The first part of series on odor management at composting and anaerobic digestion facilities is presented. This first article explores how odors are created, odor sources and approaches to regulating their emissions and control. Aerobic decomposition is the cornerstone of composting. At each stage of the decomposition process, there are a variety of different organic compounds, each with its own volatility characteristic. An odor is a volatile chemical gas. Volatility is the tendency of a substance to vaporize, which is proportional to a substance's vapor pressure. As an organic material decomposes, the mix of volatile compounds change, so the mix of vapor pressures changes, which in turn can change the characteristic odor. Decomposition began when the living in a material now making up the feedstock died. So odor formation begins almost immediately with the onset of decomposition. Virtually every composting facility has had to deal with a very odorous feedstock coming into the receiving area.