How is air quality regulated in the US?

The 1970 Clean Air Act Amendments was the first major step in federal legislation to control air pollution. The Act required states to submit State Implementation Plans (SIPs) to meet the National Ambient Air Quality Standards (NAAQS) for pollutants considered harmful to public health and the environment. Current regulations to monitor air pollution and enforce emissions limits is based on updates to the Clean Air Act in 1977 and 1990.

For detailed look at measurement standards, air quality index, and new air pollution sensor technologies, go to this page.

Some states also have their own air quality regulatory agency. In the state of California, the Air and Resources Board specify additional standards and enforcement to the national mandate. In spite of having the most stringent standards in the country, California has eight out the ten of the most polluted cities in the US. This is due to the state’s large population (33 million) and unique geography.

Most cities in California lie in plains or in valleys surrounded by mountains. These mountains trap air pollution and prevent the air from circulating. On some days a phenomena known as temperature inversion (where the air closer to the ground becomes cooler than the air above) occurs. This prevents air near the ground from dispersing upward.

What are the standards for air quality in the US?

The EPA requires the NAAQS to be reviewed and updated every five years to account for better understanding of the health effects and welfare risks from air pollution.

The NAAQS consists of two standards:

  1. Primary standards that provide public health protection for at-risk populations, including asthmatics, children, and the elderly.
  2. Secondary standards that provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.
PollutantStandardAveraging TimeLevelForm
Carbon Monoxide (CO)primary8 hours9 ppmNot to be exceeded more than once per year
primary 1 hour35 ppmNot to be exceeded more than once per year
Lead (Pb)primary and
secondary
Rolling 3 month average0.15 μg/m3Not to be exceeded
Nitrogen Dioxide (NO2)primary1 hour100 ppb98th percentile of 1-hour daily maximum concentrations, averaged over 3 years
primary and
secondary
1 year53 ppbAnnual Mean
Ozone (O3)primary and
secondary
8 hours0.070 ppmAnnual fourth-highest daily maximum 8-hour concentration, averaged over 3 years
PM2.5primary1 year12.0 μg/m3Annual mean, averaged over 3 years
secondary1 year15.0 μg/m3Annual mean, averaged over 3 years
primary and
secondary
24 hours15.0 μg/m398th percentile, averaged over 3 years
PM10primary and
secondary
24 hours150 μg/m3Not to be exceeded more than once per year on average over 3 years
Sulfur Dioxide (SO2)primary1 hour75 ppb99th percentile of 1-hour daily maximum concentrations, averaged over 3 years
secondary3 hours0.5 ppmNot to be exceeded more than once per year

Current National Ambient Air Quality Standards (Source: EPA)

How is the air quality assessed?

The Air Quality Index (AQI) is an indicator of overall air quality and its impact on health. Each country has its own AQI which reflects the country’s air quality standards. In the US, the AQI is calculated for five pollutants: ozone, particulate pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide.

The US AQI ranges from 0 to 500 with a ranking of 0 to 100 for each individual pollutant. The higher the AQI value, the greater the overall level of air pollution. Health impacts are roughly correlated to the AQI. For example, an AQI value of 50 represents good air quality with little potential to affect public health, while an AQI value over 300 represents hazardous air quality.