What the air quality index means for your breathing

The US Air Quality Index, or AQI, turns the main air pollutants into one number from 0 to 500. Higher means more pollution and more concern for health. Here is how to read it.

The bands

• 0 to 50, Good.
• 51 to 100, Moderate. Unusually sensitive people may notice symptoms.
• 101 to 150, Unhealthy for sensitive groups, which includes many people with asthma or COPD.
• 151 to 200, Unhealthy. Everyone may feel effects.
• 201 to 300, Very unhealthy.
• 301 and up, Hazardous.

Source: AirNow: Air Quality Index basics

What drives it

Most bad air days come from two things. Fine particles, known as PM2.5, come from wildfire smoke, traffic and industry. Ground-level ozone builds on hot, sunny afternoons.

What Feelcaster shows you

Feelcaster shows the index hour by hour, using EPA AirNow monitor readings in the US. If you watch for asthma, COPD or allergies, it alerts you before the air gets worse.

What you can do on a bad air day

• Keep windows closed and run a purifier if you have one.
• Move workouts indoors, or plan them for the hour with the lowest number.
• Follow the plan you made with your doctor.