This year, hundreds of square miles in the western US has been on fire, resulting in terrible air quality which often has exceeded EPA guidelines.  Dr Kevin Shilling is a long time ultrarunner, was 10th at the HR100 this year, and is a pulmonary physician based in Salt Lake City.  So we ask him an important question:

When does air pollution get so bad it's better not to exercise?

"That depends on how badly you want to go for a run."

He really did say that!  Kevin provides intelligent, balanced, nuanced, and personal answers.  Four factors are:

How healthy are you, are you experiencing any problems, how does it feel?

What is your level of effort? An easy hour is different than a hard workout.

When was the last time you ran?  If yesterday, maybe take a day off and wait for better conditions; if you've already taken 3 days off, well ...

What is the AQI?  Over 100 might be unhealthy for some people while over 150 can still be tolerated by most; an AQI over 200?

"I wouldn't do it."

Check the Air Quality Index where you live right now.  If you live in some parts of the west, checking the AQI may become as common as checking the weather report.