To be an informed (and healthy) citizen, you should know about the air you breathe. What is in air and the quality of it are essential for your existence, so are those things that might endanger it. Because air is a complex mixture of substances, knowing about it requires familiarity with certain fundamental chemical facts and concepts. Therefore, this chapter begins by considering the chemical composition of air, its major components and minor constituents including pollutants, and how their presence is expressed. Although pollutants are present, there is good news about the dramatic improvement in U.S. air quality over the past 30 years. The matter of air quality raises a theme that will frequently recur in this text — the difficult challenge of risk assessment.

In addition, you will learn about the structure of the atmosphere, and encounter the ways in which chemists categorize matter into elements and compounds and the symbols and formulas used to represent these substances. A consideration of atoms and molecules then provides a submicroscopic view of matter that helps further our comprehension. Elements and compounds, atoms and molecules undergo an amazing range of transformations. Such chemical reactions are at the very heart of chemistry, and we will focus on a few of the more important reactions that occur in the atmosphere. These, in turn, will provide an opportunity to explore the powerful shorthand of the chemist — chemical equations. Thus prepared, we will examine reasons for the significant enhancement in the quality of air in the U.S. over the past 30 years and consider the sources of the most important air pollutants in the United States and abroad. Also included is a brief discussion of indoor air quality. The chapter ends by reexamining, at the molecular level, the breath that initiated it.