Air Pollution Basics

Learn about the fundamental types of air pollution and where they come from.

What is smog?

"Smog" is a common term for visible air pollution, which has two main components: ground-level ozone and particulate matter. However, very often people use "smog" to refer specifically to ozone pollution while "soot" refers to particulate matter.

What is ozone?

Ozone (O3) is a gas composed of three oxygen atoms. Ozone found at ground level is a major component of smog, and it can damage lung tissue and cause other health problems. Ozone is generally not released directly into the air. Rather, it forms by a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight. Ozone levels are highest on hot summer days when there is little wind. Ground level ozone, or ozone pollution, should not be confused with stratospheric ozone, located high up in the atmosphere, which helps to block harmful ultraviolet rays. Here is EPA's description of ozone. Here is CARB's description.

What is particulate matter?

Particulate matter (PM) is composed of small particles of dust, smoke, and soot that are released or kicked up into the air. PM can also be formed by reactions in the air between chemicals, including some of those that form ozone. Particulate matter smaller than ten microns in diameter, known as "PM-10," is about one-seventh of the diameter of a human hair. Levels of PM pollution vary depending on seasonal activities, rainfall and wind conditions. Here is EPA's description of PM. Here is CARB's description.

Where does air pollution come from?

VOCs (Volatile Organic Compounds) generally come from evaporated petroleum products. The main sources of VOCs are pesticides and fertilizers, motor vehicles, farming operations, petroleum production facilities, surface coatings, and solvents.

NOx (Nitrogen Oxides) come from fuel combustion, both in motor vehicles and industrial processes. Power plants, factories, cars, trucks, and stationary engines all create NOx.

Particulate matter (PM) comes from dirt roads, farming operations, industrial crushing and grinding, and windblown dust, among other sources. Combustion of fossil fuels, as well as burning garbage and agricultural products, also releases particulate matter into the air. Additionally, PM can be formed when chemicals such as NOx, sulfur dioxides, and others (including many of the same that form ozone) react and condense in the atmosphere.

Emissions Inventory
For detailed information on sources of pollution in a specific region of California, visit the California Air Resources Board's emission inventory website.

Compare emissions inventories for selected California regions:

What are the health effects of ozone and particulate matter?

Health effects of ozone

When inhaled, ozone irritates the respiratory system and can cause shortness of breath, wheezing, coughing, and chest pain, as well as exacerbate allergies and respiratory diseases such as asthma. (Read more). In fact, recent studies have shown that ozone can actually cause asthma in children who are active outdoors in smoggy areas. Over 50,000 Californians are hospitalized yearly because of severe asthma attacks, and more young children are hospitalized every year for asthma than for any other cause. (Read more). Ozone not only aggravates the respiratory system temporarily; prolonged inhalation of unsafe levels of ozone can reduce lung function and development in children and permanently damage lung tissue.

Health effects of particulate matter (PM)

Nationwide, air pollution causes between 50,000 and 100,000 premature deaths per year, and particulates account for a majority of these. In fact, PM accounts for more deaths than homicides and automobile accidents combined every year. Non-fatal health impacts of PM include reduced lung function, heart problems, and aggravation of respiratory illnesses, such as asthma, bronchitis, emphysema, chronic obstructive lung disease, and pneumonia. PM can accelerate death from other causes, such as lung cancer, and exposure to the small particles, even for short periods of time, can cause heart damage and trigger heart attacks. Read more here.

Who suffers from the impacts of air pollution?

- Greenpeace

"Growing up in the Valley, I believed that having asthma was like having a common cold. I grew accustomed to seeing inhalers in people's backpacks. They blended in with my classmate's yellow pencils and erasers."
Fresno Bee Editorial by former Valley student (3/16/05). Read the entire editorial here.

Although smog is harmful to everyone's lungs, certain groups are disproportionately at risk. They include:

Children: Children spend more time outdoors than adults, and are at high risk because their lungs are still developing. Children are also more adversely impacted by air pollution than adults because they breathe more pounds of air per body mass. As a result, children suffer more than adults from asthma and other respiratory ailments. In fact, asthma is one of the leading causes of school absenteeism, accounting for over 10 million missed school days per year.

The elderly: Pollution-induced asthma attacks can be especially dangerous, and even fatal, to the elderly who suffer from more respiratory tract infections and previous lung damage than younger adults.

Adults who are active outdoors: Even the healthiest adults who exercise or work outdoors can experience lung damage when pollution levels are high.

People who suffer from asthma and other respiratory diseases: Around 2.2 million Californians suffer from asthma. Asthmatics are at risk because they have decreased lung function, and pollution can greatly exacerbate the severity of attacks.

Low-income and people of color: These communities are at a greater risk because they often lack access to culturally- and linguistically- responsive health care, so respiratory ailments, such as asthma, often go undiagnosed and untreated. 29 percent of Latino children lack health insurance and thus lack access to both treatment and preventative care. In California, the asthma death rate for African American children is over four times greater than that of white children.

How does air pollution affect agriculture?

California is the nation's most productive agricultural state, boasting sales of $32 billion in 2005. Yet agricultural operations contribute significantly to California's air pollution problems, particularly in the San Joaquin Valley. The pollution caused by agricultural activities such as land cultivation, pesticide application, and crop harvest do more than harm the health of Valley residents: it also costs Valley farmers as much as $270 million each year in damaged crops.

The Air Quality Effects Laboratory is currently researching ozone impacts on plants in California.

Read an article about the effects of pollution on agriculture: Farmers' foe: Smog damage to crops costs billions (7/16/06).