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What AQI Numbers and Colors Mean for Outdoor Activity

The U.S. AQI translates measured pollution into six health-concern categories from green to maroon; the pollutant, current conditions, forecast and personal risk all matter when planning outdoor activity.

Timeline

  1. Before outdoor activity: Check the forecast, current AQI and dominant pollutant.
  2. When AQI rises: Use the category guidance and reduce exposure according to health risk and symptoms.
  3. As conditions change: Recheck the NowCast because smoke and ozone can vary during the day.

The U.S. Air Quality Index is EPA's public scale for communicating outdoor air pollution and health concern. Higher numbers mean more pollution and greater potential risk. The index creates a common language for five regulated pollutants: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide. A displayed AQI should therefore be read together with the pollutant driving it. [1][2]

The six categories are green 0–50, good; yellow 51–100, moderate; orange 101–150, unhealthy for sensitive groups; red 151–200, unhealthy; purple 201–300, very unhealthy; and maroon 301 or higher, hazardous. At or below 100, conditions are generally considered satisfactory, although unusually sensitive people may react in the moderate range. Above 100, concern expands as the number rises. [1]

Sensitive groups vary by pollutant. AirNow identifies people with heart or lung disease, older adults and children as being at greater risk from particle pollution. People with lung disease, older adults, children and people active outdoors may face greater risk from ozone. Individual medical advice and symptoms still matter; a color category cannot predict the exact response of every person. [3]

A forecast and a current reading answer different questions. State and local forecasters predict the next day's AQI using monitoring, weather, satellite and modeling information. EPA's NowCast converts recent hourly ozone or particle readings into the AQI scale to describe the most current hour available. Conditions can shift, so the current value is useful before activity even when the morning forecast looked acceptable. [2]

The daily index also uses different averaging periods. AirNow explains that the ozone AQI is based on an eight-hour index and particle pollution on a 24-hour index, while the NowCast weights multiple recent hours. A value shown by a monitor is consequently not a direct instant measurement of exposure everywhere nearby. Smoke plumes, wind, terrain and distance from a monitor can create local differences. [2][4]

When pollution rises, exposure can often be reduced by shortening or rescheduling strenuous outdoor activity, choosing a less intense activity or moving it indoors when indoor air is cleaner. The appropriate step depends on the pollutant, category and person's risk. AirNow publishes separate activity guides for ozone and particle pollution because their sources, timing and health advice are not identical. [4][5]

For a practical check, find the official local AQI, note whether the figure is a forecast or NowCast, identify the dominant pollutant, and read the matching health statement. Recheck during smoke or changing weather. Seek medical help for serious symptoms and follow public-health instructions during hazardous episodes; an AQI app is a planning tool rather than a diagnosis or replacement for emergency guidance. [1][2][3]

Sources

  1. EPA AirNow — AQI Basics
  2. EPA AirNow — Using the Air Quality Index
  3. EPA AirNow — Your Health
  4. EPA AirNow — Activity Guides
  5. EPA AirNow — What You Can Do

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