The Air Quality Index (AQI) is a tool for reporting daily air quality. It assigns numbers on a scale between 0 and 500 and is used to help people understand how clean or polluted the air is in their area. The AQI is divided into six categories, each of which corresponds to a different level of health concern and has a specific color. The categories are:

• Good (0-50): Air quality is considered satisfactory, and air pollution poses little or no risk.
• Moderate (51-100): Air quality is acceptable; however, pollution in this range may pose a moderate health concern for a very small number of individuals.
• Unhealthy for Sensitive Groups (101-150): Members of sensitive groups may experience health effects, but the general public is unlikely to be affected.
• Unhealthy (151-200): Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
• Very Unhealthy (201-300): Health alert: everyone may experience more serious health effects.
• Hazardous (>300): Health warnings of emergency conditions. The entire population is more likely to be affected.

For each pollutant, an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health. AQI values at or below 100 are generally thought of as satisfactory, while values above 100 indicate unhealthy air quality. When AQI values are above 100, air quality is unhealthy at first for certain sensitive groups of people, then for everyone as AQI values get higher.