Is Mercury a Criteria Air Pollutant? Where It Sits in the Regulatory System
The short answer
No. Mercury is not one of the six criteria air pollutants and has no national ambient air quality standard. EPA's criteria air pollutants page names the six pollutants that carry ambient standards, and mercury is not among them — it is instead handled as a hazardous air pollutant, under a separate part of the Clean Air Act that regulates emissions from specific source categories rather than the concentration in outdoor air.
Two lists, two mechanisms
The confusion is understandable, because both systems regulate things released into the air. They work in opposite directions:
- Criteria pollutants get an ambient ceiling. EPA sets a numeric concentration limit, published in the NAAQS table with a level, averaging time, and form, and regions are judged against it by monitoring the outdoor air.
- Hazardous air pollutants get source-based emission limits. Rather than a number the air must stay under, facilities in listed categories must meet technology-based standards on what they may emit.
Our page on criteria pollutants vs. hazardous air pollutants covers the split in full, and what 40 CFR Part 63 is covers the rule family where the air toxics standards live. Mercury sits firmly on the hazardous-air-pollutant side of that line.
Is it primary or secondary?
If what you actually want is the primary/secondary classification, mercury is emitted directly — that makes it a primary pollutant in the usual sense of the term. Directly emitting sources include fuel combustion, waste incineration, and certain metals and manufacturing processes, alongside natural releases from volcanic activity and the weathering of mercury-bearing rock.
That said, mercury is unusually mobile once released. It does not simply settle where it was emitted; it changes chemical form in the atmosphere, can travel long distances, and undergoes further transformation after deposition to land and water. So while the emission itself is primary, the compound you might eventually measure somewhere downstream is often not the compound that left the stack. Our page on primary vs. secondary air pollutants explains why that distinction gets blurry for pollutants with complex atmospheric chemistry.
Why there's no mercury AQI
Because the AQI is built only from criteria pollutants. AirNow describes the index as a reporting tool covering a fixed set of pollutants with ambient standards, and each of those pollutants has a breakpoint table tying concentrations to index values. Hazardous air pollutants have no such breakpoints and no ambient standard to anchor them, so they never appear in a daily AQI number. An AQI reading tells you nothing at all about mercury in the air — not that it is low, simply that it was never part of the calculation.
This is a general point worth internalizing: the daily air quality report is a narrow instrument. It covers the pollutants that are widespread, monitored nationally, and standardized. Air toxics are tracked through different programs, different monitoring networks, and different reporting.
Where to look mercury up
For a plain-language chemical fact sheet, the ATSDR ToxFAQs collection maintained by CDC/ATSDR has an A–Z index with an entry for mercury covering what it is, how people are exposed, and what is known about its effects. Our walkthrough on looking up any chemical in ToxFAQs shows how to navigate the index and what each section of a fact sheet is for. The World Health Organization also publishes fact sheets on individual heavy metals — its lead poisoning fact sheet is a useful model of how an international health body frames a metal's exposure pathways, if you want a sense of the format before reading one.
For emissions rather than health, the relevant records are facility-level reporting and the source-category rules, not the ambient monitoring data that feeds the AQI.