunately, certain organizations such as the United States Environmental Protection Agency (USEPA) have placed restrictions on sulfur dioxide emissions. The United Kingdom has similarly implemented regulations on the burning of coal to prevent anything like the Great Smog of London from ever happening in that region again.
Photochemical smog
Most modern smog is photochemical smog. This type of air pollution is often associated with Los Angeles’ ambient haze, but it is prevalent in cities all over the world. Strangely enough, photochemical smog does not necessarily have to contain either smoke or fog. Instead, photochemical smog is usually composed of ground-level ozone and particulate matter. Ozone, while beneficial in the upper (stratospheric) atmosphere, is harmful at ground level.
The ground-level ozone in smog primarily comes from a chemical reaction between volatile organic compounds (VOCs) and nitrogen oxides (NOx). VOCs are carbon-containing compounds that usually evaporate into the atmosphere. They can come from paints, pesticides, and various consumer products. VOCs also form from fuel combustion from motor vehicles and even certain lawn equipment. Fortunately, catalytic converters have dramatically reduced the amount of VOCs emitted from motor vehicles.
Nitrogen oxides, most notably nitric oxide (NO) and nitrogen dioxide (NO2), primarily form from vehicle emissions. NO forms due to the heat that occurs during fuel combustion, and quickly oxidizes in the atmosphere to form NO2. NOx can also come from power plants, off-road equipment, stoves, ovens, and indoor heaters. Nitrogen dioxide is largely responsible for giving photochemical smog its yellowish-brown color. Sunlight causes volatile organic compounds to react with nitrogen oxides to form ground-level ozone.
Particulate matter also forms from combustion sources such as vehicles, diesel engines, and industrial facilities, and is more prevalent in urban areas. Nitrogen oxides undergo chemical reactions, which further result in particulate matter formation as well. The mix of particulate matter, nitrogen oxides, VOCs, and ozone all contribute to photochemical smog.
Health and environmental effects of photochemical smog
Ground-level ozone negatively impacts both human health and the environment. Ozone can cause difficulty breathing and exacerbate existing conditions such as asthma and lung disease. It can also lead to worsened allergies, and even premature death. Although greater levels of ozone have more harmful effects, there is no safe level of ozone. Ground-level ozone is also a greenhouse gas, trapping heat and exacerbating global warming. Ozone can also harm plants, reducing photosynthesis and inhibiting growth, which harms both ecosystems and crops.
Nitrogen oxides harm human health by irritating the upper respiratory tract, causing coughing, shortness of breath, nausea, and tiredness. Repeated exposure to NOx can eventually lead to asthma. Children are particularly at risk from these dangers. Furthermore, NO and NO2 can combine with water vapor in the atmosphere to form nitric acid, contributing to acid rain.
Volatile organic compounds can harm human health by causing irritation of the eyes, nose, and throat, as well as inducing nausea, dizziness, headaches, and difficulty breathing. Long-term exposure to VOCs can damage the liver, nervous system, and kidneys. Particulate matter also causes a variety of previously mentioned adverse health effects in addition to obscuring visibility.
Reducing photochemical smog
Organizations such as the US Environmental Protection Agency have established limits on certain pollutants that make up photochemical smog. In particular, the US EPA has established ground-level ozone, nitrogen dioxide, and particulate matter as criteria pollutants with their own regulations.
Photochemical smog remains pervasive in many cities. There are additional things that can be done on the individual level to reduce this pollution, such as replacing gas appliances, cars, and equipment with electric versions, and using alternatives to driving. Using fuel-efficient vehicles, properly caring for cars with regular tune-ups, and fueling up in the cooler hours of the day can help reduce smog. Lastly, be sure to avoid using products that release large amounts of VOCs when possible.
Looking Forward
One of the first steps in effective smog reduction and regulation is pollution monitoring. Gathering data promotes informed action from governments, organizations, and communities. Learn how to build a solution with Clarity.
