The Environmental Protection Agency (EPA) released its 2019 report on Our Nation’s Air that details the levels of criteria pollutants that exist in our air, which have decreased by 74 percent between 1970 and 2018. Criteria pollutants are precursors of acid rain and are a cause of detrimental diseases such as asthma. During this period, the U.S. economy grew 275 percent, energy consumption increased almost 50 percent, Americans drove more miles, and population increased.


Pollutants Over Time
Source: Environmental Protection Agency

Criteria Pollutants

The Clean Air Act requires EPA to set national ambient air quality standards (NAAQS) for specific pollutants to safeguard human health and the environment. EPA established standards for six common air pollutants, which are referred to as “criteria” pollutants. They are called “criteria” pollutants because the EPA sets the criteria for permissible levels. The six criteria pollutants are:

  • Carbon monoxide (CO)
  • Lead (Pb)
  • Nitrogen dioxide (NOx)
  • Ozone (O3)
  • Particulate matter (PM2.5 and PM10)
  • Sulfur dioxide (SO2)

Between 1970 and 2018, the combined emissions of the six common declined by 74 percent, while the economy increased by 275 percent, energy consumption increased by 49 percent, vehicles miles traveled increased by 191 percent, and population increased by 60 percent—a noteworthy achievement.

America’s improving air quality is an untold success story. Even before Congress passed the Clean Air Act Extension of 1970, air quality had been improving for decades. And since 1970, that improvement has continued with the six criteria pollutants declining significantly, even though the generation of electricity from coal-fired plants has increased by 63 percent and the generation of electricity from natural gas-fired plants has increased by almost 300 percent.

According to the Energy Information Administration, sulfur dioxide emissions from U.S. power plants were reduced by 82 percent between 2007 and 2017, and nitrogen oxide emissions were reduced by 58 percent. These pollutants are the principal pollutants that cause acid precipitation (colloquially known as acid rain). Their emissions react with water vapor and other chemicals in the air to form acids that fall back to earth. Prior to controlling for these emissions, power plants produced most (about two-thirds) of the sulfur dioxide emissions in the United States. About 50 percent of nitrogen oxide emissions came from cars, buses, trucks, and other forms of transportation, with power plants contributing about 25 percent. The remainder came from other sources, such as industrial and commercial boilers.

According to the EPA, between 1990 and 2018, the criteria pollutants in the United States declined significantly as follows:

  • Carbon Monoxide (CO) 8-Hour, 74 percent
  • Lead (Pb) 3-Month Average, 82 percent (from 2010)
  • Nitrogen Dioxide (NO2) Annual, 57 percent
  • Nitrogen Dioxide (NO2) 1-Hour, 50 percent
  • Ozone (O3) 8-Hour, 21 percent
  • Particulate Matter 10 microns (PM10) 24-Hour, 26 percent
  • Particulate Matter 2.5 microns (PM5) Annual, 39 percent (from 2000)
  • Particulate Matter 2.5 microns (PM5) 24-Hour, 34 percent (from 2000)
  • Sulfur Dioxide (SO2) 1-Hour, 89 percent

Technology Matters

The huge reductions in criteria pollutants have come about mainly because technology was developed at reasonable cost to reduce these elements from fossil-fired generating plants. Flue gas desulfurization units (scrubbers), electrostatic precipitators, fabric filters (baghouses), select catalytic reduction systems, advanced carbon injection systems, and direct sorbent injection systems have reduced sulfur dioxide and nitrogen dioxide emissions.

Furthermore, new generating technologies such as supercritical coal-fired power plants being built in China and in other countries substantially reduce these pollutants. China has been building cleaner coal plants for a decade or so, at a time when government policies such as the Obama administration’s “War on Coal” precluded the effective construction of new and cleaner coal plants in the United States. As a consequence, China, and not the United States, is building the lion’s share of new supercritical coal-fired plants around the world.


The United States has cleaner air now than in the past. Legislation (e.g., the Clean Air Act and its amendments) has contributed to longstanding pollutant reductions trends. In the power sector, technology has made this possible, reducing pollutant levels at a reasonable cost that has enabled electricity to remain affordable.

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