Compliance · US EPA
US EPA Air Quality Index (AQI): PM2.5 breakpoints (2024)
Ambient air: public air quality reporting (USA)
The US EPA Air Quality Index (AQI) turns pollutant concentrations into a 0-to-500 scale with six color-coded categories, so the public can read air quality at a glance. EPA revised the PM2.5 breakpoints in 2024 with the tighter annual NAAQS. For 24-hour PM2.5, the top of each category is 9.0 µg/m³ for Good (AQI 50), 35.4 for Moderate (100), 55.4 for Unhealthy for Sensitive Groups (150), 125.4 for Unhealthy (200), and 225.4 for Very Unhealthy (300); above that is Hazardous, and AQI 500 is 325.4 µg/m³.
The standard
What the Air Quality Index Is
The AQI also covers ozone, PM10, carbon monoxide, sulfur dioxide, and nitrogen dioxide, each with its own breakpoints. For real-time reporting, EPA's NowCast weights recent hours so the index follows fast changes such as wildfire smoke.
Limit values
EPA AQI Limit Values
Every value US EPA publishes for the parameter below, with its averaging period, as the standard states it.
Who it applies to
Who Uses It
Air agencies report daily air quality through AirNow, schools and employers use it to decide on outdoor activity, and some regulations use it as a trigger: California's wildfire smoke rule for workers (section 5141.1) applies from a current AQI of 151.
Monitoring
How Monitoring Relates to It
The official AQI comes from regulatory monitors. A local monitor converted to the AQI with the same breakpoints shows conditions where people actually are, which can differ sharply from the nearest regional station during a smoke event; it is screening information rather than an official index.
Recommended
best match-
Industrial monitor for indoor and outdoor sites. Built-in sensors cover the essentials, and hot-swappable gas sensor cartridges and a differential-pressure option add more.
Built-inPM2.5Gas 1
Free: one more gas sensor cartridge
Gas 2
Free: one more gas sensor cartridge
MultiSense
Free: a MultiSense, stock or built to order
Expansion
Free: differential pressure
Also measures 12 more
Dew PointHeat IndexRelative HumidityIlluminance / LightParticle Count (by size)PM1PM10Total PM / Mass Conc.Barometric PressureEquivalent Continuous Level (Leq)Sound Pressure Level (SPL)Air Temperature
Who measures each parameter
| Parameter | Aethair PRO |  Aethair IAQ |  External |
|---|---|---|---|
| PM2.5 | Built in | Built in | 3 instruments via Thiamis |
Devices
Devices That Cover EPA AQI
How much of this standard one Aethair monitor covers, followed by the third-party instruments in the sensor catalog that measure its parameters. Each connects to Environet through a Thiamis gateway, which adds whatever the monitor leaves out.
Aethair monitors
Covers it on its own
Solutions
Solutions Judged Against EPA AQI
Outdoor Monitoring
Continuous outdoor air quality networks that sit alongside reference monitors
Wildfire Air Monitoring
Smoke plumes, black carbon, and community exposure
Article Library
Articles on EPA AQI
Wildfire Smoke Air Quality Monitoring: Detection & Response
Wildfire smoke monitoring for EHS and public health teams: real-time PM2.5, the AQI 151 respirator trigger, and how to alert workers before smoke arrives.
Indoor Air Quality in Schools: Standards & Monitoring 2026
Indoor air quality in schools: which standards apply, what to monitor, and how CO2 and ventilation affect student attendance, health, and test scores.
Types of Air Quality Monitors: Indoor, Outdoor, and Combined
Learn the three types of air quality monitors (indoor, outdoor, and combined), what each one measures, and when to use each one.
OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000
OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers.
PM2.5 Exposure Limits & Monitoring: OSHA and NAAQS Standards
What are the OSHA and NAAQS exposure limits for PM2.5? Fine particulate thresholds explained, plus how continuous monitoring keeps workplaces compliant.
Questions
EPA AQI FAQ
What PM2.5 level is Unhealthy on the AQI?
From 55.5 to 125.4 µg/m³ as a 24-hour average (AQI 151 to 200) under EPA's 2024 breakpoints; 35.5 to 55.4 µg/m³ is Unhealthy for Sensitive Groups.
What PM2.5 level is Good on the AQI?
0 to 9.0 µg/m³ as a 24-hour average, AQI 0 to 50, under the 2024 breakpoints.
What is the NowCast?
EPA's method for the current, hourly AQI, which weights recent hours more heavily so the index follows quickly changing conditions.
Monitor Against EPA AQI
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