At many composting sites odors originate with the incoming waste streams, which may have been stored anaerobically (without oxygen) for a week or more before transport to the site. Once these waste streams are incorporated into the composting system, subsequent odor problems are usually a result of low oxygen or anaerobic conditions. Anaerobic odors include a wide range of compounds, most notoriously the reduced sulfur compounds (e.g., hydrogen sulfide, dimethyl sulfide, dimethyl disulfide, and methanethiol), volatile fatty acids, aromatic compounds and amines. Ammonia is the most common odor that can be formed aerobically as well as anaerobically, and thus has its own set of management options.