Air Quality Index Calculators

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The Air Quality Index (AQI) is a standardized scale used by the US EPA (and similar systems globally) to report daily air quality and communicate associated health risks to the public. AQI values range from 0 to 500, with higher values indicating more air pollution and greater health concern. The AQI is calculated for five major air pollutants regulated by the Clean Air Act: ground-level ozone (O₃), particle pollution (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). Each pollutant has its own AQI calculation based on its National Ambient Air Quality Standard (NAAQS) breakpoints.

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AQI Categories

  • 0–50 (Good): Air quality satisfactory; little or no risk
  • 51–100 (Moderate): Acceptable; some individuals unusually sensitive to certain pollutants may be affected
  • 101–150 (Unhealthy for Sensitive Groups): Children, elderly, those with heart or lung disease at risk
  • 151–200 (Unhealthy): Everyone may experience health effects; sensitive groups at more serious risk
  • 201–300 (Very Unhealthy): Health alert; everyone may experience serious health effects
  • 301–500 (Hazardous): Health emergency; entire population at risk

How AQI is Calculated

For each pollutant: AQI_p = [(IHi − ILo) / (BPHi − BPLo)] × (Cp − BPLo) + ILo. BPHi, BPLo = concentration breakpoints; IHi, ILo = corresponding AQI values; Cp = measured pollutant concentration. The overall AQI for a location = maximum AQI among all monitored pollutants.

PM2.5 Health Effects

PM2.5 (fine particulate matter, diameter < 2.5 μm) is the pollutant of greatest health concern: penetrates deep into lungs; crosses into bloodstream; causes: cardiovascular disease; respiratory disease; lung cancer; premature death. Long-term NAAQS (2024 revision): annual PM2.5 = 9 μg/m³; 24-hour = 35 μg/m³.

Data Sources

AirNow.gov (US): real-time AQI data from EPA monitoring network and PurpleAir sensors. WHO Air Quality Guidelines: PM2.5 annual mean 5 μg/m³ (more stringent than US NAAQS).

Glossary

Air Quality Index (AQI)
A 0–500 EPA scale reporting air quality and health risk from 5 major pollutants; 0–50 = Good; 51–100 = Moderate; 101–150 = Unhealthy for sensitive groups; > 150 = unhealthy for all.
PM2.5
Fine particulate matter with diameter < 2.5 μm; the most health-significant air pollutant; penetrates deep into lungs and bloodstream; causes cardiovascular and respiratory disease; annual NAAQS = 9 μg/m³.
NowCast
A weighted average of the past 12 hours of PM2.5 or PM10 hourly measurements used to calculate AQI; gives more weight to recent readings when concentrations are changing rapidly.

Frequently Asked Questions

The Air Quality Index (AQI) is a 0–500 scale developed by the US EPA to communicate daily air quality and health risk to the public. It covers five major pollutants: ground-level ozone (O₃): forms from NOₓ + VOCs in sunlight; worst in summer afternoons. PM2.5 (fine particles < 2.5 μm): from combustion, wildfires, dust; the most health-significant pollutant. PM10 (particles 2.5–10 μm): dust, pollen. Carbon monoxide (CO): vehicle exhaust, indoor combustion. Nitrogen dioxide (NO₂): traffic, industry; indicator of combustion pollution. The reported AQI is the maximum across all measured pollutants; the 'responsible pollutant' is identified alongside the AQI value.

The AQI for PM2.5 uses the NowCast concentration (a weighted average of the past 12 hours of hourly measurements, giving more weight to recent hours when concentrations are changing). The AQI_PM2.5 formula: AQI = [(IHi − ILo) / (BPHi − BPLo)] × (Cp − BPLo) + ILo. Example breakpoints: 0–9.0 μg/m³ → AQI 0–50; 9.1–35.4 → AQI 51–100; 35.5–55.4 → AQI 101–150; 55.5–125.4 → AQI 151–200. For a PM2.5 reading of 20 μg/m³ (in the 9.1–35.4 breakpoint): AQI = [(100−51)/(35.4−9.1)] × (20−9.1) + 51 = (49/26.3) × 10.9 + 51 = 1.863 × 10.9 + 51 = 71.3 → Moderate.

Sensitive populations affected at lower AQI than the general public: Children: developing lungs; more hours outdoors per body mass; cardiovascular and respiratory effects at AQI > 100. Elderly (> 65 years): reduced respiratory reserve; cardiovascular disease susceptibility; effects at AQI > 100. People with asthma or COPD: bronchoconstriction from ozone and particles; significant symptoms possible at AQI 51–100. Heart disease patients: PM2.5 and ozone increase risk of cardiac events (arrhythmia, heart attack) at AQI > 100. General population: significant effects at AQI > 150; everyone affected at AQI > 200. Practical guidance: limit prolonged outdoor exertion when AQI > 100 for sensitive groups; > 150 for everyone.

Wildfires are a major source of PM2.5 that can drive AQI to Very Unhealthy or Hazardous levels over large regions. Wildfire smoke is a complex mixture of PM2.5, CO, NO₂, ozone precursors, volatile organic compounds, and toxic compounds (benzene, acrolein). AQI during major wildfire events: AQI > 300 (Hazardous) has occurred in western US cities during large fire seasons (AQI > 500 has been recorded in some locations, beyond the standard scale). Health effects of wildfire smoke: respiratory irritation, asthma exacerbation, cardiovascular effects, eye irritation; long-term exposure linked to respiratory and cardiovascular mortality. Protection: NIOSH-approved N95 respirators (or better) provide significant protection; KN95 and surgical masks much less effective; stay indoors with windows closed; run HEPA air purifier.