The Air Quality Index (AQI) is a 0 to 500 scale that turns measured pollution into one health-based number. The reported AQI always comes from the single pollutant that scores worst at that hour, not an average of all of them. In Southern California, that worst pollutant is usually ground-level ozone on hot inland afternoons and fine particles (PM2.5) during wildfire smoke or winter stagnation. That one rule, the highest sub-index wins, explains most confusing AQI readings across Downtown Los Angeles, Pasadena and Woodland Hills.
What Does the Air Quality Index Measure?
The AQI measures how much health concern the outdoor air poses, on a 0 to 500 scale with six color-coded categories. The U.S. Environmental Protection Agency (EPA) calculates a separate sub-index for five regulated pollutants: ground-level ozone, PM2.5, coarse particles (PM10), carbon monoxide and sulfur dioxide.
The highest of those five sub-indexes becomes the AQI for that location and hour. The AQI is not a reading from one instrument, and it does not add pollutants together.
The pollutant behind the number is called the dominant pollutant, and it changes by hour and by station. A reading of 105 in the San Fernando Valley on a hot July afternoon is almost certainly ozone. The same 105 near the coast in November, after a red flag warning, is almost certainly PM2.5 from smoke. The number alone does not tell you which one you are breathing.
To see the dominant pollutant, check the label next to the number on AirNow or the South Coast Air Quality Management District (South Coast AQMD). South Coast AQMD regulates air pollution across the Los Angeles Basin, Orange County and the western halves of Riverside and San Bernardino counties.
| AQI range | Category | Color | General guidance |
|---|---|---|---|
| 0 to 50 | Good | Green | Air quality poses little or no risk |
| 51 to 100 | Moderate | Yellow | Acceptable; unusually sensitive individuals should consider limiting prolonged exertion |
| 101 to 150 | Unhealthy for Sensitive Groups | Orange | Children, older adults, and people with heart or lung disease should reduce prolonged outdoor exertion |
| 151 to 200 | Unhealthy | Red | Everyone may begin to experience health effects |
| 201 to 300 | Very Unhealthy | Purple | Health alert; everyone may experience more serious effects |
| 301 to 500 | Hazardous | Maroon | Health warning of emergency conditions; the entire population is more likely to be affected |
How Does a Pollutant Concentration Become an AQI Number?
A pollutant concentration becomes an AQI number through EPA breakpoint tables. Each table maps concentration ranges, in parts per billion or micrograms per cubic meter, onto the 0 to 500 scale. Each pollutant has its own table because each has different health thresholds. A monitor measures the raw concentration, and a formula places it within the right range to produce that pollutant's sub-index.
The tables make different pollutants comparable. An ozone AQI of 100 and a PM2.5 AQI of 100 carry the same health concern, the top of the Moderate category, even though the underlying concentrations use different units and averaging periods.
Ozone AQI uses an 8-hour average. PM2.5 AQI blends a 24-hour average with recent hourly readings so it responds faster to smoke. Compare AQI values, not raw concentrations. An orange sky is not an AQI reading; see why the sky turns orange during wildfire smoke.
The concentration breakpoints for each pollutant are listed in what PM2.5 measures in LA wildfire smoke and how ground-level ozone builds up in Southern California.
Which Pollutants Usually Drive Southern California AQI?
Ground-level ozone drives Southern California AQI on sunny, hot afternoons from late spring through early fall. Ozone forms when sunlight reacts with vehicle and industrial emissions over several hours as they drift inland. As a result, ozone readings peak in the mid-to-late afternoon in inland areas such as Woodland Hills and the San Gabriel Valley.
PM2.5 takes over in two situations. The first is wildfire smoke, when a fire in the region or farther north pushes smoke over the Los Angeles Basin.
