The Air Quality Index (AQI) is a standardized tool used by governmental agencies to communicate current or forecasted air pollution levels to the public. It transforms complex scientific data about air quality into a single, understandable number, informing individuals about potential health risks. As pollutant concentrations increase, the AQI value rises, indicating a greater health concern. This index helps the public make informed decisions about outdoor activities and health protection.
Key Pollutants Measured
The Air Quality Index measures several major air pollutants known to affect public health. Ground-level ozone forms when sunlight reacts with emissions from industrial facilities and vehicle exhaust. While beneficial in the upper atmosphere, elevated ground-level ozone can cause respiratory issues. Particulate matter, including PM2.5 and PM10, consists of tiny solid or liquid particles suspended in the air, originating from sources like construction, fires, and industrial processes. These can penetrate the respiratory system, posing health risks.
Carbon monoxide is an odorless gas from incomplete fuel combustion, harmful in high concentrations. Sulfur dioxide, mainly from burning fossil fuels, impacts respiratory health. Nitrogen dioxide, a toxic gas from fossil fuel combustion and industrial activities, contributes to urban smog. Air quality stations continuously monitor these pollutants, providing real-time data for AQI calculations.
Converting Pollutant Concentrations to Individual AQI
Air quality monitoring stations collect raw concentration data for each pollutant. These measurements are expressed in units like parts per billion or micrograms per cubic meter. Before use, data undergoes calibration and quality assurance to ensure accuracy.
The conversion of concentrations to an individual AQI value uses predefined “breakpoints.” These are specific concentration ranges linked to AQI values, often in increments of 50. Environmental agencies establish breakpoints based on scientific studies correlating pollutant levels with health impacts. For instance, a PM2.5 concentration range might be assigned an AQI of 51-100, indicating moderate health concern.
If a measured pollutant concentration falls precisely on a breakpoint, its AQI value is assigned directly. However, concentrations rarely align perfectly with these values. To determine the AQI for a concentration between two breakpoints, a mathematical technique called linear interpolation is used. This approach ensures a smooth and precise conversion, accurately reflecting where the measured concentration sits proportionally within that specific breakpoint range.
For instance, if a pollutant’s concentration is measured between the lower breakpoint for “Moderate” air quality (AQI 51) and the upper breakpoint (AQI 100), linear interpolation calculates its precise AQI value. This calculation involves taking the difference between the upper and lower AQI values and dividing it by the difference between their corresponding concentration breakpoints. This ratio then helps determine the proportional AQI value for the measured concentration, which is then added to the lower AQI breakpoint. This consistent process standardizes pollutants, despite their differing measurement scales, onto a uniform AQI scale ranging from 0 to 500.
Determining the Overall AQI
After calculating individual AQI values for each pollutant, the next step is determining the overall Air Quality Index for a specific location and time. The overall AQI is determined by selecting the highest individual AQI value among all monitored pollutants, a methodology known as the “maximum method.” This ensures the reported AQI reflects the pollutant posing the greatest health threat. It signals the most pressing air quality concern to the public.
For example, if ground-level ozone has an AQI of 75 and particulate matter (PM2.5) has an AQI of 120, the overall reported AQI would be 120. By focusing on the highest individual index, the system highlights the worst-case pollution scenario, guiding public health advisories and individual actions to reduce exposure.
Understanding AQI Categories and Health Implications
The final Air Quality Index number is translated into distinct categories, each assigned a specific color and a descriptive term, to convey its meaning to the public. These categories range from “Good” to “Hazardous,” providing a quick visual assessment of air quality and associated health concerns.
- Good (Green): AQI 0-50. Air quality is satisfactory with minimal or no health risk.
- Moderate (Yellow): AQI 51-100. Air quality is acceptable, though a small number of unusually sensitive individuals might experience some health concern.
- Unhealthy for Sensitive Groups (Orange): AQI 101-150. At this level, individuals such as children, older adults, and those with heart or lung diseases may experience health effects, while the general public is less likely to be affected.
- Unhealthy (Red): AQI 151-200. Meaning the general public may begin to experience some adverse health effects, and sensitive groups could face more serious impacts.
- Very Unhealthy (Purple): AQI 201-300. Signaling a health alert where the risk of health effects is increased for everyone.
- Hazardous (Maroon): AQI 301+. Representing emergency conditions where the entire population is more likely to be affected, necessitating avoidance of all outdoor physical activity.
These categories help individuals understand when to take precautions, such as reducing outdoor exertion.