Air pollution has been identified as a major problem in modern society, although in its conventional form of smoke and fumes, its origins date back to the Middle Ages, being closely associated with the Industrial Revolution and the use of coal. The wind can spread the acidic solution hundreds of miles; for example, much of the acid rain that falls in Scandinavia is caused by air pollution generated in the UK. Air pollutants may be classified in many ways but typically take either gaseous or particulate forms. Unwanted chemicals or other materials can enter the atmosphere either from natural sources, such as volcanoes, dust storms and forest fires, or be from human-made, known as anthropogenic, sources. In an emissions inventory, data are collected for the three types of sources (line, area and point) within the selected geographical area. The protocol aims to cut emissions from industrial sources, combustion processes and waste incineration.