Exercise guide

Is it safe to exercise outside today?

Air quality thresholds shift depending on how hard you're working. A gentle walk and a race pace put very different demands on your lungs — so pick your activity and get a straight answer.

What are you planning?

We use live station data from the World Air Quality Index project.

Air quality in — · updated —
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US AQI · dominant pollutant —

Why air quality matters more when you exercise

When you exercise, you breathe faster and deeper, pulling more air — and more of whatever is in it — into your lungs. A pace that feels effortless on a clean-air day can leave you coughing or short of breath when pollution is high, even if you're perfectly healthy. That's why the "safe" threshold isn't one fixed number: it depends on how hard you're working.

How the thresholds work on this page

We adjust the acceptable air quality ceiling based on your selected activity level. A light walk tolerates a higher AQI than an all-out training session, because gentle breathing draws in far less polluted air per minute than sprinting or racing. The three tiers above roughly correspond to:

What actually causes the risk

The main concern during exercise is fine particulate matter (PM2.5), which is small enough to reach deep into the lungs and even enter the bloodstream. Ground-level ozone is the second major factor — it tends to peak on hot, sunny afternoons, which is worth remembering if you're planning a midday run.

General tips for training on higher-pollution days

Frequently asked questions

Is running in moderate air quality bad for you?+
For most healthy adults, occasional runs at moderate AQI (51–100) carry minimal risk. People with asthma or heart conditions may notice mild symptoms and should consider lowering intensity.
What AQI is too high to exercise outside?+
As a general guide, most people should avoid intense outdoor exercise above AQI 150, and everyone should avoid it above AQI 200. Sensitive groups should lower that ceiling further, as covered on our sensitive groups guide.
Does time of day affect air quality for exercise?+
Yes. Ozone typically peaks in the afternoon on sunny days, while traffic-related pollutants like NO₂ often peak during morning and evening rush hours. Early morning is frequently the clearest window.
Should I wear a mask while exercising outdoors?+
Standard masks can make breathing harder during intense effort and aren't generally recommended while exercising. On high-pollution days, it's usually better to move the workout indoors. See our mask guidance page for non-exercise situations.