Air Quality Index: the what, how and why

The What

An Ozone Alert is issued when ozone levels are expected to be in the “orange” range or higher on the Air Quality Index (AQI). But what does that really mean, and why does the AQI stay the same even when the National Ambient Air Quality Standards (NAAQS) have changed over the years?

The AQI is a health-based scale meant to represent how healthy the air is each day. In general we only hear about the AQI during ozone season (April 1–Oct.31 of each year) in the Kansas City area, but the AQI is actually calculated year-round. The U.S. Environmental Protection Agency (EPA) calculates an AQI for five pollutants:

air_quality_index

  1. carbon monoxide
  2. nitrogen dioxide
  3. ozone
  4. particulate matter
  5. sulfur oxides

The How

The AQI also alerts the public about which pollutant is expected to have the highest concentrations that day. As I mentioned above, ozone is Kansas City’s main pollutant, but on days where our air is impacted by fire (i.e., wildfires or rangeland burning) or in areas where there is a high percentage of residential wood-burning furnaces, particulate matter can be the main pollutant. There is only one AQI issued for a metropolitan area at a time, so while a cold day in January will have ozone levels well into the green category, particulate matter could cause a city’s AQI to creep into the yellow range. This doesn’t happen often in Kansas City, but it is not unheard of. You can always find the AQI at the AirNow website, and during ozone season on the MARC website (as well as Twitter, Facebook or your own email!)

The Why

Why does the AQI stay the same when the NAAQS (pronounced “nacks”) change? The numbers on the AQI scale correspond to the current national standards for each pollutant. For example, the current ozone standard is 75 parts per billion (ppb). If a daily air quality forecast indicates that the ozone level is expected to exceed 75ppb averaged over an eight-hour period, the AQI would be 101 or higher. That’s when we’ll issue an Ozone Alert to call attention to and help everyone understand the health impacts of poor air quality on that day. Last year’s summer was particularly hot and dry, and in Kansas City we saw 29 days that exceeded 75ppb. Particulate matter also has a level that is considered unhealthy (higher than 12 micrograms per cubic meter on an annual basis) and that level would also be indicated by an AQI of 101 or higher.

By using a consistent, color-based scale for all pollutants, it’s easier for us to communicate whether the air is healthy or not. The AQI is your one-stop shop for health-based air quality information. Still have questions? Email us at AirQ@marc.org!

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