Understanding the US AQI: categories, PM2.5 concentrations, and indoor action levels

How the AQI is calculated from PM2.5

The AQI is a number from 0 to 500 plus that describes air quality in a single index. It is calculated from six pollutants: ground-level ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide, PM10, and PM2.5. Each pollutant has its own breakpoint scale, and the AQI is the highest of these six.

For indoor air quality, PM2.5 usually dominates the index. Ozone is an outdoor pollutant that does not accumulate indoors when outdoor levels are normal. For PM2.5, the relationship is linear: an AQI of 100 corresponds to 35.4 micrograms per cubic meter of PM2.5.

An air quality monitor that reports only PM2.5 is measuring what matters most indoors. The AQI calculation itself happens in the monitor or on the app that displays the reading. If you see a monitor that shows both PM2.5 concentration and AQI, the AQI should match the PM2.5 value when ozone is near zero.

Good and Moderate: when you might not need a purifier

An AQI below 50 (PM2.5 below 12 micrograms per cubic meter) is good air. Most days in areas without pollution sources, indoor PM2.5 stays in this range if windows are closed. You do not need to run a purifier.

Moderate air (AQI 51 to 100, PM2.5 12 to 35 micrograms) is fine for most people. If you have allergies or asthma, or if you can smell pollution like cooking smoke, this is when to turn on a purifier. Running it on low speed is usually enough.

Unhealthy for Sensitive Groups: when to start acting

At AQI 101 to 150, anyone with asthma, allergies, or heart disease should run a purifier. This includes children and older adults. PM2.5 at 35 to 55 micrograms is when most people notice the air is not clear.

At this level, close your windows and run the purifier on medium speed. If the source is cooking or wildfire smoke, running on high is more effective. The goal is to reduce indoor PM2.5 to below the moderate threshold before the pollution peaks, because once PM2.5 builds up, it takes longer to clear.

Unhealthy and Very Unhealthy: severe outdoor pollution

At AQI 151 and above, outdoor air is dangerous. This usually happens during wildfire season or in rare pollution events in cities. At this level, close all windows and doors, run your purifier on high, and avoid outdoor activity.

Very Unhealthy air (AQI 201 to 300) is rare except during major wildfires. Hazardous air (AQI above 300) is extreme. If you live in a region where outdoor PM2.5 reaches these levels, you need a purifier large enough to handle your space: undersizing will not keep up with infiltration.

Indoor PM2.5 when windows are closed

Even with windows and doors closed, outdoor PM2.5 will eventually reach your indoor air. The rate depends on air tightness, infiltration area, and time. In a typical home, indoor PM2.5 can reach 50 to 70 percent of outdoor levels within a few hours of closed windows.

A purifier running on high can prevent this buildup. Running high for two hours when outdoor AQI is unhealthy, then switching to low, is more effective than letting indoor levels climb all day. The key is early action: once PM2.5 builds up to 100+ micrograms indoors, it takes hours to clear even with a strong purifier running.