Air Pollution Primer


What are the gaseous components of air pollution?
Why does particle size matter?
What are the environmental impacts of air pollution?
What are the visual impacts of air pollution?

What are the gaseous components of air pollution?

While PM and ozone are the largest contributors to illnesses, EPA’s other four criteria pollutants play distinct role in health.
Carbon monoxide (CO)
Carbon monoxide (CO) is a colorless and odorless gas that is emitted from motor vehicles. CO emissions are the result of incomplete burning of the fuel. The focus of tracking CO is highways in urban areas where motor vehicle exhaust is concentrated. Other major sources of CO production are wood-burning stoves, incinerators, and industrial facilities.
Nitrogen dioxide (NO2)
Nitrogen dioxide (NO2) and nitrogen oxide (NO) are two of the most consequential air pollutants and they are collectively known as NOx. NO2 is responsible for the brown hue that is seen in hazy conditions. These molecules are produced by the high temperature combustion of N2 and O2 gases from the air or by the burning of fuel that contains nitrogen elements. Of the two, NO2 is highly reactive and more toxic to the environment.
Sulfur dioxide (SO2)
Sulfur dioxide (SO2) is a colorless gas that comes from power plants that burn coal and oil, which contain sulfur. Another major source of SO2 is industrial processes that use sulfur-containing compounds. In high concentrations, SO2 leads to the formation of other sulfur oxides (SOX) and reacts with VOCs to form particulates.
Lead (Pb)
In the past, the motor vehicle gasoline was the largest cause of lead (Pb) emissions but efforts by the EPA have resulted in 99% reduction in lead in the air between 1980 and 2014. Today, the major sources of lead are ore and metals processing and aircrafts that use leaded aviation fuel. Waste incinerators, utilities, and lead-acid battery manufacturers also release lead into the atmosphere.

Why does the particle size matter?

The particulate matter behaves differently depending on their size. They are classified as:

– PM10; 10 micrometers or less in diameter,

– PM2.5: 2.5 micrometers or less in diameter.

For reference, the width of a human hair is 100 micrometers. The larger PM10 particles do not remain airborne and tend to fall to the ground. Fine particulates, also known as PM2.5 can remain airborne for long periods and migrate thousands of miles.

What are the environmental impacts of air pollution?

Winds can carry particulates and gases far away from the source. When the pollutants settle on ground or water, they damage the natural and built environments through acid rain. Consequently lakes, streams, and other bodies of water become acidic. This shifts the nutrient balance in coastal waters and the soil, damaging forests and farm crops and threatening biodiversity.

To read more about pollution in water, go to this page.

What are the visual impacts of air pollution?

Air pollution produces haze, which impairs visibility. Haze is caused when sunlight encounters tiny pollution particles in the air. Since 1988 the EPA has been monitoring visibility in national parks and wilderness. In 1999, the EPA implemented the Regional Haze Rule, to improve visibility and air quality in 156 national parks.

Summary of health and environmental costs by pollutant.

Health Cost
Environmental Costs
Particulate matter (PM)
Exposures to PM, particularly fine particles referred to as PM2.5, can cause harmful effects on the cardiovascular system including heart attacks and strokes. These effects can result in emergency department visits, hospitalizations and, in some cases, premature death. PM exposures are also linked to harmful respiratory effects, including asthma attacks.
Fine particles (PM2.5) are the main cause of reduced visibility (haze) in parts of the U.S., including many national parks and wilderness areas. PM can also be carried over long distances by wind and settle on soils or surface waters. The effects of settling include: making lakes and streams acidic; changing the nutrient balance in coastal waters and large river basins; depleting the nutrients in soil; damaging sensitive forests and farm crops; and affecting the diversity of ecosystems. PM can stain and damage stone and other materials, including culturally important objects such as statues and monuments.
Ozone (O3)
Ozone exposure reduces lung function and causes respiratory symptoms, such as coughing and shortness of breath. Ozone exposure also aggravates asthma and lung diseases such as emphysema leading to increased medication use, hospital admissions, and emergency department visits. Exposure to ozone may also increase the risk of premature mortality from respiratory causes. Short-term exposure to ozone is also associated with increased total non-accidental mortality, which includes deaths from respiratory causes.
Ozone damages vegetation by injuring leaves, reducing photosynthesis, impairing reproduction and growth and decreasing crop yields. Ozone damage to plants may alter ecosystem structure, reduce biodiversity and decrease plant uptake of CO2. Ozone is also a greenhouse gas that contributes to the warming of the atmosphere.
Carbon monoxide (CO)
Breathing elevated levels of CO reduces the amount of oxygen reaching the body’s organs and tissues. For those with heart disease, this can result in chest pain and other symptoms leading to hospital admissions and emergency department visits.
Emissions of CO contribute to the formation of CO2 and ozone, greenhouse gases that warm the atmosphere.
Nitrogen dioxide (NO2)
Short-term exposures to NO2 can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms, hospital admissions and emergency department visits. Long-term exposures to NO2 may contribute to asthma development and potentially increase susceptibility to respiratory infections.
NO2 and other NOX interact with water, oxygen and other chemicals in the atmosphere to form acid rain. Acid rain harms sensitive ecosystems such as lakes and forests. NOX in the atmosphere contributes to nutrient pollution in coastal waters
Sulfur dioxide (SO2)
Short-term exposure to SO2 is linked with respiratory effects, including difficulty breathing and increased asthma symptoms. These effects are particularly problematic for asthmatics while breathing deeply such as when exercising or playing. Short-term exposures to SO2 have also been connected to increased emergency department visits and hospital admissions for respiratory illnesses, particularly for at-risk populations including children, older adults and those with asthma.
At high concentrations, gaseous SOX can harm trees and plants by damaging foliage and decreasing growth.
SO2 and other sulfur oxides can contribute to acid rain which can harm sensitive ecosystems.
Lead (Pb)
Depending on the level of exposure, lead may harm the developing nervous system of children, resulting in lower IQs, learning deficits and behavioral problems. Longer-term exposure to higher levels of lead may contribute to cardiovascular effects, such as high blood pressure and heart disease in adults.
Elevated amounts of lead accumulated in soils and fresh water bodies can result in decreased growth and reproductive rates in plants and animals.
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