Adjust PM2.5, PM10, NO₂, O₃, and CO concentrations to calculate AQI in real time. Compare against WHO guidelines and see the health category instantly.
AQI Formula
$\text{AQI}= \dfrac{I_{Hi}-I_{Lo}}{C_{Hi}-C_{Lo}}\times (C_p - C_{Lo}) + I_{Lo}$
Per-pollutant sub-index: piecewise-linear interpolation between concentration breakpoints. $C_p$ is the measured concentration (µg/m³ or mg/m³), $C_{Lo},C_{Hi}$ are the bounding breakpoints, and $I_{Lo},I_{Hi}$ are their corresponding AQI values.
The reported AQI is the largest sub-index — the "worst-offender" rule. The pollutant achieving this maximum is the dominant (or critical) pollutant and drives the health category.
What is the Air Quality Index (AQI)?
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What exactly is the AQI number I see on weather apps? It says "Unhealthy" but what does that really mean?
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Basically, the AQI is a single number that translates complex air pollution data into a simple health alert. It runs from 0 to 500. In practice, 0–50 is "Good," and anything over 300 is "Hazardous." Try moving the PM2.5 slider on this simulator above to 35 µg/m³. You'll see the AQI jump into the "Unhealthy for Sensitive Groups" category, which is a real trigger for people with asthma.
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Wait, really? So it's not just one pollutant? How do you get one number from PM2.5, ozone, and all the others?
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Great question! The reported AQI is actually the highest value calculated from all the major pollutants. For instance, on a hot, sunny day with heavy traffic, ground-level ozone might be the "critical pollutant" driving the index. A common case is winter, where PM2.5 from wood stoves can dominate. Try the simulator: set Ozone (O3) to a high value like 120 ppb, but keep PM2.5 low. The overall AQI will be based on the ozone.
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So the health categories are based on the worst pollutant. But are all pollutants equally harmful? What about Carbon Monoxide (CO)?
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They have different health impacts, which is why each has its own concentration-to-AQI formula. CO primarily affects oxygen delivery in the blood and is dangerous at high concentrations in enclosed spaces. In outdoor air, it's more of a tracer for traffic pollution. Notice in the simulator, you have to push the CO slider quite high (above 9 ppm) before it becomes the dominant pollutant and changes the overall AQI category. This reflects its different health threshold.
Physical Model & Key Equations
The core of the AQI is a piecewise linear function that converts the measured concentration of a pollutant (C) into an index value (I). The formula finds where C lies between two breakpoints.
Where: I = AQI value for the pollutant. C = Measured pollutant concentration (e.g., µg/m³ for PM2.5). Clow = Concentration breakpoint ≤ C. Chigh = Concentration breakpoint ≥ C. Ilow = AQI value at Clow. Ihigh = AQI value at Chigh.
Each pollutant (PM2.5, O3, etc.) has a unique table of these breakpoints defined by environmental agencies.
The Overall AQI is not an average. It is determined by the single pollutant with the highest calculated index value.
This "worst offender" principle drives public health alerts. The pollutant that achieves this maximum is called the "Critical Pollutant," and the health category (Good, Moderate, Unhealthy, etc.) is based solely on this maximum AQI value.
Frequently Asked Questions
The AQI is a 0–500 scale that summarizes air pollution into a single health-relevant number. 0–50 is "Good," 51–100 "Moderate," 101–150 "Unhealthy for Sensitive Groups," 151–200 "Unhealthy," 201–300 "Very Unhealthy," and 301+ "Hazardous." This calculator computes a sub-index per pollutant from its concentration and reports the maximum as the overall AQI.
PM2.5 (≤2.5 µm) penetrates deep into the lungs and bloodstream and is linked to cardiovascular and respiratory disease. PM10 (≤10 µm) is mostly trapped in the upper airway. The WHO 24-hour PM2.5 guideline is just 15 µg/m³, far below the 35 µg/m³ breakpoint at which most AQI scales already mark "Moderate."
