Air Quality Index Calculator
Convert PM2.5 and ozone concentrations into an Air Quality Index score to gauge health risk. Use it when interpreting air monitoring data or planning outdoor activities.
About this calculator
The Air Quality Index (AQI) is a standardized 0–500 scale used by the EPA to communicate how clean or polluted outdoor air is. This calculator estimates a combined AQI using the formula: AQI = (PM2.5 / 12) × 500 + (ozone / 120) × 500. Each pollutant is scaled relative to its EPA threshold for 'Good' air quality: 12 μg/m³ for PM2.5 and 0.120 ppm for ozone. Dividing by those thresholds and multiplying by 500 maps each pollutant to the AQI scale. An AQI of 0–50 is Good, 51–100 is Moderate, 101–150 is Unhealthy for Sensitive Groups, and above 150 is Unhealthy for all. Note that the official EPA method computes separate sub-indices per pollutant and reports the highest; this calculator provides a combined approximation for educational purposes.
How to use
Suppose PM2.5 is 9 μg/m³ and ozone is 0.060 ppm. PM2.5 component: (9 / 12) × 500 = 375. Ozone component: (0.060 / 120) × 500 = 250. Combined AQI = 375 + 250 = 625 — well above the safe range, indicating heavily polluted air. Now try PM2.5 = 6 and ozone = 0.050: (6/12)×500 + (0.050/120)×500 = 250 + 208 = 458. Adjust the values to match your local monitoring station readings to interpret real-world air quality.
Frequently asked questions
What PM2.5 concentration is considered safe to breathe according to the EPA?
The EPA's annual PM2.5 standard is 12 μg/m³ as an annual mean, with a 24-hour standard of 35 μg/m³. Concentrations below 12 μg/m³ fall in the 'Good' AQI category (0–50) and pose little to no health risk for the general population. The World Health Organization recommends an even stricter annual guideline of 5 μg/m³. Short-term spikes above 35 μg/m³ can cause respiratory irritation, especially in people with asthma or heart disease.
How does ground-level ozone form and why is it harmful?
Ground-level ozone is not emitted directly but forms when nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) react in sunlight. Car exhaust, industrial emissions, and gasoline vapors are the main precursor sources. Unlike stratospheric ozone, which protects us from UV radiation, ground-level ozone is a lung irritant that can trigger asthma attacks, reduce lung function, and cause chest pain. Levels are typically highest on hot, sunny afternoons in urban areas.
When should I avoid outdoor exercise based on the Air Quality Index?
For healthy adults, outdoor exercise is generally safe when the AQI is below 100 (Good or Moderate). When the AQI reaches 101–150 (Unhealthy for Sensitive Groups), children, elderly people, and those with respiratory or cardiovascular conditions should limit prolonged outdoor exertion. At AQI 151–200, all individuals should reduce outdoor activity, especially vigorous exercise. Above 200, everyone should avoid outdoor exertion and consider using air purifiers indoors.