Criteria Air Pollutants

Learn about six common air pollutants that are found everywhere and emitted from a range of sources: homes, motor vehicles, industry, power plants, and more.

Under the federal Clean Air Act, the U.S. Environmental Protection Agency (EPA) has established health- and science-based National Ambient Air Quality Standards for six common air pollutants that contribute to smog, acid rain, other environmental problems, and pose risks to people's health:

  • Ground-Level Ozone
  • Particle Pollution
  • Carbon Monoxide
  • Lead
  • Sulfur Dioxide
  • Nitrogen Dioxide

In Massachusetts, the Department of Environmental Protection (MassDEP) is responsible for developing implementation plans outlining how all areas of the state will meet and maintain the federal standards for criteria air pollutants. MassDEP also enforces its own ambient air quality standards.

Ground-Level Ozone

Ozone is a gas composed of three oxygen atoms that can be either beneficial or harmful depending on where it forms:

  • Good Ozone. Many miles above ground in the Earth's upper atmosphere, ozone occurs naturally and shields us from the sun's harmful ultraviolet rays.
  • Bad Ozone. Near the ground, ozone is formed when heat and sunlight cause emissions from cars, power plants, factories, and other sources to chemically react and form "smog."

New England's ozone season typically runs from April through September. Concentrations tend to be highest on warm summer days.

Smog-forming pollution tends to originate in urban areas, but can be carried long distances by the wind and affect less populated regions. In Massachusetts, smog is often a bigger problem in the rural west-central part of the state or over Cape Cod than it is in larger cities.

Learn more about ground-level ozone and how it affects our health and environment from EPA: Ozone Pollution 

Particle Pollution

Also known as "particulate matter" or PM, particle pollution includes a mixture of solids and liquid droplets. Some are emitted directly, while others are formed when other pollutants react. They come in a range of sizes:

  • Fine Particles (PM2.5). Particles up to 2.5 microns in diameter are  so small that only an electron microscope can detect them, although large concentrations can be seen as haze. Power plants, motor vehicles, wood burning, and some industrial processes generate fine particles. Because they are so tiny, these particles can penetrate deeply into the lungs.
  • Coarse Particles (PM10). Particles between 2.5 and 10 microns in diameter come from a variety of sources, such as wind-blown soil and airborne residue from business and industry. Individual particles cannot be seen with the naked eye, but collectively can appear as haze, dust, or soot.

Learn more about particle pollution and how it affects our health and environment from EPA: Particulate Matter (PM) Pollution

Carbon Monoxide

Carbon Monoxide (CO) is a poisonous gas that forms when the carbon in fuels such as gasoline, heating oil, natural gas, wood, and charcoal does not burn completely. We cannot see or smell CO, but is dangerous to our health and in high concentrations, even deadly.

Motor vehicle exhaust contributes roughly 60 percent of all carbon monoxide emissions nationwide, and up to 95 percent in cities. Air concentrations of carbon monoxide can be particularly high in areas with heavy traffic congestion. Other sources include industrial boilers, waste incinerators, and natural events such as wildfires.

Carbon monoxide levels tend to be higher during cold winter weather, when vehicles work harder and burn fuel less efficiently, and a strong inversion layer in the atmosphere traps pollution near the ground and prevents it from mixing with cleaner air above.

Learn more about carbon monoxide in outdoor air and how it affects our health and environment from EPA: Carbon Monoxide Pollution


Lead is a soft and highly toxic elemental metal found naturally in the environment. It is usually extracted from ore deposits along with copper, silver, and zinc. Australia, the United States, and China are the world's leading producers of lead.

Although lead is naturally occurring, most of it in the environment today got there from a range of earlier commercial and industrial uses by people. It has been widely used in cables, pipes, paints, and pesticides.

Historically, the major sources of lead emissions have been motor vehicles and industry. There is dramatically less toxic lead in our air now than there was in the late 1970s, when leaded gasoline was phased out across the country. Metal processing plants are the most significant remaining sources of lead in the air today.

Learn more about lead in the air and how it affects our health and environment from EPA: Lead (Pb) Air Pollution

Sulfur Dioxide

Sulfur dioxide (SO2) is a colorless, reactive gas that is produced when power plants, industrial boilers, and other facilities burn sulfur-containing fuels such as coal and oil.

Generally, air concentrations of sulfur dioxide are highest near large industrial complexes.

Learn more about sulfur dioxide and how it affects our health and environment from EPA: Sulfur Dioxide Pollution

Nitrogen Dioxide

Nitrogen dioxide (NO2) is one of a group of gases that contain nitrogen and oxygen in varying amounts, and react with other pollutants to form ground-level ozone.

Known collectively as oxides of nitrogen, or NOx, these gases are produced when fuel is burned at high temperatures. Primary sources of NOx emissions include motor vehicles, electric utilities, industry, commercial businesses, and homes.

Many NOx gases are colorless and odorless. But NO2 is often seen along with particle pollution as a reddish-brown layer in the air over major cities and urban areas.

Learn more about nitrogen dioxide and how it affects our health and environment from EPA: Nitrogen Dioxide Pollution

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