Air pollution is one of the world's leading environmental health risks, responsible for approximately 7 million premature deaths per year globally according to the WHO. The Air Quality Index gives the public a single, easy-to-read number that translates complex atmospheric chemistry into actionable health guidance. Understanding how it is calculated and what each category means lets you protect yourself and your family on high-pollution days.

The Six Pollutants Behind the Number

The US AQI is calculated separately for six criteria pollutants — PM2.5, PM10, ground-level ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and carbon monoxide (CO) — and the highest single-pollutant value is the reported AQI for a given location and time period. PM2.5 is typically the dominant driver in most US cities and especially during wildfire smoke events. Ozone is the primary summer concern in urban areas where vehicle exhaust and heat drive photochemical smog formation. NO₂ and SO₂ are industrial and traffic-related pollutants most problematic near highways and power plants. CO is most relevant near heavy traffic or in poorly ventilated indoor environments. Because the AQI reports the worst pollutant, not an average, it conservatively captures the full health risk from that day's air mix. The EPA updates AQI thresholds periodically as epidemiological research refines our understanding of health impacts at various concentration levels.

What Each AQI Category Means for Daily Activities

The six AQI color bands each carry specific health implications and recommended protective actions. At AQI 0–50 (Good, green), air quality poses little or no risk — outdoor activities are unrestricted. AQI 51–100 (Moderate, yellow) is acceptable for most people, though unusually sensitive individuals may notice mild effects. AQI 101–150 (Unhealthy for Sensitive Groups, orange) is where children, elderly adults, and people with respiratory or cardiovascular disease should limit prolonged outdoor exertion; healthy adults are generally unaffected. AQI 151–200 (Unhealthy, red) affects everyone — healthy adults should limit outdoor exercise, and sensitive groups should avoid it. AQI 201–300 (Very Unhealthy, purple) warrants staying indoors; everyone should avoid outdoor exertion. Above 301 (Hazardous, maroon), health emergency conditions exist — the entire population is at risk, and outdoor activity should be avoided entirely. During wildfire events, AQI values above 500 are sometimes reported by community monitors, though the official EPA scale caps at 500.

US EPA vs WHO Air Quality Standards

There is a meaningful gap between US regulatory standards and WHO guidelines, particularly for PM2.5. The US EPA national ambient air quality standard for PM2.5 is 9 μg/m³ as an annual mean and 35 μg/m³ as a 24-hour mean. The WHO's 2021 revised guidelines are significantly stricter: 5 μg/m³ annual and 15 μg/m³ for a 24-hour mean. Many American cities that meet the EPA standard would still exceed the WHO guideline, meaning their residents experience health effects not captured by the US regulatory threshold. Long-term exposure to PM2.5 at concentrations below the EPA annual standard is associated with increased risk of cardiovascular disease, lung cancer, and cognitive decline in older adults. This gap reflects both the strength of current scientific evidence and the policy trade-offs involved in setting regulatory limits. Using the WHO guidelines as a personal health benchmark — rather than waiting for official 'unhealthy' AQI designations — is particularly advisable for sensitive individuals and families with young children.