Air quality for running: how to read the AQI

Check air quality for running separately from temperature, rain and wind. A comfortable forecast can coincide with polluted air, and a favourable running score does not tell you whether a long outdoor session is a good idea. Start with the local air-quality forecast, identify the index it uses, and read the activity advice for that day's pollutant.

Read the AQI category before choosing a run

The US Air Quality Index, or AQI, communicates pollution levels and the associated health concern. Higher values mean greater concern. AirNow's AQI explanation defines these categories:

• 0–50: Good. Pollution poses little or no health risk.
• 51–100: Moderate. Air quality is generally acceptable, but unusually sensitive people may be affected.
• 101–150: Unhealthy for Sensitive Groups. People in the relevant sensitive groups may experience health effects.
• 151–200: Unhealthy. Some people in the general population may be affected; sensitive groups face greater concern.
• 201–300: Very Unhealthy. Health concern increases for everyone.
• 301 and above: Hazardous. This is an emergency health-warning category.

These are air-quality categories, not a personalised running prescription. Read the health advice alongside the number, particularly if you have a heart or lung condition, experience symptoms, or are planning prolonged exertion. A change from 100 to 101 changes the category label; it does not identify the exact point at which every runner's experience changes.

If you are comparing readings from different services, check the index, location and forecast time. A number without those details can be misleading. For runners in Britain, the official Daily Air Quality Index uses a different scale, explained below.

Duration, effort and the pollutant change the decision

A short walk and a hard running workout are different exposures. The EPA's explanation of ozone exposure describes how the concentration, amount of air breathed and time outside all matter. Exercise increases breathing, so both the duration and intensity of your session deserve attention.

Use the activity guide for the pollutant behind the reading. AirNow publishes separate advice for particle pollution and ozone. Their guidance includes shortening a session, reducing effort, taking breaks, moving it to another time, or avoiding outdoor exercise as conditions worsen. The advice differs for sensitive groups and the general public.

For example, US AQI 120 from ozone falls in the Unhealthy for Sensitive Groups category. AirNow includes people active outdoors among the groups sensitive to ozone. Being an otherwise healthy runner does not make that advice irrelevant: the guide recommends reducing duration and intensity for sensitive groups, with consideration of a morning session when ozone is lower. Everyone else should also consider a shorter, less intense activity.

Morning is an option to investigate, not a promise of clean air. Ozone is often worse later in the day, while smoke can behave differently. Compare the local forecast across the whole time you expect to be outside.

If you develop coughing or shortness of breath, reduce your activity and follow the official guidance. If you have asthma, follow your existing asthma action plan. Changing a pace target should never mean ignoring worsening symptoms.

Wildfire smoke needs a fresh local check

Smoke levels can change considerably during a day. Before a run during a smoke event, check an up-to-date local source rather than relying on yesterday's reading or the appearance of the sky.

In the US, AirNow recommends its Fire and Smoke Map, which focuses on fine-particle pollution and provides available trends and smoke outlooks. Its guide to using AirNow during wildfires explains why a general AQI dial and the smoke map can sometimes differ, including when a nearby particle monitor is unavailable or another pollutant drives the general reading.

AirNow's smoke advice recommends limiting outdoor exercise and avoiding strenuous activities such as running when smoke is visible or can be smelled. Wait for air quality to improve or choose an indoor alternative with cleaner air. Indoor air can also be affected by outdoor pollution, so moving a workout inside is useful only when you also consider the air there.

Do not use a clear-looking sky as proof of low pollution, or a breeze as proof that smoke has gone. The useful departure check is the current local reading and advice. Keep a way to change the session if the conditions worsen.

In the UK, read DAQI on its own scale

The UK's Daily Air Quality Index, or DAQI, uses 1–10, grouped as Low (1–3), Moderate (4–6), High (7–9) and Very High (10). Defra explains how DAQI is calculated. Its numbers and category names cannot be substituted for US AQI.

Use the official UK air-quality forecast and Defra's current DAQI health advice together. At Low or Moderate levels, most people can continue their usual outdoor activity. People experiencing symptoms should reduce exposure; those with heart or lung conditions may need to adapt physically demanding outdoor activity.

At High and Very High levels, Defra advises the general public to reduce exposure, especially when experiencing symptoms. Adults and children with heart or lung conditions, and older people, should adapt physically demanding outdoor activity. Follow an existing management plan, and speak to a doctor or nurse about new or worsening symptoms.

Check which index an app displays even when the location is in the UK. WeatherToRun shows US AQI beside its forecast, including for UK locations. Read that number using US AQI categories, and check the UK official forecast and DAQI advice separately.

Make air quality part of your departure checklist

Use a short checklist before settling on a start time:

1. Check the local official reading and forecast. Confirm the location, time, index and pollutant where available.
2. Read the activity advice. Consider your symptoms, health circumstances and existing management plan.
3. Match the plan to the whole session. A long run or intervals may need a different decision from a short, easy outing. Consider shortening, reducing effort, rescheduling or choosing cleaner indoor air as the advice calls for it.
4. Check the weather as well. Temperature, dew point, gusts, rain and daylight still matter. Neither a good weather forecast nor a low AQI removes every other consideration.
5. Recheck before leaving. Keep an alternative available if the forecast or your symptoms change.

WeatherToRun keeps air quality separate from its 0–100 weather score. A high-scoring hour can still have poor air quality, so look at the AQI alongside the underlying forecast rather than choosing the highest score alone. The iPhone app shows a dash when an air-quality value is unavailable; missing information is a reason to check another source.

Use the best-time-to-run guide to compare the hours you can actually use, then make the separate air-quality check. You can explore the New York forecast or London forecast, or get WeatherToRun for iPhone to compare your hourly weather, AQI and clothing suggestions in one place.