The Air Quality Index (AQI) is a 0-500 scale that rates outdoor air pollution from clean to hazardous. For runners, low numbers mean run as planned, mid-range numbers mean ease off or move indoors if you are sensitive, and high numbers mean cut intensity or stay inside. Running raises inhaled dose because fast, deep breathing pulls much more air into the lungs. The CDC and EPA note that breathing during hard exercise can move 10-20 times more air than rest.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- What do the six AQI colors mean?
- When should you shorten, slow, or move indoors?
- When should you skip the outdoor run?
- How do you check air before you lace up?
What do the six AQI colors mean?
The U.S. EPA and AirNow describe six color-coded levels on the AQI basics page. Green is Good at 0-50, yellow is Moderate at 51-100, and orange is Unhealthy for Sensitive Groups at 101-150.
Red is Unhealthy at 151-200, purple is Very Unhealthy at 201-300, and maroon is Hazardous at 301-500. Each step signals higher risk for a wider group of people. At 0-50, EPA guidance says pollution poses little or no risk. At 51-100, air quality is acceptable, but unusually sensitive people should consider cutting prolonged or heavy outdoor exertion.
When should you shorten, slow, or move indoors?
At 101-150, EPA activity guidance says children, older adults, and people with heart or lung disease should reduce prolonged or heavy exertion. Runners in those groups should shorten the workout, slow the pace, or move indoors and keep quick-relief medicine handy. At 151-200, everyone may feel effects, according to the EPA particle pollution guide in the particle pollution activity guide.
Sensitive groups should avoid prolonged or heavy exertion, while other runners should reduce exertion and take more breaks. Skip intervals, hills, and tempo runs in that red range. Swap a long run for an easy short jog, a walk, or an indoor session.
When should you skip the outdoor run?
At 201-300, EPA guidance says sensitive groups should avoid all outdoor physical activity. Everyone else should avoid prolonged or heavy exertion. Above 300, everyone should remain indoors and avoid exertion.
No workout goal outweighs that level of exposure. The reason is dose, not toughness. EPA wildfire-smoke guidance explains that strenuous outdoor exercise during smoke events sharply raises inhaled particles, as detailed in EPA strategies to reduce outdoor exposure. CDC advice is direct: avoid strenuous outdoor exercise during smoke and reduce time and intensity outside.
How do you check air before you lace up?
Check your ZIP-code AQI on AirNow.gov plus the Fire and Smoke Map before a workout. EPA guidance recommends morning and pre-run checks and scheduling activity for cooler, less-smoky times. Smoke-driven AQI can jump from safe to hazardous within hours.
Do not rely on yesterday's number or how the sky looks from your door. AQI reflects monitored ozone and particle averages, not every street-level spike. If outdoor activity is unavoidable in smoke, cloth or surgical masks give limited protection against fine particles; use a fit-tested NIOSH-approved N95.
- Check AirNow AQI and smoke map for your route
- Pick a cooler, less-smoky window for easier miles
- Have an indoor backup for orange days and above
- Cut time and intensity when smoke is present