Is the air quality safe to run in?
Use the AQI rather than the sky — and use EPA’s advice rather than a stronger version of it. Up to 150 the guidance is to make outdoor activity shorter and less intense, not to stop. Moving indoors enters at 151 for sensitive groups, and at 201 for everyone else.
Guidance from US EPA AirNow
Conditions
| Conditions | Risk | What to do |
|---|---|---|
| AQI 0–50 — Good | Go | A great day to be active outside. |
| AQI 51–100 — Moderate | Go | A good day to be active outside. Unusually sensitive people: consider making the run shorter and less intense, and watch for coughing or shortness of breath. |
| AQI 101–150 — Unhealthy for Sensitive Groups | Caution | Sensitive groups: make outdoor activity shorter and less intense. EPA is explicit that it is still OK to be active outdoors, taking more breaks. EPA gives no separate advice for anyone else at this level. |
| AQI 151–200 — Unhealthy | High risk | Sensitive groups: avoid long or intense outdoor activity; consider rescheduling or moving it indoors. Everyone else: reduce long or intense activity, and take more breaks. |
| AQI 201–300 — Very Unhealthy | Do not run | Sensitive groups: avoid all physical activity outdoors. Everyone else: avoid long or intense activity; consider rescheduling or moving it indoors. |
| AQI 301 and above — Hazardous | Do not run | Everyone: avoid all physical activity outdoors. |
What to do
- Use a current AQI forecast to plan the run, rather than judging by how the sky looks.
- Make it shorter and less intense before you consider cancelling it. Choosing a less intense activity, shortening it, and rescheduling are the three steps EPA lists, in that order.
- Take more breaks than you normally would when the AQI is elevated.
- Exercise away from busy roads.
- If you have asthma: follow your asthma action plan and keep quick relief medicine handy.
- If you have heart disease: treat palpitations, shortness of breath or unusual fatigue as a reason to contact your health care provider.
Practical habits from us rather than instructions from US EPA AirNow. Useful, but not the same kind of claim as the bands above.
- Check the reading again just before you go. Smoke moves quickly, and the morning forecast may not describe the evening.
- Lower the intensity as well as the distance. Ventilation rises steeply with effort, so an easy six miles takes in considerably less than a hard six — EPA’s "less intense" is doing more work than "shorter".
- A treadmill session or a rest day are both legitimate answers, and better than a hard hour in smoke.
- Wheezing, chest tightness, or a cough that starts during the run.
- Unusual breathlessness at an effort that normally feels easy.
- Burning eyes, throat or chest.
- A headache or lightheadedness that arrives with the smoke rather than with the effort.
Why
Air quality is the safety judgement runners most often make by eye, and the eye is unreliable. Wildfire smoke can measure badly on a day that merely looks hazy, and ozone — which peaks on bright hot afternoons — is invisible entirely.
Why the bands apply more sharply to runners
At rest you move perhaps six litres of air a minute. Running hard, that can rise by an order of magnitude, much of it through the mouth rather than the nose. So a runner takes in dramatically more of whatever is in the air, and takes it deeper. That is why EPA's guidance turns on how long and how intense the activity is rather than on being outdoors at all — and why the useful question is not "can I run" but "how much less of it should I do".
The useful adjustment is intensity, not just distance
Cutting a ten-mile run to six but still running it hard barely reduces your exposure. Running six miles easily does, because ventilation scales with effort. When the air is poor and you still want to move, the easy version of the run is the one worth keeping.
If you have asthma
Treat the sensitive-groups threshold as yours, carry your inhaler, and follow whatever action plan you and your clinician have agreed. Exercise-induced symptoms and poor air compound each other, and the point at which that becomes obvious is later than the point at which it starts.
Related questions
What AQI is too high to run in?
Lower than most people assume, and the honest answer is that EPA does not frame it as a cut-off at all. Its advice up to AQI 150 is to make outdoor activity shorter and less intense — it states plainly that at 101–150 it is still OK for sensitive groups to be active outdoors, taking more breaks. Avoiding long or intense outdoor activity begins at 151 for sensitive groups and at 201 for everyone else. Running is long and intense by EPA’s standard, so those are the levels that bite.
Why is running worse than just being outside?
Because you breathe far more air, and you breathe much of it through your mouth. Nasal breathing filters some particulate; mouth breathing at effort bypasses that and draws the air deeper into the lungs. The same AQI represents a much larger dose to a runner than to someone walking.
Do masks help while running?
N95 respirators filter fine particulate, but they were not designed for exercise and they make breathing harder — which is a poor trade during a run. For running specifically, reducing exposure by going indoors or postponing is the more reliable answer.
Is ozone different from smoke?
Yes. Ozone tends to peak on hot sunny afternoons, so an early morning run often avoids it. Particulate from wildfire smoke does not follow that daily pattern, so timing helps much less.
Sources
- Air Quality Guide for Particle Pollution (EPA-452/F-23-002) — the activity guidance quoted on this page — US EPA AirNow
- Air Quality Index (AQI) Basics — category boundaries — US EPA AirNow
- Wildfire Smoke and Your Health — US EPA AirNow