In its original usage, the term "Clean Coal" was used to refer to technologies that were designed to reduce emission of pollutants associated with burning coal, such as washing coal at the mine. This step removes some of the sulfur and other contaminants, including rocks and soil. This makes coal cleaner and cheaper to transport. More recently, the definition of clean coal has been expanded to include carbon capture and storage. Clean coal technology usually addresses atmospheric problems resulting from burning coal. Historically, the primary focus was on SO2 and NOx, the most important gases in causation of acid rain, and particulates which cause visible air pollution and have deleterious effects on human health.