For every pollutant the sub-index is computed as I = (I_Hi − I_Lo)/(C_Hi − C_Lo)·(C − C_Lo) + I_Lo using a fixed table of concentration breakpoints. The overall AQI is then the maximum of the sub-indices, and the pollutant that produces this maximum is reported as the dominant pollutant.
Tropospheric ozone is a secondary pollutant produced when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react under sunlight. It is not emitted directly, so concentrations typically peak on hot, sunny afternoons in urban areas with heavy traffic and industrial activity, and they fall after sunset.
Real-World Applications
Public Health Advisories: City governments use the AQI to issue smog alerts or "Spare the Air" days. When the AQI is forecast to be "Unhealthy," schools may cancel outdoor sports, and health departments advise sensitive groups (children, elderly, those with lung disease) to stay indoors.
Personal Exposure Management: Apps and wearable devices use real-time AQI data from local monitors. A runner might check it before a morning jog; if PM2.5 levels are high ("Unhealthy"), they might choose to run indoors instead to avoid inhaling harmful particles deep into their lungs.
Urban Planning & Policy: Long-term AQI trends are critical for evaluating the impact of policies like vehicle emission standards (affecting NO2 and CO), industrial regulations, or promoting electric vehicles. A sustained high AQI often triggers investment in public transit and clean energy.
Industrial Site Monitoring: Facilities like ports, refineries, or construction sites install air quality monitors. They track pollutants like PM10 (dust) to ensure they comply with environmental permits. Real-time AQI readouts help them mitigate operations (e.g., spraying water to suppress dust) before levels become a public nuisance or health hazard.
Common Misunderstandings and Points to Note
"AQI is an average of all pollutants" — wrong. The reported AQI is always the single highest sub-index, not an average. Two cities can both report AQI 150, but one may be driven by PM2.5 from winter heating while the other is driven by O₃ from summer photochemistry. The recommended health response is similar, but the source-control levers are very different, which is why the dominant pollutant is reported separately.
"Indoor AQI equals outdoor AQI" — wrong. Outdoor monitors do not see kitchen emissions, secondhand smoke, mold spores, or VOCs from new furniture. A poorly ventilated room can reach PM2.5 several times the outdoor value during cooking. Conversely, a sealed room with a HEPA filter can stay near AQI 0 even during a wildfire smoke event outside.
"AQI 100 is fine for everyone" — also wrong. An AQI of 100 corresponds to a regulatory short-term standard, not zero risk. Children, the elderly, pregnant women, and people with asthma or cardiovascular disease can experience symptoms below AQI 100. The WHO 24-hour guideline for PM2.5 (15 µg/m³) is much stricter than the AQI 100 breakpoint (35 µg/m³), reflecting modern epidemiology — that is why this tool also shows the concentration-to-WHO-guideline ratio.
Enter PM2.5 concentration in µg/m³ (range 0–500) into the pm25 field
Input PM10, NO2, O3, and CO values in their respective fields using standard ambient air quality units
Click Calculate to generate Overall AQI score, Health Category classification, Dominant Pollutant identification, and WHO Guideline comparison
Review the bar chart visualization showing each pollutant's contribution to total AQI and color-coded health risk zones
Worked Example
Urban monitoring station records: PM2.5 = 45 µg/m³, PM10 = 120 µg/m³, NO2 = 65 ppb, O3 = 55 ppb, CO = 1.2 ppm. Calculator returns Overall AQI = 156 (Unhealthy for Sensitive Groups), Dominant Pollutant = PM10, and flags PM2.5 exceeding WHO 24-hour guideline of 15 µg/m³. Bar chart displays PM10 as primary driver (sub-index 185) with PM2.5 secondary (sub-index 142), guiding remediation priorities.
Practical Notes
PM2.5 typically dominates AQI in industrial zones and near highways; prioritize this parameter for health risk assessment in populations with asthma or cardiovascular disease
O3 peaks mid-afternoon in warm climates (photochemical reactions); collect samples after 14:00 for accurate ground-level ozone capture
WHO 2021 guidelines recommend PM2.5 annual mean ≤15 µg/m³; many cities exceed this by 3–5× during winter inversions
NO2 contributions surge near traffic corridors; correlate spikes with rush-hour vehicle counts for source apportionment studies