How to Read the NAAQS Table: Levels, Averaging Times, and Forms Explained
The NAAQS table is the EPA's official listing of the National Ambient Air Quality Standards — the legal limits on the six criteria pollutants in outdoor air. Each row pairs a pollutant with a standard type (primary or secondary), an averaging time, a concentration level, and a "form" that defines how compliance is judged. Reading it is mostly a matter of knowing what those four columns mean.
Column by column
Pollutant
The table covers the six pollutants regulated under the criteria pollutant program: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Particulate matter appears twice, as PM2.5 and PM10 — two size classes with separate standards, which we explain in PM2.5 vs. PM10. Several pollutants have multiple rows because they have more than one standard, each with a different averaging time.
Primary or secondary
A primary standard is set to protect public health; a secondary standard is set to protect public welfare — things like visibility, crops, buildings, and ecosystems. For some pollutants the two are identical; for others they differ or only one exists. The distinction matters enough that we gave it its own page: primary vs. secondary air quality standards.
Averaging time
No standard is a limit on an instantaneous reading. Each one applies to a concentration averaged over a defined window — an hour, 8 hours, 24 hours, or a year, depending on the row. This reflects how the pollutant does harm: short averaging times target pollutants whose effects show up quickly, while annual averages target long-term exposure. That is why one pollutant can have both a short-term and an annual standard at very different numbers, and why comparing the two directly is meaningless.
Level and units
The level is the concentration itself. Watch the units, because the table mixes three: parts per million (ppm), parts per billion (ppb), and micrograms per cubic meter (µg/m³). The first two express pollutant amount as a fraction of the air by volume, and are used for gases. Micrograms per cubic meter express mass of pollutant per volume of air, and are used for particles and lead. A ppm figure and a µg/m³ figure cannot be compared without a conversion that depends on the specific gas.
Form
The form column is the part most readers skip, and it is where the legal meaning actually lives. It defines how monitored data are processed before being compared to the level — typically some combination of a statistic (an annual mean, or a specified percentile of daily values) averaged over three years. The form exists so that a single unusual day doesn't decide a region's status, but a persistent problem does. Whether an area meets a standard, form and all, determines its designation — covered in attainment vs. nonattainment areas.
What the table is not
The NAAQS table is not the AQI. The AQI is a public communication scale that translates monitored concentrations into a 0-to-500 number and a color; the NAAQS are the underlying legal standards. The two are related — AQI breakpoints are anchored to the standards — but a day's AQI reading and a region's compliance status are computed differently, as we cover in AQI vs. air quality standards. The table also says nothing about hazardous air pollutants like benzene or mercury, which have no ambient standards at all.
A reading checklist
When you look up a pollutant in the table, take four things from its row, not one:
- Which standard you're looking at (primary, secondary, or both)
- The averaging time the level applies to
- The level and its units — noting ppm vs. ppb vs. µg/m³
- The form — the statistic that actually gets compared to the level
Read that way, the table stops being a wall of numbers and becomes what it is: six pollutants, each with a small set of precisely defined legal ceilings on the outdoor air.