The AQI is a nationally uniform color-coded index for reporting and forecasting daily air quality. In this case, the AQI is being used to communicate the level of PM 2.5 present. We reviewed what PM 2.5 is and its effects last fall. There is a national air quality standard for PM 2.5 and the 24 hour average standard is set at 35 ug/m3. The pollutant is measured hourly and those values are averaged to get the PM 2.5 value.
When related to the AQI, the 35 ug/m3 value standard is the threshold where air quality crosses over from “moderate” to “unhealthy for sensitive groups.” Rather than give you measurements of PM 2.5 which can be confusing, the EPA translates that to the AQI values to communicate risk to you. Check out the image below to see what the values are, who needs to be concerned at each level, and what they should do.
EPA Air Quality Index