What Pollutant Is Driving My AQI Today?
Ozone, PM2.5, or NO₂ — learn how to tell which pollutant is behind the number and why it changes how your run feels.
Quick Answer
Your AQI is driven by whichever single pollutant has the highest sub-index at that moment. In summer it’s usually ozone (afternoon heat); in winter or near traffic it’s often PM2.5 or NO₂. Knowing which pollutant dominates helps you choose the right time, route, and intensity.
This is general guidance, not medical advice. Consult a healthcare provider if you have respiratory or cardiovascular conditions.
How AQI Is Calculated
The Air Quality Index isn’t a single measurement — it’s derived from several pollutants measured independently. Each pollutant gets its own sub-index on a 0–500 scale. The overall AQI you see is simply the highest sub-index among them.
How Sub-Indexes Work
- Measure each pollutant — PM2.5, PM10, ozone, NO₂, SO₂, and CO are tracked separately
- Convert to a sub-index — each concentration maps to a 0–500 value using EPA breakpoint tables
- Take the highest — the single worst sub-index becomes the reported AQI
This means an AQI of 85 could be driven entirely by ozone while PM2.5 sits at 30. Or it could be the reverse. The number alone doesn’t tell you which pollutant is the problem — you need to look at the dominant pollutant.
Meet the Pollutants
PM2.5 — Fine Particulate Matter
Tiny particles under 2.5 micrometers that penetrate deep into lungs and enter the bloodstream. The most health-relevant pollutant for exercisers.
When It Peaks
Winter mornings (heating, inversions), wildfire events, near busy roads year-round
How It Feels During a Run
Reduced lung capacity, a “heavy chest” feeling, and sometimes a persistent cough after your run
Ozone (O₃) — Ground-Level Ozone
A reactive gas that forms when sunlight cooks vehicle exhaust and industrial emissions. Not emitted directly — it builds up during the day.
When It Peaks
Hot, sunny afternoons in summer (typically 1–5 PM). Drops at night.
How It Feels During a Run
Chest tightness, throat irritation, reduced performance even at moderate effort. Effects increase with breathing rate.
NO₂ — Nitrogen Dioxide
A reddish-brown gas primarily from vehicle exhaust. Concentrations spike near busy roads and in street canyons.
When It Peaks
Rush hours (7–9 AM, 5–7 PM), especially on streets with heavy traffic
How It Feels During a Run
Airway irritation, wheezing, and a scratchy throat — particularly noticeable during faster efforts
PM10 — Coarse Particulate Matter
Larger particles (2.5–10 µm) from dust, construction, and pollen. Less dangerous than PM2.5 but still irritating during exercise.
When It Peaks
Dry, windy days; near construction sites; spring pollen season
How It Feels During a Run
Coughing, eye irritation, and upper-airway discomfort, especially for allergy-prone runners
Check Your Route's Air Quality
See segment-by-segment AQI along your running or cycling route before heading out.
Try Aeriqo FreeHow to Identify the Dominant Pollutant
Most air quality apps and websites show only the overall AQI number. But a few contextual clues can help you figure out what’s driving it — even before you check the breakdown.
Quick Clues
- Hot, sunny afternoon? Ozone is likely dominant. Consider running early morning instead.
- Hazy sky or wildfire smoke smell? PM2.5 is almost certainly the driver. Shorten your run or go indoors.
- Cold, still morning with no wind? PM2.5 from overnight heating and trapped air is often the culprit.
- Running along a busy road during rush hour? NO₂ is probably elevated even if the city AQI looks moderate.
Aeriqo shows the dominant pollutant when the data is available, so you can make a more informed decision without guessing.
Why It Matters for Your Run
Different pollutants call for different strategies. A blanket “AQI is 90, reduce intensity” misses the fact that ozone-driven 90 and PM2.5-driven 90 respond to different adjustments.
Practical Scenarios
- Ozone at 90: Move your run to early morning — ozone drops dramatically before sunrise. Afternoon runs are the issue, not running itself.
- PM2.5 at 90: Time of day helps less. Choose a route away from traffic and through parks. If smoke is the source, indoor training is the safest bet.
- NO₂ at 90: Shift your route 200+ meters from major roads. Even a parallel side street can cut exposure significantly.
Knowing the driver lets you adjust smartly — change timing, change route, or change venue — instead of canceling your workout entirely.
Frequently Asked Questions
Can the dominant pollutant change during the day?
Yes, frequently. PM2.5 or NO₂ might dominate in the morning rush, then ozone takes over by afternoon as sunlight builds. Checking before you head out is important because conditions from a few hours earlier may no longer apply.
Does Aeriqo show the dominant pollutant?
When the data source provides a pollutant breakdown, Aeriqo displays the dominant pollutant alongside the overall AQI. This helps you understand what’s driving the number and adjust accordingly.
Is ozone or PM2.5 worse for runners?
Both are concerning but in different ways. Ozone directly irritates airways and reduces lung function during exercise. PM2.5 penetrates deep into lungs and the bloodstream, with both short-term and cumulative health effects. Neither is safe to ignore at elevated levels.
Why does my city AQI say 'Good' when I can smell pollution?
The monitoring station may be far from your location. AQI varies block by block — you might be near a busy intersection while the station is in a park. Route-level AQI gives a more accurate picture of your actual exposure.
Should I worry about pollutants other than the dominant one?
The dominant pollutant drives your biggest risk at that moment, but cumulative exposure to all pollutants matters over time. If the overall AQI is elevated, reducing exposure is worthwhile regardless of which pollutant leads.
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