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Take a deep breath! Now think about what you just inhaled. The air we breathe is full of many substances, some of which can harm you. EPA has established six principal pollutants used to monitor air quality for the entire country, as well as 187 hazardous air pollutants that are found in a variety of places and from a variety of sources. Air pollutants are usually referred to as emissions if they are coming from a source and ambient pollutants once they are in the air. In addition to having a significant impact on human health, these pollutants affect the environment by impacting water quality, ecosystems, and the built environment. The following are links to materials that explore a variety of pollutant topics and the current quality of Pennsylvania's air.