Commission on Pollution

A plain-English reference on air and water pollution terms, standards, and reports

AQI vs. Air Quality Standards (NAAQS): What's the Difference?

Two different tools that answer two different questions

People often use "air quality standard" and "air quality index" interchangeably, but they are separate things with separate jobs.

  • The National Ambient Air Quality Standards (NAAQS) are legal limits. They are the concentrations of certain pollutants that outdoor air is not supposed to exceed, set by the U.S. Environmental Protection Agency under the Clean Air Act. You can see the current levels and averaging times in the official NAAQS table.
  • The Air Quality Index (AQI) is a communication tool. It translates measured pollutant concentrations into a single 0–500 number and a color, so that today's air can be described to the public without anyone needing to know what a microgram per cubic meter is. The basics are laid out on AirNow's AQI page.

A useful shorthand: the standards are about whether a place meets the law over time; the index is about what the air is like today.

What the standards actually regulate

The NAAQS cover the six "criteria" pollutants — ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead — which the EPA describes on its criteria air pollutants page. We cover what each pollutant is in our guide to the six criteria air pollutants.

Each standard has three parts, and all three matter when you read the table:

  • A level — the concentration itself, such as parts per billion or micrograms per cubic meter.
  • An averaging time — one hour, eight hours, 24 hours, or a year. A pollutant can have more than one standard with different averaging times, because short spikes and long-term exposure are different problems.
  • A form — the statistical rule for deciding whether the standard is met, often based on several years of monitoring data rather than a single bad day.

That last part is why one smoky afternoon does not, by itself, put a county out of compliance. Attainment is judged over years, not hours.

How the AQI is built on top of the standards

The AQI is not independent of the standards — it is anchored to them. By design, an AQI value of 100 for a pollutant generally corresponds to the level of its short-term national standard. Below 100, air is meeting that benchmark for the day; above 100, it is worse than the benchmark. The exact conversion tables, called breakpoints, are published in the EPA's technical assistance document for reporting the daily AQI.

This anchoring is also why the AQI changes when the standards change. When the EPA tightens a standard — as it has done over the decades as health research accumulates — the breakpoints are revised so that 100 still lines up with the current benchmark. The same physical concentration of a pollutant can therefore map to a different AQI number in different years. The air did not change; the yardstick did.

Which one should you look at?

It depends on your question.

  • "Is the air okay today?" — Look at the AQI. That is exactly what it is for, and our guide on how to read the AQI walks through the categories.
  • "Does my area have a long-term air problem?" — Look at whether your area meets the NAAQS. The EPA designates areas as attainment or nonattainment for each standard, and nonattainment areas must have plans to improve.
  • "What is the actual legal limit for ozone?" — Go straight to the NAAQS table, and note the averaging time next to the number, because quoting the level without it is how most misunderstandings start.

The two systems are designed to work together: the standards define what clean air means in law, and the index tells you, day by day, where today's air sits relative to that definition.

Sources

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