About Criteria Air Pollutants

Although levels of outdoor air pollutants have fallen dramatically in California over the last 30 years, many parts of the state are still riddled with air quality problems. In fact, Los Angeles County remains one of the most polluted regions in the nation.

Efforts to control pollution are necessary to ensure that our air meets state and federal air quality standards. Standards are set by both the Environmental Protection Agency (EPA) and locally, by the California Air Resources Board (CARB). The EPA establishes standards for six common air pollutants, also known as "criteria air pollutants", that all states must abide by. In California, CARB, an agency of the California EPA, imposes additional air quality regulations (California Ambient Air Quality Standards); in many cases, CARB's standards are even stricter than those implemented by the EPA.

The six criteria air pollutants regulated by the EPA include particulate pollution, ground-level ozone, lead, carbon monoxide, nitrogen oxides and sulfur oxides. These pollutants are felt to be of particular importance because of their potential to cause health problems in humans and damage the environment.

Particulate Matter

Particulate matter, also known as particulate pollution, is a mixture of extremely small solid particles suspended in liquid droplets. Particulate matter varies significantly in composition. It also varies dramatically in size; some particles like dust, dirt or smoke are large enough to be visible while others are so small they can only be seen with a microscope. When it comes to health, smaller particles are more worrisome than larger ones because they are able to pass through the nose and mouth and get deep into the lungs.

Exposure to particulate pollutants affects both the heart and lungs. It has been linked to respiratory symptoms like coughing and breathing difficulties, as well as chronic bronchitis. In people with asthma, exposure to particulates aggravates symptoms of their disease; in individuals with heart disease, it raises their risk of a heart attack. Prolonged exposure to elevated levels of particulate pollution actually increases the risk of death from both cardiovascular and respiratory diseases. A recent study by the California Air Resources Board estimated that particulate pollution causes over 9,000 deaths in California annually.


In the stratosphere, high above the earth's surface, ozone forms a protective layer that helps screen us from the sun's harmful rays. At ground-level, however, ozone is a harmful pollutant.

Ground-level ozone is created when sunlight strikes pollutants called nitrogen oxides found in the air. (Major sources of nitrogen oxides include cars, trucks and buses and power plants.) Because sunlight is brightest in the summer months, ozone often becomes a particular problem at this time of year, reaching harmful concentrations in the air.

Exposure to ozone inflames the linings of the lungs and damages lung function. It can trigger a variety of health problems including chest pain, coughing, throat irritation and congestion, as well as aggravate lung diseases such as asthma, bronchitis and emphysema. Repeated and prolonged exposure to high levels of ground-level ozone may permanently damage lung tissue.


Lead is a metal that occurs naturally in soil and rock; however, a variety of human activities also release lead into the environment, contaminating our air. Motor vehicles used to be the primary source of lead emissions but, with the phasing out of leaded gasoline, emissions from cars and trucks have declined dramatically. Currently, major sources of lead emissions include industrial processes like ore and metal processing and non-road equipment that utilize lead-containing fuel such as recreational vehicles, farm and construction machinery, lawn and garden equipment, boats, and trains. (Note: This is by far and away the largest contributor in California)

Prolonged exposure to lead can cause levels in the blood to rise and trigger a host of harmful effects. The frequency and severity of medical symptoms increases with the concentration of lead in the blood. Elevated lead levels can cause symptoms such as irritability, fatigue, loss of appetite and headache. And, as it accumulates in the body, organs throughout the body can be affected. Infants and young children are especially sensitive to lead and even relatively small elevations in their lead levels can lead to behavioral problems and learning deficits.

Carbon Monoxide

Carbon monoxide is a gas that forms when fossil fuels are burned. Motor vehicles are the single largest source of carbon monoxide emissions. Nationwide, cars and trucks contribute to more than 50% of all emissions; in cities like Los Angeles, where traffic density is high, their contribution is even greater. Other major sources of carbon monoxide include non-road equipment and fires.

When carbon monoxide is inhaled, it enters the bloodstream and attaches to hemoglobin, a substance that carries oxygen to cells throughout the body. This prevents the blood from carrying adequate amounts of oxygen to the body’s organs and tissues and causes several health effects. At relatively low concentrations, men and women in good health may develop headaches and become fatigued; individuals with heart disease, however, might suffer more serious symptoms such as chest pain.

Nitrogen Oxides

Nitrogen oxides are a group of gases regulated by the EPA - nitrogen dioxide being among the most important. By far in a way, the leading contributors to nitrogen dioxide emissions are cars and trucks and non-road equipment.

Breathing in high levels of nitrogen dioxide can lead to respiratory problems. Nitrogen dioxide irritates the lining of the lungs and causes symptoms such as coughing and wheezing; it also impairs the body’s ability to fight off pulmonary infections and someone exposed to elevated levels of the pollutant may be more susceptible to develop colds and bronchitis.

Sulfur Oxides

Sulfur oxides are another group of regulated gases—the most closely monitored being sulfur dioxide. The main sources of sulfur dioxide emissions are power plants and industrial facilities that burn sulfur-containing fossil fuels.

Exposure to sulfur dioxide damages the respiratory system. It causes nose, throat and airway irritation, coughing, wheezing, chest tightness and shortness of breath. People with asthma and other chronic respiratory diseases are at greatest risk of developing problems after exposure to sulfur dioxide.

The Environmental Protection Agency along with state and local planning agencies identify areas of the country where air pollution levels fail to meet air quality standards. These areas, known as "nonattainment" areas, must develop plans to achieve federal standards in a timely fashion.

Currently, Los Angeles County does not meet the U.S. EPA's national air quality standards for several pollutants. To help improve air quality in the region, the Air Quality Management District is preparing the 2016 Air Quality Management Plan. The plan will set forth a comprehensive program that will help improve air quality in the county.

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