Ozone is a gas with a dual nature in Earth’s atmosphere. High in the stratosphere, it forms a protective layer that absorbs harmful ultraviolet radiation from the sun. However, ground-level ozone is a harmful air pollutant and the primary ingredient in smog. This pollutant forms through chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight and high temperatures. Because ground-level ozone can trigger health problems, particularly for the lungs, monitoring air quality is necessary for public health protection.
The Air Quality Index Framework
Air quality data is based on specific pollutant concentrations. To translate this complex information into a simple, standardized message, environmental agencies rely on the Air Quality Index (AQI). The AQI is the primary tool used by the U.S. Environmental Protection Agency (EPA) to communicate the level of air pollution and its associated health risks. This index synthesizes the concentrations of five major pollutants, including ground-level ozone, into a single, easily understandable numerical and color-coded scale.
The numerical scale ranges from 0 to 500; a higher number indicates greater air pollution and a more significant health concern. An AQI value of 100 is a significant benchmark, generally corresponding to the national standard for public health protection. Values at or below 100 are considered satisfactory, while anything above 100 signals air quality that is unhealthy for at least a segment of the population.
Decoding the Color Scale
The color assigned to a specific AQI value communicates the air quality and the level of health risk. The scale begins with Green (0-50), which represents “Good” air quality. In this range, the air is satisfactory, and pollution poses little risk to the general public. As ozone levels rise, the color transitions to Yellow (51-100), indicating “Moderate” air quality. This category is acceptable, though people unusually sensitive to air pollution may experience health concerns.
The color Orange (101-150) signals the first level of unhealthy air quality, categorized as “Unhealthy for Sensitive Groups.” At this stage, individuals with lung disease, older adults, and children are most likely to experience health effects. A shift to Red (151-200) represents “Unhealthy” air quality, indicating that health effects are becoming more widespread. Some members of the general population may begin to experience issues, and sensitive groups face more serious impacts.
When the index reaches Purple (201-300), the air quality is “Very Unhealthy,” serving as a health alert. The risk of health effects is significantly increased for the entire population at this level. The final, most severe category is Maroon or Dark Red (301 and higher), which denotes “Hazardous” air quality. This level is a health warning of emergency conditions, meaning everyone is likely to experience serious health effects.
Health Implications and Protective Actions
Exposure to ground-level ozone, especially at higher color-coded levels, can irritate and inflame the airways. This highly reactive gas can cause chest pain, coughing, throat irritation, and shortness of breath, even in otherwise healthy individuals. For sensitive groups, such as those with asthma or chronic bronchitis, high ozone levels can trigger severe attacks and worsen underlying lung conditions.
People who are active outdoors, including children and older adults, are particularly susceptible to ozone’s effects because they typically breathe more deeply and rapidly. When the AQI is in the Orange range, these individuals should limit their time outdoors and reduce prolonged or strenuous exertion. As the color moves into the Red or Purple categories, everyone should take protective measures to reduce their exposure.
The actionable response to these color warnings involves modifying outdoor behavior and activity levels. For example, during a Red day, sensitive populations should entirely avoid prolonged outdoor exertion, and the general public should limit it. On a Maroon day, the recommendation is for everyone to avoid all outdoor exertion to minimize the risk of serious health consequences. Checking the daily AQI forecast allows people to plan activities, such as scheduling exercise for times when ozone levels are lower.