Acidity is measured on the pH scale ranges from 0 to 14 with 0 being acid, 7 as neutral, and 14 as alkaline. The pH scale measures the concentration of hydrogen ions in solution, which indicates acidity. The pH scale is also logarithmic, so that a change in one unit represents a tenfold change in acidity, thus a solution of pH 4 is 10 times as acidic as one with pH 5 and 100 times as acidic as pH 6. "Natural" or unpolluted rainfall is slightly acidic and has a pH of between 5.6 and 5.8. When fossil fuels, such as coal, are combusted, sulfur dioxide (SO2) and nitrogen oxide (NOx) are released into the troposphere. The primary source of acid SO2 is electrical power plants, whereas NOx comes from industrial boilers, mineral smelting plants, and automobiles. Once airborne, these acidic gases mix with precipitation such as rain, sleet, and snow and fall back to earth as “acid” rain.