AQI stands for Air Quality Index. Many Bay Area residents now check it before opening windows, going for a run or heading out during a wildfire smoke event. The number is easy to read but easy to misunderstand. An AQI of 100 does not mean the air is twice as polluted as an AQI of 50. A reading in the "moderate" range can also still bother people who are very sensitive to pollution. Knowing how the scale works and how Bay Area weather moves the numbers makes it easier to decide what to do on a smoky or stagnant day.
How AQI Is Calculated
The AQI is a scale from 0 to 500 created by the U.S. Environmental Protection Agency. Its purpose is to describe several different pollutants with one easy number. The scale covers ground-level ozone, fine particles called PM2.5, coarser particles called PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide.
Each pollutant is measured in its own units, such as micrograms per cubic meter for particles or parts per billion for ozone. The EPA sets a series of breakpoints for each one, based on health research, and converts the measured amount into an index value. The overall AQI for a place is simply the highest of those values. If ozone works out to 60 and PM2.5 to 110, the AQI is 110, and PM2.5 is listed as the main pollutant.
Because of those breakpoints, the scale is not a straight line. The jump from 50 to 100 does not double the pollution, and different stretches of the scale cover very different amounts of a pollutant. In 2024, the EPA tightened the PM2.5 part of the scale based on newer health studies. A PM2.5 level that once counted as "good" can now show up as "moderate."
In the Bay Area, two pollutants drive most high readings. PM2.5 dominates during wildfire smoke and on still winter nights when wood smoke builds up. Ozone, the main ingredient of smog, rises on hot summer afternoons in the inland valleys. It forms when sunlight cooks the fumes from cars and other sources.
AQI Categories and What They Mean
The scale is split into six color-coded categories, each with its own health message. The table below shows what each range means for most people.
| Color | AQI | Category | What it means |
|---|---|---|---|
| Green | 0 to 50 | Good | Little or no risk |
| Yellow | 51 to 100 | Moderate | Fine for most people, though a few very sensitive people may notice symptoms |
| Orange | 101 to 150 | Unhealthy for Sensitive Groups | People with heart or lung disease, older adults and children may be affected |
| Red | 151 to 200 | Unhealthy | Anyone may start to feel effects, and sensitive groups more strongly |
| Purple | 201 to 300 | Very Unhealthy | Higher risk of health effects for everyone |
| Maroon | 301 and higher | Hazardous | Emergency conditions in which everyone is likely to be affected |
On most days outside fire season, Bay Area readings stay green or yellow. Wildfire smoke can change that within hours, even when the fire is far away. During the Camp Fire in November 2018, smoke from Butte County pushed AQI readings across the Bay Area above 200 for several days. Schools closed, cable cars stopped running and many outdoor events were canceled. Still winter days with heavy wood burning can push some inland valleys into the orange range.
PM2.5: The Most Important Bay Area Pollutant
PM2.5 refers to particles 2.5 micrometers across or smaller, about 30 times thinner than a human hair. Particles that small slip past the body's defenses, reach deep into the lungs and can pass into the bloodstream. Breathing them can worsen asthma, reduce lung function and raise the risk of heart and lung problems. Over many years of exposure, they are linked to shorter lives. Children, older adults and people with heart or lung disease are the most vulnerable.
The Bay Area's biggest PM2.5 sources are wildfire smoke in summer and fall and wood smoke in winter. Traffic, ships, industry and cooking add to the total every day. On winter nights, a temperature inversion can trap that pollution close to the ground. A lid of warmer air sits above the cold valley air and keeps the smoke from rising and spreading out.
Readings can differ a lot from one neighborhood to the next. On a still winter evening, a coastal monitor might read 30 while a monitor in an inland valley reads 120. Near the coast, the sea breeze keeps bringing in clean ocean air. Inland valleys surrounded by hills can hold on to smoke and exhaust for hours. That is why the nearest monitor is a better guide than a single regional number.
Smoke high in the sky does not always show up in the AQI. On September 9, 2020, smoke from wildfires across Northern California turned Bay Area skies a deep orange and kept streetlights on at midday. Yet many monitors that morning showed only moderate readings. Most of the smoke sat above the marine layer, thousands of feet up, while the cool, damp air near the ground stayed fairly clean. As the marine layer mixed out later in the day and the following days, more smoke reached the ground and readings rose. The lesson is that the color of the sky and the AQI measure different things, and only the AQI tells you what you are breathing.
Deep Dive: How Is an AQI Number Calculated?
An AQI number is a measured pollution level stretched onto a common 0 to 500 scale. Each pollutant has its own set of breakpoints, and the AQI is drawn as a straight line between them. Real-time maps then smooth the latest hours of data, and low-cost sensors need a correction before their readings can be trusted.
How Does the Breakpoint Formula Work?
For each pollutant, the EPA pairs ranges of concentration with ranges of the index. Since May 6, 2024, PM2.5 of 0 to 9.0 micrograms per cubic meter counts as good, or 0 to 50. The moderate band, 51 to 100, covers 9.1 to 35.4 micrograms.
Within a band, the AQI rises in a straight line. The formula finds how far the reading sits through its concentration band and moves the same share of the way through the matching index band. By our calculation, a PM2.5 reading of 20 micrograms is about 41 percent of the way through the moderate band, which gives an AQI of about 71.
The bands are not the same width, which is why the scale bends. The moderate band spans about 26 micrograms for 50 AQI points. Higher up, the 2024 update set the unhealthy band at 55.5 to 125.4 micrograms, so the same 50 points cover a far wider range of pollution.
How Do Real-Time Maps Turn Hourly Data Into AQI?
The official AQI for particles is based on a 24-hour average, but nobody wants to wait a day during a smoke event. The EPA's AirNow system uses a method called NowCast instead. It averages the most recent 12 hours, giving the latest hours more weight when pollution is changing quickly. It needs valid data for at least two of the three most recent hours.
Low-cost sensors fill in the gaps between official monitors, but their raw readings run too high, especially in humid air. In 2021, EPA scientists Karoline Barkjohn, Brett Gantt and Andrea Clements published a nationwide correction for PurpleAir sensors in Atmospheric Measurement Techniques. It includes a humidity term, and it cut the typical error from about 8 to about 3 micrograms per cubic meter.
That correction is why the AirNow Fire and Smoke Map can show thousands of sensor readings alongside official monitors. When the map and a raw sensor reading disagree, the corrected map value is usually the better guide.
How to Use AQI Day to Day
Official readings come from regulatory monitors run by the Bay Area Air Quality Management District and posted on AirNow. These instruments are accurate but spread out, and they often report averages over an hour or longer. Low-cost sensors such as PurpleAir fill the gaps between them and update every few minutes. Their raw numbers can run high in smoke, so maps such as the AirNow Fire and Smoke Map apply a correction before showing them.
When the AQI climbs into the orange range, sensitive groups should cut back on hard outdoor exercise. In the red range, everyone should take it easier outdoors, and an N95 or KN95 mask helps if you need to be outside. Surgical and cloth masks do little against fine smoke particles. Indoors, closing windows and running a HEPA air purifier, or a box fan with a furnace filter strapped to it, can cut particle levels sharply.
On days when ozone or PM2.5 is expected to be high, the air district issues a Spare the Air alert. Summer alerts ask people to drive less, and winter alerts make it illegal to burn wood in fireplaces and wood stoves. Checking for an alert takes a few seconds and tells you whether the next day is likely to bring unhealthy air.