The second is winter stagnation, when high pressure traps cool air near the ground for days and particles from traffic and wood burning build up. South Coast AQMD declares No-Burn Days to limit wood-burning smoke during those episodes. Its monitors cover the coast, central Los Angeles, the San Gabriel and San Fernando valleys and the Inland Empire, and the dominant pollutant often differs from one area to the next.
Why Can Nearby LA Neighborhoods Report Different AQI Values?
Nearby LA neighborhoods report different AQI values because of monitor location, wind, terrain, traffic and the averaging period. Ozone generally rises moving inland, because the afternoon sea breeze pushes coastal pollution deeper into the basin. On a hot afternoon, a station in Pasadena often reads higher ozone than a coastal station at the same hour. Wildfire smoke is even less even: it moves in plumes, so one side of a valley can sit under thick smoke while the other stays clear.
The type of device matters too. Regulatory monitors run by South Coast AQMD or the California Air Resources Board (CARB) follow federal reference methods and set official health advisories.
Modeled maps estimate AQI between monitors using weather and satellite data, which fills gaps but misses very local differences. Low-cost citizen sensors show general trends but are less accurate and can drift, especially in heavy smoke. Give official monitors the most weight when planning time outdoors at a specific address.
What Should You Do at Each AQI Level?
At each AQI level, AirNow gives separate advice for the general public and for sensitive groups, and the two split at 101. In the Good (0-50) and Moderate (51-100) ranges, outdoor activity is fine for nearly everyone; Moderate flags a small risk for people unusually sensitive to one pollutant. At Unhealthy for Sensitive Groups (101-150), children, older adults, pregnant people and people with heart or lung disease, including asthma, should cut back on long or heavy outdoor exertion. Everyone else can usually continue as normal.
At Unhealthy (151-200), AirNow extends the caution to everyone and recommends shorter, lighter outdoor activity. At Very Unhealthy (201-300) and Hazardous (301-500), avoid long outdoor exertion, and at the highest levels avoid outdoor activity entirely. The right response for any one person depends on their health, the pollutant and time outside, so use AirNow's category guidance and a doctor's advice for personal decisions.
At What AQI Should You Stay Indoors?
Sensitive groups should start limiting long outdoor exertion once the AQI passes 100, and everyone should once it passes 150, according to AirNow. The strongest advice to stay indoors and keep outdoor air out applies at Very Unhealthy and Hazardous levels, an AQI of 201 and above. These are general thresholds: a healthy adult and a person with asthma can reasonably draw the line at different numbers.
What AQI Level Is Considered Good?
An AQI of 0 to 50 is Good, the EPA's cleanest category, with little or no health risk. Moderate, 51 to 100, is acceptable for almost everyone but carries a small caution for people unusually sensitive to one pollutant, such as ozone for some people with respiratory conditions.
Should You Run the AC When Air Quality Is Bad?
Running the air conditioner on recirculate helps when air quality is bad, because it pulls in less outdoor air than open windows or fresh-air settings. Public health agencies recommend it as a basic step during smoke events. For filtration and portable air cleaner advice, use AirNow's smoke guidance and any instructions South Coast AQMD issues during an episode, since the advice depends on the pollutant and how long the event lasts.
Where Should You Check Current Southern California Air Quality?
Check current Southern California air quality on AirNow, the EPA's national site, and South Coast AQMD, which runs the densest monitoring network in the Los Angeles Basin. Both show the AQI, the dominant pollutant, the station or model behind the number and the time of the reading. The California Air Resources Board publishes statewide forecasts, and NWS Los Angeles/Oxnard and NWS San Diego issue the red flag warnings and smoke advisories that often come before a PM2.5 spike.
AQI can change within hours as the wind shifts or a smoke plume moves, so treat any reading as a snapshot of one station at one hour, not a forecast for the day.
The AQI follows one rule: the worst pollutant sets the number, and the label tells you which pollutant it is. Before spending long stretches outdoors in Downtown Los Angeles, Pasadena or Woodland Hills, check the current AQI, dominant pollutant and forecast for that neighborhood on WeatherLA.