Without more specific information, it is difficult to determine what is causing bad air quality today. Air quality can be affected by a variety of factors, including weather patterns, traffic volume, and local sources of pollution such as buildings and industrial areas. The Air Quality Index (AQI) is a measure of how polluted the air is and what health effects might be a concern for you. The AQI correlates levels of different pollutants to one scale, and the higher the AQI value, the greater the health concern. The AQI is divided into six categories, each corresponding to a different level of health concern, ranging from "Good" to "Hazardous". It is important to monitor air quality and take necessary precautions when levels of pollutants are high, especially for sensitive groups.