Criteria air pollutants

Scope

The Clean Air Act identifies two types of NAAQS (40 CFR Part 50). 

1. Primary standards provide public health protection, including protection of the health of “sensitive” populations like asthmatics, children, and the elderly.
2. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

Regulatory citations

• 40 CFR Part 50 — National primary and secondary ambient air quality standards

Key definitions

• Carbon monoxide (CO): A colorless, odorless gas that can be harmful when inhaled in large amounts. CO is released when something is burned.
• High wind threshold: The minimum wind speed capable of causing particulate matter emissions from natural undisturbed lands in the area affected by a high wind dust event.
• Lead: A naturally occurring element found in small amounts in the earth’s crust. While it has some beneficial uses, it can be toxic to humans and animals, causing health effects.
• Nitrogen dioxide: One of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). It mainly gets into the air from the burning of fuel.
• Ozone: A gas composed of three atoms of oxygen. Ozone occurs in the earth’s upper atmosphere and at ground level. It can be good or bad, depending on where it is found.
• Particle pollution: A mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen by the naked eye. Others are so small they can be detected only by using an electron microscope.
• Sulfur dioxide: A colorless gas with a characteristic, irritating, pungent odor. Exposure to sulfur dioxide may cause irritation to the eyes, nose, and throat.

Summary of requirements

The standards for criteria air pollutants are as follows:

Carbon monoxide

• Averaging time of eight hours (primary)
  • Level: 9 parts per million (ppm)
  • Form: Not to be exceeded more than once per year
• Averaging time of one hour (primary)
  • Level: 35 ppm
  • Form: Not to be exceeded more than once per year

Lead

• Rolling three-month average (primary and secondary)
  • Level: 0.15 micrograms per cubic meter of air (µg/m³)
  • Form: Not to be exceeded

Nitrogen dioxide

• Averaging time of one hour (primary)
  • Level: 100 parts per billion (ppb)
  • Form: 98^th percentile of one-hour daily maximum concentrations, averaged over three years
• Averaging time of one year (primary and secondary)
  • Level: 53 ppb
  • Form: Annual mean

Ozone

• Averaging time of eight hours (primary and secondary)
  • Level: 0.070 ppm
  • Form: Annual fourth-highest daily maximum eight-hour concentration, averaged over three years

Particulate matter (PM) pollution

PM_2.5 (fine particles)

• Averaging time of one year (primary)
  • Level: 12.0 μg/m³
  • Form: Annual mean, averaged over three years
• Averaging time of one year (secondary)
  • Level: 15.0 μg/m³
  • Form: Annual mean, averaged over three years
• Averaging time of 24 hours (primary and secondary)
  • Level: 35 μg/m³
  • Form: 98^th percentile, averaged over three years

PM₁₀ (inhalable particles)

• Averaging time of 24 hours (primary and secondary)
  • Level: 150 μg/m³
  • Form: Not to be exceeded more than once per year on average over three years

Sulfur dioxide

• Averaging time of one hour (primary)
  • Level: 75 ppb
  • Form: 99^th percentile of one-hour daily maximum concentrations, averaged over three years
• Averaging time of three hours (secondary)
  • Level: 0.5 ppm
  • Form: Not to be exceeded more than once per year

Process for new or revised NAAQS. After EPA sets a new NAAQS or revises an existing standard for a criteria air pollutant, the Clean Air Act requires EPA to decide whether areas throughout the country meet the new standards. Within a year of setting a new or revised standard for a criteria pollutant, states and tribes submit recommendations to the EPA as to whether an area is attaining the standard. They base these recommendations in part on current air quality data from monitors.

EPA works with the states and tribes and considers all this information. The agency then designates an area based on whether it meets the standards. If it meets or is cleaner than the national standard, it is called an attainment area. If it doesn’t meet the standards, it’s called a nonattainment area.

Once designations go into effect, state and local governments with nonattainment areas must make implementation plans describing how areas will attain and upkeep the standards to lower air pollutant emissions. Tribes may decide to make these implementation plans but are not mandated to do so.